Aquastat hi limit controlQuestions & Answers on the High, Low, & DIFF Settings on a Heating Boiler Aquastat

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Best settings for a heating boiler aquastat control:

Here we explain how to choose the best settings for a heating boiler aquastat - the combination control that sets boiler temperature and may also control hot water production via a tankless coil on the heating boiler.

This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

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Questions & Answers About What Settings To Use on a Heating Boiler Aquastat Combination Control

Honeywell R8182D internal (C) Honeywell At AQUASTAT CONTROLS and in the more detailed article AQUASTAT HI LO DIFF SETTINGS where most of these questions and answers were first posted, we explain how aquastats work, defining the functions and dials of the aquastat HI LO and DIFF control along with the reset button often found on these heating boiler controls.

The photo at page top shows a the "HIGH" or "HI" setting on a Honeywell R8124A combination heating control, also called an "aquastat".

[Click to enlarge any image]

Question: how to set the DIFF control

(Sept 2, 2011) Joe P. said:
In the section on keeping the tankless heater hot, don't you mean turn the the Diff control to a higher setting and set the lo control to the lowest settng (120)?

This would increase the on time for the boiler, making the water hotter, thus using less hot water during a shower, and allowing longer shower times.

(Sept 14, 2011) said:

I have read this over and over again... so the boiler w/ tankless Hot water should be set w/ HI at say 190 LOW at 170 with DIFF at 10 in summer and 25 in winter?


Joe, in our text above in discussing the DIFF setting, we stated:

Where a tankless coil is installed on a heating boiler to make domestic hot water (for washing and bathing) we prefer to set the differential (DIFF) to its highest number (usually 25 degrees).

But if you want the most hot water, you'll want to set the LO up as high as you can to keep the boiler hotter for making more and hotter water (storing more heat in the boiler). The highest you can set the LO is to 20 degrees BELOW the HI setting (to avoid circulator lockout).

If you set the LOW DOWN instead of UP then when you are not calling for heat the boiler will be kept at a comparably lower temperature and you'll have less hot water than otherwise.

Thanks for the question, I'll review our text again and adjust it for clarity as needed.

Recent Aquastat Setting Questions & Answers

On 2017-01-02 23:12:50.810472 by (mod) - working on the aquastat's wiring or relays


If you can open the aquastat keeping conscious that touching the wrong contact can kill you by electrical shock, then you'll see controls like the HI Limit dial shown in the article on this page.

On 2017-01-02 15:15:34.293611 by mike 3

Have a burham 200 series gas burner my temp control is set at 220 top how do i adjust this. It a 2002 model

On 2016-12-26 17:54:16.949963 by (mod) - how to get more hot water from Aquastat Settings

Good question TMan.

The boiler (furnaces are forced air heat) turns off at the LO (you set it to 180) IF the room thermostat isn't calling for heat.


But really with a tankless coil most users liking hotel-style long hot showers will always run out of hot water. The boiler is designed for home heating, not for making hot water. The incoming cold water through the coil will almost always suck heat out of the boiler faster than the burner can put heat in - so you'll run out of hot water.


On 2016-12-26 17:45:24.607819 by TMan

Thank you for explanation of Diff control. My furnace isn't responding how described. I have 10 yr old Peerless furnace with 8 gpm Diversified coil and Honeywell controller that looks just like the pictures (206625A?).

We aren't getting enough hot water. HVAC guy suggested new coil. But, with someone in the shower, the temp goes way down before the furnace kicks on. So, I'm thinking controller problem. With settings at 180 lo, 200 hi, 25 diff, on the way down it kicks on at 170, as expected.

Here is the problem, the furnace turns off (assuming no tstat calling for heat) at the lo temp. I thought it was supposed to keep running until it gets to lo+diff=205 (but hi would prevent that). At 180, it stops. With the big coil, that isn't hot enough (we like hotel style high pressure hot showers and are willing to pay for it!). If I move it up to 190 lo, 210 hi, and leave diff at 25, the furnace still doesn't kick on until 170. With 190 lo, 210 hi, and 10 diff, furnace kicks on at 180 and our hot water is working much better.

The High hi is making me a little nervous, so I ordered a new coil. Any ideas on why furnace turns off at lo on the way up?

On 2016-12-13 14:49:18.728481 by (mod) - what should the High/Low on the aquastat Honeywell LB124A,C LB151A be set.

HI 180-200

LO 140-160

Running as hot as you can helps the distant radiators, as would insulating water heating lines. But running too hot - over 200, is unsafe.

On 2016-12-11 17:49:43.019121 by Choke51

I have a oil hot water with radiator system It has the tankless hot water but I do not use it now If I have a power outage I Have the furnace hooked up to run off a generator so I would then switch to the tankless water heat.

My question is what should the High/Low on the aquastat Honeywell LB124A,C LB151A be set.

My issue is that some radiators that are the farthest from the boiler do not get as hot as the others.I have purged all air from the system.The radiators closest to the boiler provide excellent heat

On 2016-07-12 15:58:30.684632 by (mod) - heating boiler gauge temperature reading accuracy

I agree, Alex, except I might add that monitoring the boiler temperature by its own gauge doesn't reassure me entirely: the gauge is also not precise lab-grade instrumentation. If you set the gap between HI and LO to a bit more than the 20 degF minimum then you can help assure there is no inadvertent lock-out of the boiler.

Lower temperatures waste fuel and thus are inefficient.

On 2016-07-12 03:46:12.617537 by Alex

I think with the inaccuracies of the temperature sensing equipment, after setting 200hi, 180 low diff 25 (domestic max temp 195) one should monitor the boiler to make sure these temps are reached. 5degs is small buffer if there is inaccurate guage. On my setup i just noticed our "anti-scald" valve says (right on it!) "not anti-scald"

Just some random screw based temperature mixing valve. Very hard to know what temp it's set to. Just do it by feel... 120-160 range. Set roughly middle. Would a real one make for more stable temperatures?
Thank you, i learned a lot.

On 2016-04-19 15:11:05.595920 by (mod)

I would use the "best" settings recommended in the article above. In summer when the thermostats never call for room heat, the boiler will be kept hot by the LO and DIFF settings.

On 2016-04-19 15:08:10.232279 by suzanne

I just blew thru 150 gls in 6 weeks with warm weather. I found the furnace turning on and off. Checked the high and low...the setting hi was just under 180 and low 170 and diff set at 10...I have house hold hot water coming off the boiler. What should the high low and diff be set for summer?

Question: wood burning heating boiler tied into oil fired heating boiler controls

(Oct 17, 2011) Phillip said:

I have just installed an outside wood burning furnace that ties into my oil Furnace via a water to water heat exchanger. The wood furnace cycles Hot water constantly. When the house calls for heat the oil furnace turns on for about 5 minutes or longer. I assume this a result of the colder radiator Water cycling through the heat exchanger.

Once the oil furnace gets up to temp it stops and the wood furnace keeps up. I would like to have the Oil furnace burner not turn on at all. Can I unplug the connector from the Aquastat to the burner? And will the two circulating pumps still work for the Hot water and heat.

(Dec 7, 2011) Jeff said:

I have just installed an outside wood burning furnace that ties into my oil Furnace via a 30 plate water to water heat exchanger. The wood furnace cycles
Hot water constantly at 170. My aquastat settings are HI 130 LO 110 DIFF 10. I have tried to adjust the settings in many ways to eliminate the oil furnace from running at all, but have not been able to do this.

When the house calls for heat the oil furnace turns on for about 2- 5 minutes. I know this a result of the colder baseboard
Water and the reservoir in the oil furnace cooling between calls for heat. Once the oil furnace gets up to temp it stops and the wood furnace keeps up. There are times when the circulator pump runs without the furnace running which is ideal.

I would like to have the Oil furnace burner not turn on at all. I know I can unplug the connector from the
Aquastat to the burner or add a switch and the circulating pump will still work but I was hoping to achieve the never use oil system without having to do this. What settings would you make for the HI, LO, DIFF and at what temperature would you run the wood furnace? If I bring the HI to 120 can there be be a 20 degree difference between the LO?



I agree that the shot of cold water from the home's radiators will drop temperature in the boiler and cause its burner to turn on.

If the system is wired as usual, if you disconnect electrical power to the oil burner itself the circulator pumps will continue to function normally, Phillip. In fact one could install an electrical switch just for that purpose.

Just watch out: if you leave the burner off and also leave the home such that your wood burner runs down you'll risk frozen pipe damage.

Here's an alternative to consider:

Normally for boiler efficiency we like to set the aquastat HI to 200.

If your aquastat HI limit is set to 200 F (about as high as is safe), as long as the boiler temp stays above about 180F the burner won't come on until boiler water drops below that point. The burner "on" point is fixed at about 20 degF. below the HI (assuming you are not using a tankless coil for domestic hot water and the LO limit and DIFF have been disabled).

But if you temporarily drop the HI down to 120, the "ON" point will be around 100 - that too might prevent unnecessary boiler cycling when the wood burner is working.

If you do use a tankless coil and the aquastat's LO and DIFF are left in service be sure to also keep the LO set to at least 20 degF below the new HI setting.

(Oct 18, 2011) Phillip said:

Thanks DanJoeFriedman for the response. I have a one zone house with about 18 radiators. The amount of cold radiator water is quite large to cycle through the furnace. If I understand you correctly no matter what setting I use for the Hi/Lo and differential I would still expect that the oil furnace would kick in until the temp reached the diff limit. Today I set the hi to about 140 the low to 120 and the diff to 10 degrees. My thought was that with the lo set at 120 the furnace would kick off at about 130 degrees and with the heat exchanger helping out heat the cold return water that that would minimize the amount of time the oil furnace is on.

Ideally I would like the oil burner to never kick on unless the wood furnace burns out or has a failure with its circulating pump. If there is no harm in just unconnecting the burner cable from the aquastat (there is a nice connector in the middle of the cable) then I might try that this weekend. I can always reconnect during the cold months as a backup. So just to be clear on a typical installation there is no feadback to the Aquastat from the burner unit that would cause the Aquastat from not turning on the circulating pumps when there is a call fro heat from the house or the hot water tank?


Not quite Phillip

Please review the explanation of HI LO DIFF

The boiler will start it's oil burner if on a call for heat at the thermosta the boiler temp is about 10F below the HI

The Lo and diff settings maintain heat in the boiler when there is no call for heating, in order to heat a tankless coil in summer.

I'm skipping details found in the articlesnon this topic.

Question: How to stop using the tankless coil

(Oct 27, 2011) Marc Ruland said:

I stopped using the tankless coil for domestic water last year upon having a large external tank installed (smart 40 system). I read the above article describing a disconnect of the low setting. Wouldn't that be a bad idea for winter heating as the house would call for heat and the furnace would run and run to try and bring the base boards up to temp.? Also is there a more efficeint system or way to run my current system for winter.

As it is now with my Weil-McLain Oil Boiler at Hi 200 Low 160 my furnace runs almost every 15 minutes. Is that just the way it's supposed to work? Each new winter season I always seem to forget how much that thing runs and oil isn't getting any cheaper.

(Nov 17, 2011) Josh said:
I have an old boiler in the house I just bought. It has a reckless coil for hot water. I notice that it turns on about 10 times a day for a few minutes at a time. This happens even when the heat is off (during the summer and this fall). Is it turning on just to heat water? Seems like a lot to me. On the recommendation of this article I changed the settings of the aquastat, but not sure if they are related.


Take a look at the instructions found
in AQUASTAT LO & DIFF DISABLED if you want to abandon using a tankless coil

Question: Burnham oil boiler with a honeywell aquastat set at 180/160/20 diff

(Feb 15, 2012) Nick said:

Currently I have a Burnham oil boiler with a honeywell aquastat set at 180/160/20 diff. I am planning on installing an electric hot water heater in series with the boiler so that in the winter the coil is used to preheat the water coming into the electric hot water and in summer I can fully turn off the oil boiler and only rely on the electric hot water heater, thus using no oil but still having hot water. IN the winter, when I am using the boiler to heat the house- using copper/aluminum baseboards- what are going to be my best settings.

We tend to keep the house on the cool side, as this is a guest-house that isn't always occupied. We live in Maryland, so there are some very cold days, and some not so cold. My feeling is that settings along the lines of 190/120/25 diff might be good. It seems that the low should be set very low- on a warm winter day where there is minimal call for heat, there is no reason to keep the water warm, and everything suggests that the high should be high with this type of baseboard as it is much more efficient at a high temperature. My goal is to maximize efficiency, ie minimize oil usage. What are peoples' thoughts on this??? I'm unclear what my best diff setting is. Does the diff only effect the low?

(Feb 15, 2012) Nick said:
On reading this, maybe more appropriate settings for me would be 190/120/10 diff. Thoughts?


(Feb 28, 2012) Buddy said:

Nick, I installed an electric water heater in parallel with my tankless water heater last year. You can see my comments on January 12, 2012 on this web-site under "Guide to Heating System Boiler Aquastats (AQUASTAT CONTROLS) , Their Settings & Wiring." Installing your electric water heater in series after the tankless seems to makes sense since your hot water usage is probably relatively low. My old settings were 190 hi/160 low/ 15 diff in order to ensure adequate domestic hot water temperature. This winter I have my Aquastat set at the lowest possible, 130 hi/ 110 low/ 10 diff although the actual boiler temperature seems to run about 150 hi and 120 low. I am using the electric water heater but not the tankless.

These settings have been adequate to re-heat the house with copper/aluminum finned baseboard and set-back thermostats set at 58 degrees night and 68 degrees daytime, while maintaining house temperature even on the coldest days. It does take longer to heat up the house now but the temp no longer swings up past the thermostat setting and rooms that ran colder than others before now seem to benefit from the lower boiler temperature and longer circulator run times. If your guest-house is well insulated you can probably get by with these settings. If not, you can always bump up the high temp.

The lower aquastat settings should save oil due to reduced heats loss out the flue and from the boiler casing and also from the higher heat transfer efficiency from the burner flame to the colder boiler water. The baseboard will now transfer less heat per hour per square foot at the lower temperature but the heat transfer efficiency will still be 100 percent. Efficiency will not change with water temperature, only the heat transfer rate. Unintended consequences - 1) The basement is now cooler in the summer and more humid. Running a dehumidifier may kill my savings. 2) The internal cast iron boiler sections may be more rapidly corroding due to not being heated.

(June 12, 2012) Paul said:

Nick, I have a Weil-Mclain Gold Oil Burner Model P-WTG04 series 3 boiler. I have 4 heats zones controlled by an Agro controller. I also have an Amtrol Boiler mate for hot water connected to the Argo priority zone. What should I set the Honeywell 8124 A aquastat. I am confused as the what differential and Low limit should be used, It is presently set 160 H 140 Low and 15% DIF. Thank you for your help!

(June 14, 2012) DanJoeFriedman (mod) said:

Nick, for reasons we explain in the article above, we don't prefer your aquastat settings for a boiler that includes a tankless coil.

Buddy, thanks for the helpful detail and update.

On most points including your advice to Radhames, we agree completely. Contact me by email if you want to be more fully identified as a reviewer/commentator.

(Nov 13, 2012) Buddy said:
I guess I didn't properly proof read my comment of a couple of day ago. Efficiency is actually "output divided by input". Not the other way around as I stated. Too many late nights.

Question: Would the 200/180/25 diff settings still be the best

(July 5, 2012) Dan said:

Great article. I have a somewhat more unique situation that I could use your advice on for setting the aquastat. It's a newish burnham tankless coil burner, but in a summer home so the room heat is never turned on.

Would the 200/180/25 diff settings still be the best for maximizing hot water and minimizing energy consumption, or can you recommend more appropriate settings for a burner just used to heat the tankless coil for showers, etc?


Dan - good question. You're asking: if we never call for heat, just for hot water via a tankless coil on the boiler, do we still want those "high" settings on the aquastat?

The short answer is yes. You don't want to set the HIGH ever above 200 F as it risks overheating the boiler and spilling at the TP relief valve.

You want as much HOT water as you can get for washing and bathing. But if we never call for heat, the system is always going to operate off of the LO and the DIFF settings.

180 is as high as you can set the LO - since it has to be 20 degF below the HI should proper heating ever be needed in the future.

In sum, keep the settings high and use a mixing valve or tempering valve to avoid getting scalded in the shower.

At those settings when never calling for heat, the standby losses at the boiler are very small - since the controls and boiler only have to heat up the relatively small volume of water inside the boiler itself - not the water in heating baseboards or radiators.

Question: higher boiler aquastat settings vs outdoor air temperature sensors

(Sept 17, 2012) Bob said:
Raising the temperature to save $$ seems to directly contradict the reasons why you would save by installing an outdoor air reset system. What is true?

(Oct 18, 2012) Anonymous said:
"The thermal conductivity of heating water inside of finned copper tubing baseboards or through radiator surfaces is exponentially greater at higher temperatures. In other words, hotter water actually transfers heat into the occupied space more efficiently than cooler water."

So how does an outdoor air system save money if the water temperature is lowered down to 140,150, etc. and the thermal efficiency is lowered, according to the above?


Bob, thanks for the comment. I agree that those are two contrasting approaches operating under different principles. I'm not sure there is a true/false answer when different factors are at work. Generally, though, we learn in oil burner school that longer on cycles and higher water temperatures make for higher operating efficiency of the boiler and better heat transfer.

(Nov 7, 2012) Buddy said:

Regarding (Oct 18, 2012) Anonymous comment: The relationship between heating capacities (rate of heat transfer) and average hot water temperatures for copper aluminum-finned baseboards is actually linear and proportional.

If you simply plot, on graph paper, the performance data (hot water temperatures vs. heat capacity ratings) provided by baseboard system manufacturers that follow the AHRI rating standards, you will see that the relationship is not exponential.

Simply stated, efficiency is defined as input divided by output. For a baseboard heater the term "efficiency" is actually a misnomer and should not used when discussing baseboard heat ratings. A baseboard heater merely transfers heat. The heat removed from the hot water is equal to the heat gained in the occupied space. There is no heat lost in this process. Thus, 100% of the heat is transferred. If the statement you quoted was a direct quote from a published source, I would be suspicious of any "technical" information furnished in that document.

The fuel savings produced by an ambient temperature reset system is apparently achieved by an improvement in the efficiency of the "total hydronic system" and not just one component.

Evaluation of this phenomenon requires a complex analysis of burner cycle times, heat transfer from burner flame to heating water, stack losses, heat losses during burner and/or circulating pump on/off cycles, etc. and is not easily quantified or explained. I trust that the manufacturers are correct in their analyses and actual system field tests and that actual savings can be realized using a setback system.

I do not have the money to install a setback system. However, using available internet information to determine the appropriate reset temperatures, I merely adjust my boiler's high water temperature setting several times during the heating season based on the anticipated ambient temperatures and trust that I am achieving some amount of savings.


(Nov 5, 2012) Ron Walken said:

I have a 1925 cold boiler likely with a heavier heat exchanger. It is controled with High and low setting. It never has heated the usable hot water. After learning what I could three years ago when we purchased the place I got a honeywell 3 degree set thermostat because it was only achieving 120 degrees during a cycle. It now reaches 160 plus on a cycle.

I am thinking I would like to raise the low temp setting on the control unit in the winter months to 120. My thinking is that I could select a thermostat with a 1- 2 degree swing and the home would be consistently more comfortable.

Today it swings from 68 to 72. It has a natural gas burner a good circulating pump and is installed in the center of the main floor so all heat generated would stays within the structure.

Question: L6006C Aquastat to control blower for combustion on my outdoor coal boiler.

(Nov 17, 2012) Anonymous said:
Im using a L6006C Aquastat to control blower for combustion on my outdoor coal boiler. 120v Power to terminal R , red wire to terminal B. Red wire from B to fan, white wire (neutral) other side of fan. Circuit complete.Aquestat set at 180F when it reaches 180f aquastat turns off blower? Do I have it correct?

Question: how much higher can I safely go on these aquastat settings?

(Jan 16, 2013) John B said:
Have 2 bathrooms, one shower gets enough hot water, other one begins to get cold after about 5 minutes. I've already raised the HI to 200 and LO is set at 180 on the aquastat, how much higher can I safely go on these settings? All the pipes are insulated in the basement. Thanks for your help.


You can't set the aquastat HI over 200 F without risking spilling the TP relief valve. You will want to take a look at other measures for improving hot water quantity such as flow restriction or going to an indirect fired water heater.


Question: low limit differential be set for on my electronic oil aquastat L7224?

(Feb 2, 2013) Matt said:
what should the low limit differential be set for on my electronic oil aquastat L7224?



Question: Can I set the Low to 140 to keep the furnace from turning off and on so much in warm weather?

(Mar 4, 2013) Question about settings said:

I have an oil furnace to heat the house and part of it to heat my domestic water. It is set at Hi 180 and Low 160. Diff is 25. In warm weather I have a on/off switch on my thermstat. I turn it to off because I don't need heat after April 1st. Can I set the Low to 140 to keep the furnace from turning off and on so much in warm weather? Hopefully saving some energy costs. Thanks for any help you can provide. Cliff




(Mar 4, 2013) Sal said:
I have 40 year old oil furnace heating my house, it also provides me with domestic hot water.
It is set at Hi 180 and Low 160. Diff is 25.
It seemed to work fine until It was serviced 3 months ago.
(Of course, the service folks have no clue.)

When it is working normal, the burner will go on, the circulating pump will kick in,
it will run for about 10 or 15 minutes at a time, once every hour or so.
The heat in the house seems fine, but the hot water does not last very long.

Then, this is what it does:
The burner goes on for 30 seconds, then it turns off,
one minute later,it will go on for 30 seconds, then it turns off.
and it will do it again, and again, and again, for half hour.
During this time the circulating pump is not circulating.

What is it doing ?
How do I fix it ?


(Mar 16, 2013) Buddy said:

John B.
I had a problem with our shower getting cold after a couple of minutes. I tried setting the HI at 220 and LO at 190. But since the aquastat sensor is located inside the domestic water heating coils, the rest of the boiler got very hot and the heating water exceeded 250 deg F at times. At times the boiler rumbled as if producing steam. I suggest that you NOT go above your current settings.

I found a couple of items that were causing my problem. First, the faucet in our bath tub/shower is the Delta single knob that adjusts both temperature and flow. It is about 45 years old and I have not rebuilt it in the 27 years I have owned the house. I found that even with showering at a good temperature, when the faucet was moved to the full-up high-flow position the temperature would drop. Apparently the internals are worn and there are spots that the water mix changes. Rather than spend a few hundred dollars to try to fix the problem, we no longer try for full flow and that is working just fine. Try different positions of the faucet to see if this might be the problem.

Another possibility is that the shower that gets cold is in another heating zone. My problem was that the upstairs thermostat only started the circulating pump. It would only control the temperature around the LO setting. To make matters worse, the controls were was not wired to cut out the pump when the temperature dropped below the circulator-off LO limit.

This problem was made worse since our set-back t-stat would usually turn on for the first time when we were showering and the return water to the boiler of around 63 deg F drastically dropped the boiler temperature which killed heat transfer to the domestic water. Also, the boiler controls were not wired to fire the boiler around the HI setting when the t-stat called for heat. I found that we were heating the upstairs around the LO setting . I rewired the controls and solved the problems but eventually put in and electric water heater to eliminate the frequent minor faucet adjustments needed because of the boiler's moderate temperature swings.

One last thing to check is the tempering valve that mixes heated domestic water with outside cold supply water to maintain a safe domestic hot water temperature. Mine went bad and I replaced only the internals with no soldering required.

(Mar 16, 2013) DanJoeFriedman (mod) said:

Thanks for your comments above.

First, it's never a good idea to set the HI above 200F on the aquastat - you risk dumping the relief valve.

Second, if your aquastat was above 220 your TP valve should have been spilling, so I suspect you have a safety hazard at the boiler - possibly a dangerous one.

Third, ditto my second point above if you actually saw water temperatures above 250F at times - how is this POSSIBLE unless the boiler was under pressure? Water boils at 212 F. WATCH OUT for a BLEVE boiler explosion (search InspectAPedia for BLEVE explosions for details)

Usually when hot water flow rate is poor on a tankless coil system we start by checking for mineral clogging at the tankless coil.

Just go to convenient fixture (I like sinks that use a single lever Delta type faucet handle for this trick) and run cold water at full "on", then switch over to hot at full "on". If hot flow is noticeably less than cold flow then the hot water system is probably mineral clogged, usually right at the tankless coil.

If both hot and cold flow rates are the same, then there is a different problem with system water pressure or hot water delivery piping.

If the system water piping, faucets, etc, is clogging with minerals, rust, crud, setting the temperature up doesn't fix it and actually speeds the clog procedure.

About your "... shower is in another heating zone ..." - I'm a bit confused. The tankless coil is on the heating boiler. IT doesn't know about heating zones.

However it is TRUE that IF we are enjoying a nice hot shower and a heating zone calls for heat, turning on the circulator (or opening the zone valve) for that zone sends a blast of cold water (from the cold radiators or baseboards) back into the boiler, dropping its temp and thus reducing heat for the tankless coil and for your shower). That's why the Aquastat is designed to lock out the circulator at > 20 F below HI.

Search InspectApedia for tankless coil clogging for details on diagnosing and fixing the trouble.

Question: Fleetline boilers: wood/coal, MF110/155 and oil, F-22, with a honeywell aquastat L8124

(Mar 29, 2013) Rau said:
I have two Fleetline boilers; one is a wood/coal, MF110/155 and the other is a oil, F-22, with a honeywell aquastat L8124.

My boiler technician pulled a bypass wire out of the L8124 and I may no longer use the wood/coal boiler to heat my home. The circulator will not turn on even though the oil gun is turned off by the wood/coal boiler. How do I reconnect the bypass to enable the use of the multi-fuel boiler.



Because a mistake can burn down the house or cause other unsafe conditions, I figure if your heating service technician deliberately disabled something she must have thought it was unsafe. So I can't imagine telling you to just go ahead and hook it back up.

I'd start with a call to the heating service company, speak with the service manager, and tell him your concern.

Question: 8124 aquastat set with bypass

(Mar 31, 2013) Ray said:

The 8124 aquastat was set with the bypass when originally purchased and installed and was a standard procedure for all installations so I don't think it was a safety issue at all. As of now I cannot use the wood/coal boiler at all and that defeats the purpose of having it which is not the original intent. thanks for your comment.


Ray, I'm sorry but I can't say I have a certain and clear picture of what you have installed and thus you must understand the need for caution.

The only bypass wire I know about on the L8124 aquastat is not a "bypass" - at all. There is a wire that can be pulled to disable the LO limit - a feature that is not needed if a boiler is not making use of a tankless coil for domestic hot water. We discuss this and show the blue wire in the article above.

What you describe is not a tankless coil (search InspectApedia for TANKLESS COIL to see what those are) but rather an indirect-fired water heater that uses boiler water through a coil in the heater to heat a tank or reservoir of hot water. (Search InspectApedia for Indirect-Fired Hot Water Heaters to see how those heaters work)

If your indirect fired water heater is properly hooked up, it runs as just another heating zone - you boiler doesn't know it's heating hot water rather than a room in the house.

The LO and DIFF are almost irrelevant to this operation and use, and that may be why your tech disabled that circuit. You don't need it. If you mess with the LO and DIFF and the tech did NOT disable that circuit then you could set the control improperly (e.g. setting the LO too close to HI) and you'll SNAFU the system and lose hot water.

If the LO is connected then turning it up too high locks out the circulator.

Leave the LO and DIFF alone - the indirect water heater runs as a heating zone just like any other heating zone.

Unless your tech did something odd.

Question: DIFF settings

(Feb 3, 2014) emes said:
lo 120 dif 10 - burner off at 120 burner on at 110
lo 120 dif 25 - burner off at 135 burner on at 110
lo 110 dif 10 - burner off at 110 burner on at 100
lo 110 dif 20 - burner off at 120 burner on at 100 etc.

(Mar 14, 2014) Anonymous said:
I set the high to 180 and low to 160 what should the diff be set at the boiler only heats base board in the house


As the DIF pertains ONLY to operation of the lo-limit when using a tankless coil, it's not doing anything for you. You can set it down to its lowest setting and if you are not going to disable the LO - which is not needed when there is no tankless coil, you could set that down lowest as well.

Question: cycling hot water temperature drops too low

(Mar 30, 2014) Anonymous said:

I have a oil fired HWBB with an internal hot water coil. The low is at 160, the high is at 180, the diff is at 10. When we shower, the hot water takes a reasonable amount of time to get to the shower, but after 30 secs or so, the water goes to room temperature, stays there for another 30 secs, then gets hot again and stays there. Any ideas?


Interesting; I'm not sure what's happening. On a call for hot water it's the LO and DIFF that are in control. Set the DIFF to its maximum and you should get better performance. Let us know what that does to your system.

(Oct 17, 2014) Anonymous said:

Sounds more like the water in the pipes are in a warm area then a cold area and then finally the hot water gets to your shower.... pipes carry a lot of water and if you have an efficient shower head (recommended always) then it can take some time to get to the hot water from the heater especially if they are the old pipes and your shower is a ways (by pipe not necessarily actual distance to furnace)..

You may want to trace the pipes from the shower to the furnace and see where it goes.. through walls which are on the outside of house vs inside, higher lower, etc and even turn the shower on as you hold the pipe at various places to see if what I say is true. Sometimes it is the simple obvious reason in this case just physical having noting to do with furnace, which is why some people are getting instant heaters from Europe and installing them.

Speaking of which when i was in Germany the coolest thing I saw was a stainless steal multi tubed towel rack which had the the hot water going through it in the bathroom to dry towels and cloths.. very effective and space saving in small bathrooms with toilet tanks in the wall and use half of what we do per flush and have two flushes, small flush for pee and full tank for solids.... why not in America? Because Americant's have no clue and can't stop being wasteful, can't read the instruction the law the Constitution etc and seemingly can't find or figure out how to use the other brain cell... :-} Let us know what you find .. very interested.

Question: odd thermostat operation

(July 28, 2014) colton said:

I have a dual zone heating system which is oil hotwater with a seperate electric hotwater heater if I adjust the upstairs thermostat to 70 degrees the furnace runs until it reaches that temp if I do the same on the downstairs thermostat the furnace runs and shuts off over and over again and does'nt reach desired temp i had thermostat replaced and four different service people check it's operation and was told that's how it was suppose to work.Any help or suggestions would be great Thank's.



Let's sort out what may be a wiring error by making sure the upstairs heat is OFF and the downstairs thermostat is calling for heat. If the odd behavior continues I'd lookf or a circulator or relay problem.

Question: how do I bypass or disable hot water heating on the boiler

(Oct 19, 2014) Anonymous said:

I just installed a hybrid electric hot water heater and need to know how to bypass or disable the call for Domestic Hot Water for the house and only run the oil boiler for heat. Help



In the More Reading links above the article you want is


Question: wide difference between aquastat settings and boiler temperature gauge readings

12/30/2014 Ryan said:

Great site and info. I noticed that my aquastat and my boiler temp gauge are a good 20 degrees different.

My Aquastat's HI might be set to 180 but the boilder temp gauge reads 200 before it shuts off then pending no heat is being called, it might heatsoak all the way to 220. I turned the acquastat HI down to 160 and that seems to make the boiler shut off at 185 and max heat soak i've seen was 205 in this configuration. Do I need a new aquastat or does my compensation method work?

Also, is it worth re-greasing the probe well to ensure the aquastat is in good contact with the probe well wall? Any other suggestions?

Another quick question... My boiler has a tankless water heater coil to provide hot water. Recently I had a tech install a 50gal storage tank. The storage tank has no wires going into the the aquastat of the boiler - the storage tank has it's own aquastat that turns a zone circulator on/off. The zone circulator is connected to the in/out of the boiler hot water coil.

Do I still need to set my LOW and DIFF on my boiler aquastat (and if so, can you recommend a setting)? What does the storage tank aquastat do in this set-up and how should I set that in conjuction with the boiler LOW/DIFF settings?

Currently my hot water measures a max of 155 - 160 degrees coming out of a faucet on full blast hot. Looking for some advice on settings that could save me some $$ in oil yet have little impact on current performance (I.E currently 2 simultaneous 10min showers without issue and plenty of hot left over)



While aquastats and boiler pressure/temperasture gauges are not lab-grade highly-precise instruments, a 20 degree difference may say something is wrong - or might be normal. It depends. Certainly the HI or upper limit setting, say set to 180F should result in a boiler gauge temperature reading close to 180F when the boiler SHUTS OFF at the end of a heat-on cycle.

But not always. For example if the burner is firing but the thermostat call for heat is satisfied (the thermostat then turns "OFF", at that point the boiler will stop firing even though it has not reached the cut-off temperature.

But your boiler gauge readings of 20-40 degF above the HI setting suggests something's wrong. It could be a faulty temperature sensor, poor contact between the temperature sensor in its well and the sides of the insertion well (use heat-conductive grease provided by the manufacturer) or a faulty control.

It's possible the trouble is your boiler's gauge - it might be sticking. Because typically if the boiler were truly reaching abnormally high temps, over 200F, the pressure/temperature relief valve should be spilling. I might try installing a new gauge first.

For details on how to set the HI LO DIFF optimally, in More Reading above see the article titled AQUASTAT OPTIMAL LO/DIFF SETTING - and let me know if questions remain.

12/30/2014 Ryan said:

Thanks Joe. At the moment, the aquastat is set to 150HIGH and 120 LOW which is resulting in consistent boiler temps of 180 - 185 HIGH (even when heat soak comes into factor) and 150 - 160 LOW.

I don't notice the boiler temp gauge sticking, it swings very fluidly but I'll read into what it takes to replace the boiler gauge. Since the aquastat is consistently reading LOWER temps then the boiler gauge, i'm guessing (hoping) based on your response the aquastat is not touching the inside of the probe well. I'll take a look and re-insert grease where needed.

On the other subject I posted about: Boiler coil connected to storage tank; Do you have any thoughts there?
" Another quick question... My boiler has a tankless water heater coil to provide hot water.

Recently I had a tech install a 50gal storage tank. The storage tank has no wires going into the the aquastat of the boiler - the storage tank has it's own aquastat that turns a zone circulator on/off. The zone circulator is connected to the in/out of the boiler hot water coil. Do I still need to set my LOW and DIFF on my boiler aquastat (and if so, can you recommend a setting)? What does the storage tank aquastat do in this set-up and how should I set that in conjuction with the boiler LOW/DIFF settings? Currently my hot water measures a max of 155 - 160 degrees coming out of a faucet on full blast hot. Looking for some advice on settings that could save me some $$ in oil yet have little impact on current performance (I.E currently 2 simultaneous 10min showers without issue and plenty of hot left over) "


Ryan if the gauge seems OK I suspect that the temp sensor on the aquastat is defective or it is not in good thermal contact in the sensor well. Some heating service techs deliberately skipped the manufacturers' instructions to use a thermal grease when installing the sensor, complaining that the grease gets stiff and makes later control replacement difficult. But I believe that the manufacturer knows what's most important in successful installation of their product. Further, the current thermal grease has been improved and doesn't create a stuck-sensor issue any longer.

Ryan if you are using an indirect fired water heater with its own heating zone and controls, you can abandon the tankless coil and disable the appropriate parts of the aquastat - see


OR there are other options for using both the indirect water heater AND the tankless coil. For your set-up I don't recommend this, but one can pipe the tankless coil after the outlet from the indirect water heater so that the coil only comes into play as a booster unit if the first heater has run out of hot water. I wouldn't do it. A decent indirect fired water heater is efficient, a good use of the boiler as heat source, and most likely the added support of the tankless coil is not needed.

Question: how to abandon a tankless coil & what aquastat changes are needed

Dec 30, 2014) Ryan asked

How do I get rid of a tankless coil


Dec 30, 2014) DanJoeFriedman (mod) said:
Ryan if you are using an indirect fired water heater with its own heating zone and controls, you can abandon the tankless coil and disable the appropriate parts of the aquastat - see


OR there are other options for using both the indirect water heater AND the tankless coil. For your set-up I don't recommend this, but one can pipe the tankless coil after the outlet from the indirect water heater so that the coil only comes into play as a booster unit if the first heater has run out of hot water. I wouldn't do it. A decent indirect fired water heater is efficient, a good use of the boiler as heat source, and most likely the added support of the tankless coil is not needed.

(Dec 30, 2014) DanJoeFriedman (mod) said:
Ben, thanks for your comments.

We agree and have stated that longer boiler-burner ON times are more efficient ways to run any oil fired heating system.

Our comments AND illustration of how the DIFF works on aquastats is excerpted direclty from the manufacturer's control installation manual data sheets.

About water temperatures and uncomfortable operation, indeed we, and most manufacturers, recoimmend use of an automatic tempering valve at the boiler where a tankless coil is in use, both to avoid scalding burns and to permit higher boiler temperature operation for both greater efficiency and more total hot water supply (longer hot water at the shower before running out).

Question: best aquastat settings when leaving a house unattended in winter

(Jan 3, 2015) Rick said:
Going south for 5 months. What settings are best for my aqua stat considering the boiler does not need to make domestic hot water, only has to maintain a thermostat setting of 50 deg. Hi? Lo? Diff? Leaving Monday 1/5. What do you reccomend for normal settings for optimum savings? Thank you..


(Jan 3, 2015) DanJoeFriedman (mod) said:
Rather than un-wire the control in this case, you can leave it alone. It's winter where you are; the heating thermostat settings are more important to keep the house safe from freezing without wasting fuel. The hot water features won't come into play during heating.


Question: not enough heat from the tankless coil - what's the best aquastat setting?

(Jan 13, 2015) Anonymous said:
my heat is fine but my showers are just warm for small amount of time i have a oil burner and water all in one with baseboard heat what setting would be best


Just as we suggested above in this article: HI 200 LO 180 DIFF 25


(Feb 11, 2015) Phyllis said:
Thank you so much for this. I just had work done on my oil burner and have been losing hot water in the shower ever since. It would take forever to get hot again and only at the hottest setting. Checked the settings and the diff was at 10.



Glad to assist Phyllis; your questions or comments help us to

Question: setting hi to 200, low 180 and diff to 25, I have been getting sufficient domestic hot water but when the circulator kicks on there is a lot of banging of pipes

(Feb 15, 2015) Joe said:
Since setting hi to 200, low 180 and diff to 25, I have been getting sufficient domestic hot water but when the circulator kicks on there is a lot of banging of pipes and herring the hot water run through the system. I have bleed the radiators to release any air but still the banging. Any suggestions? Btw I have a Peerless boiler with tankless domestic hot water



Usually water hammer, that can occur in heating systems too, happens when a pump turns off or when a valve closes; it's not a feature of water temperature.


Question: my high limit is 185 and my low is 170 is this the best way

21 Feb 2015 pat said:
I have a hot water peerless boiler with hydronic heat. hot water by air. my high limit is 185 and my low is 170 is this the best way to run the furnance?

Reply: NO


For a hot water heating boiler system your aquastat control setting is incorrect and locks out the circulator. You want to set the LO at least 20 degrees below the HI

Details are in the article above.

Question: boiler is short cycling

24 Feb 2015 Anonymous said:
Settings found hot water boiler which also supplies domestic hot water. Hi 180 f low 150 f dif 15 deg.going through a lot of oil,and the furnace runs for a short time and shuts off. It cycles like this often, also the thermostat temp. is not satisfied when the furnace goes off. any suggestions




we discuss oil burner short-cycling and offer some diagnostic suggestions. Take a look there and let me know how you make out.

Question: boiler works good, then shuts down and wont' come back on

(Feb 25, 2015) Luis Torres said:
When my boiler reach a set temp. and the circulator come on it works good 'till reach the set temp in the house thermostat at 70
Then the boiler shots down and do not come back on. The water cools down and not heat.
I have to go to the basement and reset
What should I do?



The fact that your boiler is shutting off on safety reset means you should not keep re-starting it as doing so is unsafe and risks a puff-back explosion or other unpleasant event. The chief cause of a boiler shutting down on reset is typically need of cleaning and maintenance but of course there could be a failing component. Call your heating service company.

Question: change the aquastat settings for summer months?

14 March 2015 Karl R said:

I have an older Weil-McLain P-366 oil-fired boiler feeding my low-pressure hot-water baseboard heat registers accompanied by an Amtrol BoilerMate WH9Z indirect-fired water heater for my domestic hot water needs (set up in it's own zone)----all controlled by a Honeywell L8124G-1046 Aquastat.

My question is - I understand I can lower the upper/lower/Diff settings on the aquastat during the summer months to reduce the boiler cycling - thereby reducing fuel costs. What would be the most cost-effective settings on the aquastat to provide 135 degree temperature domestic hot-water output from the above Amtrol unit during these summer months ?

Now going further, (and I apologize for being so long-winded)- for the winter months, after intense study of your excellent informational Wiki---I have the aquastat set at 190/170 with a Diff setting of 10. I'm not sure this is the best and most efficient setting for my setup.

I find I cannot trust the OLD settings (210/190 Diff 10), as during trouble-shooting of a triggered pressure relief valve, (and the replacement of same, along with the replacement of the expansion tank, Hy-Vent, and pressure-reducing valve on the incoming cold supply---all were leaking)..

I found that the previous owner (or perhaps his oil-service technician) had cut the Amtrol water heater out of the loop (by closing various valves) -- meaning domestic hot water supply had been coming DIRECTLY out of the boiler (and no, this is NOT a tankless-coil boiler) for at LEAST as long as I've owned the house (5 years- AND I've had the boiler inspected and safety-checked more than once in that time)...Needless to say, everything done before is suspect.

My second question is.....what is the most cost-efficient WINTER setting for the aquastat, given the above setup ?


Hi Karl - in the Article Series Contents links above take a look at


Normally in summer when we don't call for heat the boiler is running only off of the LO and DIFF and you don't need to make adjustments.

17 March 2015 Karl R. said:

Yano...the answer to the below query was in your fine wiki site, and it might explain why the Amtrol indirect-fired water tank was cut out of the system in the first's quite likely the coil in that tank has developed a leak...forcing the pressure to equalize between the street-pressure of the domestic hot water and the closed low-pressure boiler system, correct ? That said, and if that's the that coil conducive to repair (Soldering, brazing, etc) ? Or, failing that, I've seen some articles about a form of 'stop-leak' introduced into the system.....are these viable options ?


No, probably not. Please see TANKLESS COIL / HOT WATER COIL LEAKS

Question: Aquastat settings on a water heater

(Mar 16, 2015) 6006A aquastat said:
I have a 100 gal nat gas hotwater heater with a circulation pump and a 6006A honeywell aquastat. it has a temperature wheel and a diff wheel. i just want to know what the correct settings should be. Currently the temp is set at 100 degrees and the Diff wheel is set at 3. Honeywell does not tell you and refers to the hot water heater manufacturer which is AO smith and there is no mention. Can you help?

(Apr 13, 2015) need your help said:
do you need pc setting to on if your using a armtrol boilermate. Also what are the setting for L7224u aquastat



Question: Disabling the Low Limit for home heating only

(Nov 3, 2015) J R said:
Hi, I want to use oil boiler to heat the home only. So, I just want to know what are the best settings (Hi,Lo, diff) to heat the home only?


JR, just take a quick look at the section in the article AQUASTAT HI LO DIFF SETTINGS about disabling the low limit.


(Nov 16, 2015) Terence said:
Hello i think my cir has lockout.i change my expansion tank an took the probe out (being lazy didnt want to go back up an turn the cut off switch)but now my oil boiler wont fire up an the aquastat is reading c.i.r off can u help me


Sorry Terence I'm confused as I don't quite understand what you did, what probe, what switch. Try turning the system off and on again.

ALso search InspectApedia for AIR BOUND HEATING SYSTEMS

Question: Controlling a Peerless wbv-03 oil boiler with hydrostat and beckett burner

(Dec 26, 2015) Richie said:
Hey guys just had some questions. I purchased a new house with a peerless wbv-03 oil boiler with hydrostat and beckett burner no water heater.
When we moved in we noticed hot water in shower would last 1 min and then go to cold.

After adjusting the cartridge in single handle hot water came out more but died in 2 mins now. After reviewing this site and adjusting the hydrostat from 150lo/180hi to 180lo/200hi 25diff on both his and lo we are now able to take 7-8 min showers before the water goes warm. Now I noticed that hot water pressure in the house is very low.

If I take a shower the hot water in the house fades drastically if someone opens a faucet but cold water is strong. What could be my issue? Also if I take a shower the only way to get hot water is to turn my handle all the way to hot position anything below that water is cold. Any suggestions



You made your boiler hotter which will give more heat to the tankless coil in the boiler, but if the tankless coil is clogging or clogged that will reduce the hot water pressure or flow rate - though not the total quantity of hot water.

Double check the boiler temperature and see if the home is heating while you're showering.

(Dec 27, 2015) Richie said:

We do leave our Honeywell heat on. Should I be turning the heat off in the house before I shower? The hydrostat heats up when I shower but I do noticed that the gauge on boiler and hydrostat do not match if the exampl. Hydrostat says 203 but boiler at about 195. It's always about 7-10off. Now by hydrostat does havery 30 diff as max. Should be setting it up that high or is 25 diff fine?


The equipment is not lab grade precise, and measurement locations also nay vary.

You should not need to turn off heat.

Question: Summing up best Aquastat control settings: fine tuning the DIFF setting

2016/02/28 Bill said:

I have an Oil Fired Boiler forced hot water system (Hydronic) with typical baseboard radiators. Almost standard fare with 3 zones (1st for finished basement, 2nd for first floor and 3rd for second floor) in my typical New England Colonel. Honeywell L8124A Aquastat on a York AP-590 Boiler with tankless domestic hot water.

Almost normal no. 1 because the former owner installed a SuperStor domestic hot water heater. This means that there are now 4 zones, 4th being the SuperStor. This works exactly like another zone with the ‘thermoset’ that call for heat as part of the SuperStor. And while the original tankless is still plumbed up it, it was done properly such that it is cut out of the domestic hot water circuit and vented to atmosphere. Works just fine.

Almost normal no. 2 because I installed an Intellidyne IntelliCon HW+ Hot Water Hearing System Economizer some years ago. It has a feature that allows for using a SuperStor and it is all set up per the manual. Works just fine and has saved me a noticeable percentage of oil during a heating season (~15%).

In the past I had not studied nor modified my Aquastat settings. Until now. I have now read the various excellent articles on Hi, Low, Diff and disabling the Low that can be found on this site.
So ‘posting’ what I have set the Aquastat to in hopes to get comments that I did it correct or wrong.
And I think these settings apply regardless of the Intellidyne Economizer. Of note is that since I have a SuperStor, while I could conceptually disable the Low Limit, I did not want to because (1) the concern over boiler cooling down to ambient (condensation, etc.) and then being called upon as the SuperStor called for heat and (2) it seems that the SuperStor will produce domestic hot water sooner, as in longer to run out, with the family of 5 folks feeding off it (think consecutive showers). Note: with the SuperStor we have never ‘run out’ of domestic hot water, even in the summer.

I have also crafted a modified version of Honeywell’s ‘set point’ diagram. It occurred to me that a lot of my confusion over the Low Limit is simply due to their diagram showing an “up arrow” for the Differential Setting. This implies the bottom ‘line’ (switch makes R-B and breaks R-W…) is the low limit and the differential takes it to the line above. In fact it is opposite. Differential ‘drops’ the Low Limit. Add to that they show another “differential” for the High Limit that I think adds to some of the confusion, when I fact that is hard wired.

So onto my setting and justification. Looking for advice that “makes sense, this is good” or “Bill, you need to go back to school” 

High Limit Setting: set to highest possible WITHOUT exceeding 200 on Boiler. In my case this is ~ 185, after observing a number of cycles. Note: I have not only the Boiler temp gauge but also the Intellidyne has a senor on the boiler output. And when the circulator is running these closely track one another. Logic from reading on this site is that the hotter the boiler water the better the efficiency and heat transfer to the air. See above in the article for this justification.

Low Limit Setting: Set to lowest possible. In my case this is ~ 110. Logic is that (1) no tankless but do have SuperStor (see my thinking on this above) and (2) do not want to deal with issues of boiler going to ambient.

Differential Setting: Set to lowest possible. In my case this is ~ 10. Logic is that do not want the boiler to drop too low in temp. Not sure what temp things will start to condense but I figure that is the boiler only gets down to 110 – 10 = 100 that is still hot enough to prevent. If NOT, then I need to raise the Low Limit NOT the Differential.

Thanks for reading and if you have some other ideas would appreciate your comments.


Almost right in all respects, Bill.
I used to agree with your DIFF settings until I studied Honeywell's explanation of the aquastat control more closely.

But please take a look at the explanation of HI LO and especially DIFF settings found in this companion article: AQUASTAT CONTROLS - home

An excerpt from that, given below, forms an argument for setting the DIFF up rather than down.

More HI LO DIFF Setting Examples:

LO = 120 F, DIFF = 10 F: when the boiler temperature drops to 110 the burner turns on and the circulator is turned off. As the burner re-heats the boiler and the boiler temperature rises back up to 120, the burner turns off and the circulator is allowed to turn on.

LO = 120 F, DIFF = 25 F: when the boiler temperature drops to 110 the burner turns on and the circulator is turned off, just as before. But as the burner re-heats the boiler and the boiler temperature rises back up to 135 F, the burner turns off and the circulator is allowed to turn on. We calculated the 135F as follows: LO setpoint of 120 is added to (DIFF minus 10) or 120 + (25-10) = 135.

LO= 140 F, DIFF = 25 F: when the boiler temperature drops to 130 F the burner turns on and the circulator is turned off, because the burner-on temperature is always fixed at 10 below the LO, just as before. But as the burner re-heats the boiler and the boiler temperature rises back up to 155 F, the burner turns off and the circulator is allowed to turn on. We calculated the 155F as follows: LO setpoint of 140 is added to (DIFF minus 10) or 140 + (25-10) = 155.

The effect of setting the DIFF up from 10 to 25 is that when the burner is re-heating the boiler (for example while the tankless coil is in use and you're in the shower), the burner heats the boiler temperature up to a higher level before the burner is turned off and the circulator is allowed to turn back on. This gives more heat to TANKLESS COILS and therefore more domestic hot water to the building occupants.


Continue reading at AQUASTAT HI LO DIFF SETTINGS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see MANUALS for HEATING SYSTEM CONTROLS for a list of aquastat installation & repair guides

Or see these

Article Series Contents

Suggested citation for this web page

AQUASTAT HI LO DIFF SETTINGS FAQs at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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