FAQs on best settings for a heating boiler aquastat control:

These articles 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.

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

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]

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

MIke:

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.

Details are at AQUASTAT HI LO DIFF SETTINGS https://inspectapedia.com/heat/Aquastat_Settings.php

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.

Take a look at HOT WATER QUANTITY IMPROVEMENT https://InspectAPedia.com/plumbing/Hot_Water_Quantity_Increase.php

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 AQUASTAT HI LO DIFF SETTINGS.

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 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?
Thanks

Reply:

Phillip:

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?

Reply:

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 AQUASTAT HI LO DIFF SETTINGS 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.

Reply:

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?

Reply:

(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) (mod) said:

Nick, for reasons we explain in AQUASTAT HI LO DIFF SETTINGS, 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?

Reply:

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.

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Continue reading at AQUASTAT HI LO DIFF SETTINGS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see AQUASTAT HI LO DIFF SETTINGS FAQs-4 - more-recent questions & answers about adjusting a heating system aquastat

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