Aquastat High, Low, & DIFF FAQs

Best Heating boiler Aquastat Settings
Heating System Noises vs Aquastat Settings or Failures
Seasonal changes to Aquastat Settings

Best Aquastat Settings FAQs

Sum up the Best Aquastat control settings: fine tuning the DIFF setting

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. 2016/02/28 Bill

Reply:

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:

inspectapedia.com/heat/Aquastats.php 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.

What are the Optimal LO & DIFF settings for moderate hot water use in summer?

What would be the optimal setting for the LO & DIFF settings for fuel savings with low and with moderate hot water usage in the summer months when the boiler functioning for heat does not come into play. If you would please identify the low usage from the moderate usage settings it would be greatly appreciated. - Mark

This is an excellent article on the DIFF setting as it pertains to using a Tankless Coil. During the summer in North Carolina when outside temperatures are in the 90's and my system never calls for heat, I am only running my Burnham boiler to provide hot water. It is not unusual to burn 1 gal of oil/day (30 gals / month) just for residential hot water. With oil prices well above $3.50/gal, $100/ month for hot water is a little extreme.

I am curious if some combination of LO Limit / DIFF might cause the burner to run less while still maintaining adequate heat to provide hot water. For example, if my current settings are LO 160/ HI 180/ DIFF 10, might adjusting the LO/DIFF settings either up or down lower oil consumption? I suppose another option could be to use an electric hot water heater with a tank during the summer months. - Tony

I have a VERY old system. Have two separate aquastats, one used for LO and one used for High. No Diff. I have a separate hot water heater. Because of the old system the oil company technician feels that I should keep the furnace on during the summer so that there isn't a "shock" to the system when it is turned on in the fall. Question: What are the best HI-LO settings in the summer and in the heating season.? - Steve

Reply: Summer Aquastat Setting Recommendations

Mark and Tony and Steve: For summer use, if you set the LO to its lowest temperature setting (typically 102 F) you will be keeping minimal heat in the boiler to support the tankless coil in summer and will lower your heating fuel consumption by as much as 30%.

For summer use, if you set the DIFF to its lowest setting you will also lower your fuel consumption.

The result of these settings will also be a reduction in the quantity and also the maximum temperature of hot water supplied to the home - which may be OK for cases when hot water use is only moderate.

If you have an automatic tempering valve or mixing valve installed on your hot water supply that will increase the total hot water quantity available by drawing heated hot water from the boiler only at the rate needed.

Tony:

You can leave the HI setting on your Aquastat alone in summer; since the system will never be calling for heat, the HI will be asleep all summer.

If you set the LO down to a lower number in summer you will be keeping less heat in the boiler, and in that sense burning less oil; but if your mixing valve is a MANUAL one you may need to adjust it so that water at the tap is hot enough.

If you keep the DIFF to its lowest number in summer you're also minimizing the oil burner on-time. Or conversely, if you set the DIFF to a higher number (say in summer), quoting from the article above:

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.

Steve,

Given that you are not using your heating boiler for making domestic hot water, the reason a service tech would recommend keeping it on in summer is most likely because some older cast iron boilers can develop leaks between the boiler sections if the boiler is allowed to cool off to stone cold. Such a leak may not "heal" on its own when heat is restored, and it can lead to worse boiler damage.

For summer use on such a system, keeping just about any heat at all in the system, say 100 °F should be sufficient. If your LO is currently set above that number, try setting it down for the summer - but don't forget to restore it at the start of the heating season.

I am inferring that the HI on your system is an upper limit on a call for heat and won't be operating in summer. If I'm mistaken, just keep the HI 20 °F above the LO.

What are the best Hi Lo DIFF settings on an aquastat? We are using too much heating oil.

Right now I have baseboard heat and I have the boiler set at HI-170 LO-150 and DIFF at 10. Is that a good Setting. I'm trying to save money on oil. thank you - Matt

I'm still not 100% sure on what my settings should be. I have a Weil Mclain boiler that's about 6 years old with a DHW coil installed.

I had a 30 gallon electric hot water heater installed after the coil, and during the summer I would shut off the boiler and use only the hot water heater. During heating season the water would be pre-warmed by the coil and the electric heater wouldn't be doing much.

Problem is, we seem to be using a lot of oil. The boiler is constantly running, every time we use the shower, run the dishwasher, wash our hands, etc. It's also constantly kicking on to keep hot water in standby.

During the early heating season, it's a waster, since the boiler isn't on very much to provide heat to the house. What should my settings be? Will I save more money by just not using the coil and telling the aquastat there's no coil? - Matthew

I live in Maine and the winter season is here. What would be a good Hi and Low setting for my aquastat? Its is starting to dip below 10 degrees at night. Thanks. - Nick

Reply:

Nick and Matt:

Under AQUASTAT CONTROL FUNCTIONS (the article above on this page) you'll find good aquastat HI LO and DIFF setting recommendations in the article t

itled Optimal LO/DIFF settings save fuel

Matt: you can significantly save on heating costs by lots of other measures including using a thermostat setback to lower temperatures at night or when the home is unoccupied.

See HEATING COST SAVINGS for our suggestions of a variety of ways to save on heating costs, and importantly some priorities: what heat cost savings steps should we address first at a building?

What is the maxmum temperature setting on a heating boiler aquastat? I think it's 210°F & 190°F

I have a Burnham V73 boiler with an L8124C control.

The boiler has a gauge on the front that shows water pressure and temperature.

I've read in the v73 manual online that the controller is supposed to be set at 210 hi and 190 lo but i have it set at 180 and 160 because at those temps it shows on the gauge on front it got up to 210 and sometimes tries to go higher and goes 20 to 25 psi.

The relief valve was also blowing out steam when setting hi to 190 or above.

Maybe i need a new aquastat control since the temp gauge and steam seem to be telling me it's getting too hot?

I forgot to say about the V73 that I am not using the tankless water heater since i think the coil inside has a leak that leaches water directly from the boiler into the water as far as I can tell.

This boiler is in a home where the people left and all pipes froze and burst as i can remember water was still in the boiler also.

Boiler could have some hairline crack inside or would water freezing inside it be enough to damage it or just the copper tankless coil? (Jan 21, 2014) Hank

Reply: Wrong: do not set the aquastat HI over 200°F or you risk relief valve opening; Do not set the LO less thanm 20°F below the HI

Hank

On usual residential heating boilers, temperatures over 200 F risk spilling the releif valve. I'm surprised by that 210 F citation.

Set your HI limit to just under 200F and the LO to at least 20 degrees lower than that.

Complete details are at AQUASTAT HI LO DIFF SETTINGS

What's the best low limit differential to set on my oil boiler aquastat L7224?

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

Reply:

Please see AQUASTAT OPTIMAL LO/DIFF SETTING

What's the best aquastat setting for home heating only?

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? (Nov 3, 2015) J R

Reply: - disable the LO & DIFF circuits

JR,

Set the HI to 180 or better in my opinion, 200 F.

Then take a quick look at the section in the article AQUASTAT LO & DIFF DISABLED about disabling the low limit.

Seasonal Aquastat Setting Changes FAQs

Should I Change the aquastat settings for summer months?

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 ? 14 March 2015 Karl R

Reply:

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

AQUASTAT HI LO DIFF SETTINGS

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.

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 place,

It'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 case is 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 ?

Reply:

No, probably not.

Please also see TANKLESS COIL / HOT WATER COIL LEAKS

Aquastat was HI 180 LO 170 DIFF 10 - What should be HI LO DIFF for Summer?

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? On 2016-04-19 by suzanne

Reply by (mod) - wrong aquastat settings increase heating costd

Watch out: your aquastat is nto set correctly - the LOW is set much too close to the HI (just 10 degrees difference). The HI and LO must be at least 20 degrees apart to avoid locking ouit the circulator pump.

The mistake in your aquastat settings would indeed increase your heating cost because without the circulator pump running to speed hot water delivery to your heating baseboards or radiators, the boiler has to run much longer and the heating ON cycle will be longer to satisfy the room thermostat.

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.

Would the 200/180/25 diff settings still be the best when heat is turned OFF in Summer?

\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? (July 5, 2012) Dan

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.

Higher boiler aquastat settings vs outdoor air temperature sensors

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? (Sept 17, 2012) Bob

"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 AQUASTAT HI LO DIFF SETTINGS? - (Oct 18, 2012) Anonymous

Reply:

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.

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 7, 2012)

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

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

Reply:

Yes, but do read the other Thermostat setback discussion above on this page. Savings may not be quite what you expect.

Also see THERMOSTAT SETBACK ADVICE - energy savings by thermostat setback adjustments.

Also read Alan S. Hoback, EXAMINING BOILER SETPOINT EFFECT [PDF] ASHRAE Journal, March 2022, pp. 36-44 - local copy saved as Boiler-Setpoint-Hoback.pdf

discussed in our Thermostat Set Back advice article cited just above.

Aquastat Settings / Failures vs Heating System Noise FAQs

Tip: See our complete diagnosis and repair advice on heating system noises found at HEATING SYSTEM NOISE DIAGNOSIS 

How do I fix a buzzing boiler aquastat?

My my furnace turns on, the acuastat buzzes for a few mins then it is fine. What could be causing this? It has been doing this ever since we lost power a month ago. On 2017-12-01 by Tina Barbour

by (mod) - acuastat buzzes for a few mins then it is fine

Often a failing relay or transformer is what's buzzing - and while that part can be replaced, few boiler technicians will try to do so - it's quicker and less labor so possibly less expensive to just replace the whole aquastat.

by Brian

Tech said assistant relay needs replacing and I also think control switch also needs replacing

Reply by (mod) -

If the tech wants to and can try replacing just the relay, I'd give that a shot.

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

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. (Feb 15, 2015) Joe said:

Reply:

Joe

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.

See WATER HAMMER NOISE DIAGNOSE & CURE 

Aquastat setting causing heating pipe vibration?

Can an improperly set aquastat cause pipes to vibrate (Jan 12, 2016) Anonymous

Reply:

I doubt it. The aquastat turns on burner and circulator.

Pipe vibration may be traced to a bad circulator pump or to poor pipe mounting or routing or even to water hammer.

So you might think the vibration noise is caused by the aquastat but more likely it's an indirect connection: the aquastat control turns on the burner or possibly a circulator that in turn causes pipe vibration noises.

Burnham furnace with domestic hot water delayed ignition and booming sounds

Have a Burnham furnace with domestic hot water.

Was having delayed ignition when the furnace was starting but the service man replaced the transformer and that problem stopped. I am still getting an occassional boom out of the furnace inbetween heating cycles without a furnace start-up.

Just a loud boom without the furnace starting or a demand for hot water or heat.

Also, the Honeywell aquastat control always has a humming sound when the furnace isn't running. Any suggestions? (Dec 30, 2014) Mike Reilly

Reply:

Mike a boom sound can presage a dangerous oil burner puffback. I'd ask for a service call. I suspect incomplete combustion or burner sloppy startup or shutdown.

At the aquastat there may be a failing relay or transformer

Also see BANGING BOOMING NOISE DIAGNOSIS & CURE - home

Boiler rumbling, overheating, producting steam - vs Aquastat settings

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) Buddy

(Mod) said: SAFETY WARNING

Buddy, Thanks for your comments above. But ...

Watch out: 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.

See CLOGGED TANKLESS COIL or PIPES, LIME SCALE  for details on diagnosing and fixing the trouble.