Boiler Aquastat Diagnostic FAQs

Definition of HI LO & DIFF Settings
Aquastat settings if no tankless coil
Aquastat settings to save on heating cost

Aquastat Settings When No Tankless Coil is in Use or is Abandoned FAQs

Tip: If your heating boiler has no tankless coil or the tankless coil has been abandoned, see the aquastat setting procedure at AQUASTAT LO & DIFF DISABLED

Also see these AQUASTAT SETTINGS if NOT USED for DOMESTIC HOT WATER FAQs - if your boiler doesn't make hot water at all

When Abandoning a Tankless Coil: Should We Change the Aquastat Primary Boiler Control Settings

I am currently installing a hot water heater to replace a tankless coil in my boiler. Do I need a different aquastat to keep my boiler from trying to heat the coil? Can I replace my present aquastat #l8148a with #l8124a??? - John Haines

We installed an electric hot water heater, do we have to change anything to make the boiler function just for heat and not hot water (for bathing, dishes etc) ? - Ernie

I have replaced the tankless coil in the boiler with a gas water heater thus eliminating the need for the boiler to heat water for bathing,washing etc. question does the aquastat still needs to be connected for my boiler to run to heat the house? - jorge gonzalez

Reply: Disable the LO & DIFF settings OR set them to their lowest setting when abandoning a tankless coil

John:

Take a look at the instructions that come with the aquastat and you'll see a list of devices it can replace. But most likely you do NOT have to replace your aquastat with a different model just because you are ceasing to use the tankless coil for domestic hot water.

On the aquastats discussed here, the instructions (link given below) describe a simple process for disconnecting a wire that will disable the low-limit control, thus eliminating that function when a tankless coil is no longer in use.

Ernie, When you abandon or stop using a tankless coil on a heating boiler, you can save on heating fuel consumption by disabling the LO / DIFF feature on your aquastat. The effect is that the aquastat (primary boiler controller) will stop keeping the boiler hot when there is no call for heating the home itself.

If you leave the LO/DIFF working, the boiler will keep heating itself even in summer when there is no call for building heat - because it thinks it's staying hot to support use of the (now abandoned) tankless coil.

Jorge, you still need the aquastat control, since it has to manage the boiler on and off temperatures when the wall thermostat(s) call for heat in the home itself. But as we explain above, the aquastat controls can be changed to stop keeping heat in the boiler to support the tankless coil that is no longer in use.

The procedure for disabling the tankless coil heating system is in our Aquastat Settings article at DISABLING LO & DIFF On an aquastat.

How to stop using the tankless coil

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. (Oct 27, 2011) Marc Ruland

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. (Nov 17, 2011) Josh said:

Reply:

Take a look at the instructions found

in AQUASTAT LO & DIFF DISABLED if you want to abandon using a tankless coil

Do these aquastat settings make sense to you? HI 200, LO 110, DIFF 10 ? We have a SuperStor and no tankless coil.

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 AQUASTAT HI LO DIFF SETTINGS 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. (Feb 28, 2016) Bill

Reply: almost but not quite: HI 200 LO 180 DIFF 25

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 CONTROL - 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.

Reader follow-up:

Got it. I missed the 10 degree delta part on the low side. Ok so temp will drop (with no call) to LO - 10 degrees and heat back up to (LO - 10) + DIFF. So in my case with the numbers listed and no call:

* Will drop to 110 - 10 = 100 (the 10 has nothing to due with the DIFF :)
* Will rise to (110 - 10) + 10 = 110

And since I do not have a Tankless, is this temp too low?

Any concerns about condensation building up, particularly in the summer where the only calls will be the SuperStor?

I suspect the boiler will drop to its lowest frequently in summer time...

Thanks again,
Bill

Reply: what is gained by continuing to use the LO limit on an aquastat when no tankless coil is installed?

Ah

With no tankless coil what you're gaining by keeping the LO hooked up at all is a slightly faster supply of heat to re-heat the indirect fired water heater.

My opinion is that that gain is not much, since when we are only heating the water inside the boiler itself (that is not also all of the water in the hot water piping and rads or baseboards) the boiler comes up to heat quickly.

There are other reasons to keep heat in the boiler such as avoiding leaks in some older cast iron boiler models, and avoiding condensation as you mention. That's no worry IMO when the boiler runs from time to time to make hot water.

Your Superstor indirect water heater operates as its own separate zone - it looks like a heating zone to the boiler, so the boiler's HI will take effect. I imagine the only summer difference you may see is a slightly-longer time to re-heat the indirect water heater. I guesstimate less than a 10 minute difference from winter, since most of the re-heat time will be heating the larger volume of water in the indirect water heater.

But it would be interesting for you to conduct experiments to actually measure the re-heat time at different starting temperatures in both the boiler and in the indirect water heater. To make comparisons one would need to not run hot water during the tests.

Boiler's burner runs when not needed - no tankless coil in use

I have a Honeywell L8124a c aquastat with zone controls bypassed so that the thermostat wires run directly to the aquastat.

The when the thermostat is turned down, the burner runs for a while, then shuts down, sometimes for a minute, sometimes for 5-10 minutes, then fires back up.

The circulating pump is running whether or not the burner is firing.

When the burner is during, it shuts down when I turn down the upper limit switch, and acts just like when it shuts down as above. It keeps running like this until it gets so warm I have to shut off the power.

I realized while working on this, that when the burner shuts down and I turn up the thermostat, the burner doesn't start running, I gather, until it calls for hot water again.

When the burner is off, but the thermostat is turned up, there is no voltage at the burner terminals, but the circulator is still running. Does this make any sense?

I did replace the old thermostat. On 2019-03-22 Bob

by (mod) - Honeywell L8124a c aquastat, zone controls bypassed, wires run direct to the aquastat.

Bob

take a look at AQUASTAT LO & DIFF DISABLED

and let me know if that does what you need.

by Bob

I appreciate your help. That link says to disable the low/dif control if not using a tankless coil to heat domestic hot water. This boiler does heat hot water, but I would actually like to disable it because an electric hot water heater was installed in series with it because it didn't heat water fast enough for my sister's beauty shop.

I wanted to clarify this because of the notice in the link about disconnecting the blue wire if thete us not a tankless coil. Thank you for your help. Five servicemen have been called but nobody has fixed it, which is why I'm now the designated repair guy!

Reply by (mod) -

Thanks for the followup- yes go ahead and disable the low limit as we describe. "not a tankless coil" needs to be edited to clarify that that includes the presence of a tankless coil that is not in use.

Heating Cost vs Aquastat Settings FAQs - how to cut heating cost / fuel use

Tip: See AQUASTAT SETTINGS vs HEATING COST

and HEATING COST SAVINGS METHODS - home

How Much Will I Save on Heating Cost by Lowering the LO/DIFF Settings on the Aquastat?

Thanks for answering my questions on my recent hot water heater installation.

I disconnected the differential wire (see DISABLING LO & DIFF) on my aquastat and now my system works perfectly. - Tony

Thanks for answering my question from June 28, 2011. I did, in fact, reduce the LO limit setting this summer from 160 to 120 and it reduced my oil consumption for heating hot water by about 30 percent. I left the DIFF setting unchanged at 10.

To be honest, I was a little unclear from your response about the effect raising/lowering the DIFF setting would have on overall oil consumption during the summer months.

However, adjusting the LO setting definitely had the desired effect.

It was not necessary to adjust my manual mixing valve because the shower water was at a comfortable level. Thanks for taking the time to answer my question and for the valuable information contained on this website! - Tony

Reply: Oil Consumption Savings Reported to be 30% When LO set Back

Thanks for the feedback that lowering the LO setting will save heating fuel during summer months. We have edited the HI, LO, and DIFF functions above to make that text more clear.

Basically, by lowering the LO to its lowest point (120) you reduced that amount of heat kept in the boiler during the summer (when the only function of the boiler is to stay hot in order to heat the tankless coil used to make domestic hot water). Like you I'd have also set the DIFF to 10 as well.

If you are continuing to use a tankless coil for your home hot water supply, you might want to set the LO back up to 20 degrees below the HI in winter because in cold weather the tankless coil is receiving colder incoming water from the building public or private water supply.

Comment: heating cost savings from aquastat settings is confirmed

Hi Dan. ..I just wanted to clear up that the 30% savings you mentioned in the article were those that I reported in my post as a follow up to the June 28, 2011 post. ...

I can confirm the excellent point you made about setting the LO back up to 20 degrees below the HI in winter because of colder incoming water from the public water supply. It is now late October in NC and I still have the LO limit setting from this summer at 120 with the DIFF setting unchanged at 10. I have not begun to use the system for heat yet and am only running the boiler for hot water.

I have noticed a definite decrease in shower water temperature and will shortly need to reset the LO back up to 20 degrees below the HI. Again, thanks for all the valuable information you provide on your website! - Tony [October 2011]

Are my aquastat settings correct to optimize heating fuel use? HI-190 LO-150 DIFF-15

Trying to get the best optimal settings for my Weil McLain boiler here in northwest NJ. I do not have a hot water heater or a wood burning ng stove. The burner was just replaced last year with a carllin, EZ pro model #J98022 EZ-hpw.

Just replaced the filter. Here is a pic of my settings.

I believe they are set @

high 190

low 140

Diff set @ 15

Is this correct? Just trying to save a few bucks and optimize my fuel consumption. Thanks in advance and have some great holidays! - On 2022-12-22 by Joseph Donahue scrunchd@gmail.com -

Reply by InspectApedia Publisher - how do I optimize my fuel consumption

@Joseph Donahue scrunchd,

No, the settings you quote for that aquastat don't match the photo: In the photo the aquastat settings are

HIGH 190°F

LO 150°F

DIFF 15°F

How to get the "most" hot water from a tankless coil on a boiler

To get the most hot water from a tankless coil system on a boiler you'd want to keep the boiler as hot as possible (close to 200 degF) so that the maximum amount of heat is stored in the boiler and thus is available for transfer to the domestic hot water supply through the coil.

So you'd set the HI to 200F. (No higher or the temperature relief valve will open).

And you'd set the DIFF down to its lowest number - say 10 - so that as boiler temperature drops the burner will come on as soon as possible.

That's not the most-economical setting, it's the "most hot water" setting.

How to set the Aquastat for the the Most Economy

Here we have conflicting parameters.

First: Arguments for higher boiler temperatures:

1.a. The thermal conductivity of hot water in heating baseboards or radiators is exponentially greater at higher temperatures

- that is, at hotter temperatures you get more-efficient heat transfer out of the boiler's water through the radiating device and into the occupied space.

1.b. A heating boiler operates most-efficiently when it's up to full operating temperature.

So short "ON" cycles, especially less than 5 minutes for oil fired boilers and similarly for gas fired boilers, are inefficient. When the boiler comes on you want it to run for a longer period.

For example if your boiler turns on, heats up, and shuts off after 4 minutes that's quite inefficient, especially for oil-fired systems. A lot of your heating btus went up the chimney as incompletely-burned oil. Even if the boiler ran for 6 minutes, only 1 of those minutes was run at full efficiency (for oil).

So longer boiler on-cycles is better.

Second: arguments for lower boiler temperatures

Some add-on heating boiler efficiency improvement controls adjust the boiler operating temperatures in accordance with changes in the outdoor temperature, on the assumption that the control can still provide an adequately-long on-cycle while running the boiler at lower temperatures when outdoor temperatures are higher.

In my OPINON independent research supporting the net gain from these devices is still needed - citations are invited.

Finally:

How to minimize heating cost for your building

1. Stop air infiltration losses

2. Install energy-efficient windows

3. Improve insulation, especially in attic and then in walls

4. Have the heating boiler cleaned and adjusted for optimum performance every year

See details at

AQUASTAT SETTINGS vs HEATING COST

HEATING COST SAVINGS METHODS - home

Read those articles and post your further questions, opinions, or criticism on those pages or here - we'll be glad to continue the discussion.

Honeywell R8182D - is 160 High and 140 Low with a difference of 15 efficient? Will it save heating oil?

For Honeywell R8182D - is 160 High and 140 Low with a difference of 15 efficient? Will it save heating oil? On 2022-11-18 by Anonymous -

Reply by InspectApedia (Editor) - what aquastat settings will save the most on heating oil?

@Anonymous,

No.

Watch out: if you set a DIFF of less than 20 degF. you risk locking out the circulator pump and your heating system wont' work properly.

Not only will that not save heating oil, it might actually use more heating oil, as without the circulator pump in operation people will crank up the thermostat even higher trying to get heat - which is arriving only by natural convection as the circulator isn't pulling hot water through the heating zone.

Take a look at AQUASTAT SETTINGS vs HEATING COST

for details on the question of "what aquastat settings will save the most on heating oil?"

You'll see that there are some conflicting factors:

Some companies like an automatic set-back that is controlled by monitoring outdoor temperature,
but
Running the boiler at a lower temperature, IF its on-cycle is too short, actually wastes oil as the burner and combustion chamber never get up to full operating temperature.

Take a look at that article for our best answer.

How do I bypass or disable hot water heating on the boiler

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 (Oct 19, 2014) Anonymous

Reply:

anon

At the ARTICLE INDEX at the end of this article the article you want is

AQUASTAT LO & DIFF DISABLED

Rusty crud found on the bottom of the heating boiler firebox caused by trying to save on heat cost

As Tony said in his comment above, I also was using about 1 gal of oil per day during the non-heating months. In Connecticut, we are paying close to $4.00 per gal now. I installed an electric water heater in parallel (NOT series) with the boiler's tankless heater. Either the tankless or the electric can be used since the water does not go through one and then the other.

(Glad I still had the tankless piped up since I could still have hot water during the recent 10 day power outage in Connecticut. The 4000 watt generator could run the boiler but not the 4500 watt electric heater.) I found that we are using about 11 kwh/day for domestic hot water by timing how long the 4500 watt heater elements were on each day - a 240 volt relay activating a battery powered clock.

The cost at $0.17/kwh is about $1.90/day - better than 1 gal/day at $4.00. I turned off the boiler when I turned on the electric water heater.

My concern is that when I went to tune-up the boiler for the 2012 heating season, I found about one cup of rust (some of the material was magnetic) on the bottom of the firebox below the cast iron boiler sections. I open the firebox for cleaning each year and have not seen this much rust before. I have brush cleaned the flue passageways twice since new and don't recall seeing this type of material.

The boiler is a Dunkirk that was new in 2001. Water leakage does not appear to be a problem. Should I be worried about damaging the boiler (corrosion) by letting it go stone cold during the non-heating season? - Buddy 1/12/12

Reply: leaving boiler shut down to save on heat can, for SOME boilers, cause rust damage or leaks

Buddy, thanks for the important comments & question. Indeed on many boilers, if you leave the unit shut down for months there can be problems with rust and also with solidification of soot and crud deposits that make seasonal cleaning more difficult (and perhaps costly).

If the boiler and tankless coil is left "on" but not in use, it will run only very little, as it runs just to keep itself hot - it's never cooled down to a longer on-cycle by having moved its hot water out into heating zones.

Alternatively at least it's a good idea to have the annual service/cleanout done in the spring at the end of heating season - don't let it sit over the summer.

Aquastat Setting Definitions for HI LO & DIFF DIALS FAQs

I'm sorting out HI LO and DIFF - what setting does what?

There seems to be some conflicting info on the site. In some articles, you say that the DIFF is the temperature rise over the LO setting.

For example, DIFF 15 and LO 120 would mean you'd have a range of 120-135. In this article, it's explained that there always a 10 degree drop below LO and a rise above LO of DIFF-10, meaning DIFF 15 and LO 120 would give you a range of 110-125.

Which is true? And what about aquastats that have a 5 degree DIFF setting? - Brian

Reply: Here's a summary of HI LO DIFF definitions and functions on an aquastat

Thank you for the clarification request, Brian.

We've reviewed our HI LO DIFF setting articles to be sure that the text is now self-consistent.

Summarizing the definitions of HI, LO, & DIFF controls on an aquastat control

HI Limit Definition: The HI is the cut off point on a call for heat. You can think of the HI dial as the "heat control" in this regard: as long as the room thermostat is calling for heat, when the boiler temperature falls 10 degrees below the HI the burner will (normally) turn on. That 10 degrees is hard-wired into the control - it's not adjustable.

LO Limit Definition: the LO is a control function added to keep heat in the boiler when the room thermostat is not calling for heat. This feature is important on heating boilers that use a tankless coil immersed in the boiler to produce domestic hot water.

The DIFF Definition: the DIFF is an added control that defines the cut-off temperature (above the LO) during a burner-on cycle initiated by the LO control - that is, we are not calling for heat, and we're keeping heat in the boiler to make domestic hot water. It works as you described.

Circulator lockout: There is an interaction between the LO and DIFF settings and the delivery of heat to the building however. That is that the LO operating range on an aquastat also will lock out the heating circulator pump - stopping delivery of hot water out of the boiler and into baseboards or radiators - if the boiler temperature is too low. (That's why aquastat instructions specify that the LO should always be at least 20 °F below the HI setting).

The intent is to give priority to the tankless coil and thus the person in the shower. For heating boilers at homes that do not use a tankless coil to make domestic hot water, it is sometimes (not always) reasonable (and a savings on fuel during summer months) to disable the LO/DIFF controls - as we explain at Disabling LO & DIFF

See Aquastat control HI LO settings for details about how the HI and LO settings work.

See Aquastat control DIFF settings for details about the DIFF setting and how it works

See Optimal LO/DIFF settings save fuel for some recommended settings to save on heating fuel cost