FAQs about choosing the best settings for a heating boiler aquastat control:

This article series explains exactly 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

These questions & answers were posted originally

at AQUASTAT HI LO DIFF SETTINGS - please be sure you review the advice given there.

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 2022-12-22 by Joseph Donahue scrunchd@gmail.com - are my aquastat settings correct to optimize heating fuel use?

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 2022-12-31 by InspectApedia Publisher - how do I optimize my fuel consumption

@Joseph Donahue scrunchd,

No that's not quite right.

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 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.

On 2022-12-19 by Adam

Hi, this is helpful, thank you! I will be traveling this winter for a week and while away want (1) to burn as little oil as possible while (2) not having pipes freeze. Heating is forced hot water.

What is the _lowest_ setting for LO on my aquastat? I may have misunderstood, but thought I'd read that if it's too low, the circulators won't run when the thermostat call for heat.

Thanks!

On 2022-12-19 by InspectApedia (Editor) - HI limit controls your boiler on a call for heat

@Adam,

The HI limit is what controls your boiler on a call for heat. The boiler's burner will turn on when boiler temperature drops 15 deg (typically) below the HI setting.

Please read through the article above on this page where we explain the HI, LO and DIFF settings and then don't hesitate to ask if any of that leaves you with questions.

The LO is there to maintain heat in the boiler if the home uses a tankless coil for domestic hot water.

On 2022-12-16 by JIM Hickey - Dual oil / wood boiler aquastat settings

Hi, I have a duo oil boiler with a wood boiler.It has worked fine for years.

Last year I replaced the aquastat on the oil boiler and when the wood boiler should have taken over the oil boiler came on and stayed on. so we were burning wood and using oil.

I turned off the oil boiler and had to wait for the wood boiler to go below the 180 degree mark before the oil took back over. I replaced the wood aquastat same thing happened. The question is what settings should I have on both. Maybe the wood boiler should be at a higher setting??

On 2022-12-16 by InspectApedia (Editor) - why won't wood boiler come on

@JIM Hickey,

Forgive me but I'm too nervous about safety issues concerning combination wood-oil fired heating boilers to give advice that's a bit of a guess.

Let's start with the specific heater's brand and model so that we can review how the manufacturer thinks we should control the operation of the two heaters.

Also identify which aquastat brand and model was on the boiler and what brand and model you used to replace it.

I suspect that it's not just an aquastat setting question but also an aquastat model and wiring method.

On 2022-12-12 by InspectApedia (Editor) - what the low and differential settings should be

@Donald SANDERS,

Those setting on your aquastat are reasonable.

In the article above please be sure to read the section titled

Guide to Best Settings for the HI Limit on a Honeywell R8182D Heating Boiler Aquastat & Similar Controls:

On 2022-12-31 by Donald D SANDERS

@InspTectApedia (Editor), Thanks! Keeping my high at 190 and low at 150 and changing the diff from 20 to 25, will that give me hotter water for dish washer and tub.

Hi, I have a 50-year-old boiler that heats water through coils. I set my hi at 190 and my low at 150 and my diff at 20. Trying to save on oil. Not sure what the low and differential setting really effect. Would there be a better setting.

Another question I have is, put an electric hot water heater in to save not running furnace in spring, summer, and fall months. also added mini splits. Too cold to run splits during our winter months. If I don't shut off the water heater and let that hot water run in to my coil when calling for hot water, is that a good idea and what would my settings be.

I tried that at 190 H and 150 L and 20 diff, but water is really hot.

On 2022-11-18 by Anonymous - 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 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.

On 2022-10-04 by Clare Martinez - I think my aquastat is not set correctly

I just bought a home with a 20 year old Thermo Dynamics Boiler. Former homeowner said he had a technician change the Aquastat in July.

Tried to heat the home and failed today. It is a problem with the Aquastat he installed just three months ago.

For some reason when the temp differential is set to 20 degrees High above low the system doesn't function.

It shuts itself off. The tech manually adjusted the differential so the High is 180 and the low is 200 and the house heats up now.

What is the issue with the Aquastat? Theoretically this means they're crossed, and the system shouldn't function.

On 2022-10-04 by InspectApedia-911 (mod) - new Aquastat won't heat home

@Clare Martinez,
You are absolutely correct that the aqua stat is not set properly. The way that it's set the circulator will never turn on. The high must always be at least 20° higher than the low. Set your high to 180 or 200 and the low at least 20° below that point.

A reasonable starting place would be to set the high to 180 and the low to 155.

Now with the aquastat set correctly, set your thermostat well above room temperature to be sure that it's calling for heat.

The circulator should come on and you should feel the water and the pipes returning to the boiler get hot.

Let me know how that works and if you still don't have heat we will continue from that point.

On 2022-01-17 by Sam

Good afternoon, I have Burnham Series 2A with 2 zones circulating pumps, 1 zone for the main house and one zone for one of the bedroom. The main heat work fine, the bedroom I replaced the circulating pump and the thermostat but the circulating pump wont run.

I tested the pump by connecting it to 120V and it worked fine. Replaced the old Taco Circulating pump007_F4 with a new model F5

On 2022-01-17 by Inspectapedia Com Moderator - circulating pump wont run

@Sam,

If you are absolutely certain that the circulator pump does not run when installed then also ran normally when connected by itself directly to power, there's either a wiring error or a bad control relay.

Watch out: These circulators are so quiet that they can be running without you realizing it.

So the problem could be in air bound heating system zone.

On 2022-01-14 by Bill

If my Accused that swag it set on 180 what should be the bottom number for Low

On 2022-01-14 by Inspectapedia Com Moderator

@Bill,

Please take a look at the article AQUASTAT HI LO DIFF SETTINGS as it is a more complete answer to your exact question.

On 2022-01-08 by Jeff f

My HI is set to 180 and my house isn’t heating up to what the thermostat is set for, the heat stays below 60° if I set it to 200 it’s better but still has a hard time reaching what the thermostat is set for?

On 2022-01-11 by Inspectapedia Com Moderator - house not heating up enough

@Jeff f,

1. Check your Aquastat settings as advised in the article AQUASTAT HI LO DIFF SETTINGS particularly, if you set the LO too close to the HI then the circulator can stay locked out and you won't get heat; the LO must be at least 20 deg F below the HI.

2. If the Aquastat settings are correct and the circulator then runs on a call for heat, your system may be air-bound.

In that case see AIR-BOUND HEATING SYSTEMS - home

On 2021-09-20 by inspectapedia.com.moderator - keeping some heat in cast iron boiler even when not heating the building can avoid condensation

@Aydar,

You are correct that for cast iron boilers keeping some heat in the boiler even in summer time when you're not heating the building can avoid condensation and also leaks associated with Cooling and move the can and some boilers cause leaks at the push nipples between sections.

Elsewhere we recommended keeping some heat in the boiler for those systems. However that was never the purpose of the LO control, it's simply a side benefit for those particular heating boilers prone to leakage between sections.

On 2021-09-20 by Aydar

About LO setting on cast-iron boilers. I read that LO setting is set to let cast-iron boiler heat up without running circulator, in order to prevent a lot of condensation and rust. So LO setting lets boiler heat up first and only then to turn on the circulator.

On 2020-12-21 by danjoefriedman (mod)

It sounds to me as if your aquastat is not correctly set. Be sure to review the different purposes of the high limit and low limit and differential settings as explained in this article so that you understand what each of them does.

On 2020-12-21 by Doug McGarrett

My aquastat has one dial with two pointers on it. Also, it is not accurate, but be that as it may, I would never never never want the high limit set to 180 degrees! It was recently set to 140 by a service man, and the hot water will severely scald you. I believe that
if you can keep the water at the kitchen sink faucet around 125 to 130 it is more than hot enough.

On 2020-11-13 - by (mod) - number of variables determine how efficiently we burn heating oil and what our heating cost is

Thank you for your comment, Jim.

I agree that there are a number of variables that determine how efficiently we burn heating oil and therefore what our heating cost is. The greatest Improvement in most buildings is from stopping air leaks.

However there's no getting around science which tells us that the thermal conductivity of water and thinned copper baseboards is exponentially greater at higher temperatures.

That means we got a more efficient heat transfer out of the boiler system and into the occupied space when the Border Water is hotter. That doesn't speak to the heat loss rate for the building at all.

Aye, that I read recently really stuck with me. Science tells us truth whether we choose to believe it or not.

On 2020-11-13 by Jim McMahon

Setting a high limit at 180 or above is wasteful. I set my high limit at 160. The lower boiler temp is unnoticeable in the home heating. Actually my oil bill dropped 25%. Low limit is always 20 degrees less than high limit. The furnace does not come on all the time as before.

My circulators stay on longer to move heat to baseboard pipes. The heat is very even in the house now. No pinging pipes. A water circulator is low power and costs 90% less to run than the boiler. I am completely happy and promote this to everyone I meet

On 2020-11-06 by Bill

How best to set the aquastat controls if heating with a woodstove?

What are the best settings for aquastat to keep furnace from running too much in summer when not using hot water from it?

On 2020-09-02 - by (mod) - how the low and diff settings interact

Steve,

Thank you for helpful question.

Take a look at the yellow colored area on the graph I include on AQUASTAT HI LO DIFF SETTINGS that shows how the low and diff settings interact for an explanation.

On 2020-09-01 by Steve

The Honeywell documentation (form 95-6571-10) states the burner will break at the low set point when the differential is set to a value of 10. This is not true on our 8124 aquastat as the burner always breaks 10 degrees above the low set point. An example would be as follows: low set point at 120 degrees and differential set at 10.

Burner makes at 110 degrees (caused by the built in low differential) and breaks at 130 although the instructions state the burner should break at the low set point when the differential is set to 10. Please advise.

On 2020-03-25 - by (mod) -

David

Probably the way your aquastat is wired it thinks you've got a tankless coil in use for domestic hot water, so it keeps the boiler hot even when there's no call for heat.

See AQUASTAT LO & DIFF DISABLED

for some suggestions

On 2020-03-25 by david

If I set the Hi at 180 and the Lo at 160 what should I set the Diff. at. I only use the boiler for heat. What could be causing the boiler to short cycle every now and then?

On 2020-03-23 - by (mod) -

The best solution for too-hot water coming from a heating system using a tankless coil (suggested by the page where you've posted your comment, Al) is the installation of an anti-scald valve or tempering valve.

Details are at ANTI SCALD VALVES & TEMPERATURE CONTROL / MIXING VALVES

On 2020-03-23 by al crawley

hot water is too hot

On 2020-01-22 - by (mod) - settings for the triple aquastat and for the single aquastat

I don't, Adam, but you can give the company a call

Benjamin Heating Products
166 Junction Road, PO Box 2079
Springhill, Nova Scotia, Canada B0M 1X0
Tel: 1-800-565-5495 · Fax: (902) 597-3797
furnaces@eastlink.ca

and perhaps the company can give you a manual;

or see these product specs

FS 140 COMBINATION OIL/WOOD HOT AIR FURNACE [PDF] retrieved 2020/11/22 original source: http://benjaminheating.com/files/8113/1550/1314/FS_140-Eng.pdf

On 2020-01-22 by Adam

I bought a house with a Benjamin wood and oil combo boiler and cannot seem to heat it very well with just wood, does anyone know what the settings are for the triple aquastat and for the single aquastat

On 2019-12-20 - by (mod) -

Tim:

If the boiler temperature is actually over 200F then on a normal residential heating boiler the temperature and pressure relief valve should open and spill.

So if that is NOT happening either the temperature isn't really 220 (the gauge is inaccurate) OR the TPR valve is unsafe and risks a dangerous, even fatal boiler explosion.

For these reasons you want to satisfy yourself that you know what's going on - the solution is NOT in the Aquastat itself it first is in measuring boiler temperature and in inspecting, testing, and if necessary replacing the temperature-pressure-relief valve (TPR valve).

Once those critical life-safety issues are resolved we can worry about

- temperature/pressure gauge accuracy (search InspectApedia.com for these phrases to read details)

- proper aquastat settings (article on this page)

- proper aquastat installation including of its temperature sensor that is inserted into a well on the boiler (maybe you're missing temperature-conductive grease)

On 2019-12-17 by Tim Clark

I just put a new temperature gauge on front of my boiler and for an Aquastat setting of 180 for high the gauge temp goes up to 220 .

I set the aquastat to 160 low and high temp reaches 200 .So it's about 40 degrees off . The aquastat is a Honeywell L8124C and probably about 15 to 20 years old . Can i run it like this with the lower setting as long as it doesn't change ?

Where the Aquastat gets it's reading is from a tankless coil area that I don't use because i figured it was burst or broken inside when i moved in. So i set my differential to 10 down from 15 since i have a broken coil that i'm not using and trying to save heating fuel and money here in Maine where it's a long winter usually .

Going to check the temp reading when boiler comes back on,set to 140 low to still give it 20 degree diff from the high even though the high temp is way off maybe due to hard water buildup or something on the sensor on the aquastat .What might be a better Aquastat if I ever change the one I have ?

On 2019-12-17 by Tim Clark - Honeywell L8124C settings

I just put a new temperature gauge on front of my boiler and for an Aquastat setting of 180 for high the gauge temp goes up to 220 .I set the aquastat to 160 low and high temp reaches 200 .So it's about 40 degrees off . The aquastat is a Honeywell L8124C and probably about 15 to 20 years old . Can i run it like this with the lower setting as long as it doesn't change ?

Where the Aquastat gets it's reading is from a tankless coil area that I don't use because i figured it was burst or broken inside when i moved in. So i set my differential to 10 down from 15 since i have a broken coil that i'm not using and trying to save heating fuel and money here in Maine where it's a long winter usually .

Going to check the temp reading when boiler comes back on,set to 140 low to still give it 20 degree diff from the high even though the high temp is way off maybe due to hard water buildup or something on the sensor on the aquastat .What might be a better Aquastat if I ever change the one I have ?

On 2019-12-20 by (mod) - If the boiler temperature is actually over 200F that is unsafe

Tim:

Watch out: If the boiler temperature is actually over 200F then on a normal residential heating boiler the temperature and pressure relief valve should open and spill.

So if that is NOT happening either the temperature isn't really 220 (the gauge is inaccurate) OR the TPR valve is unsafe and risks a dangerous, even fatal boiler explosion.

For these reasons you want to satisfy yourself that you know what's going on - the solution is NOT in the Aquastat itself it first is in measuring boiler temperature and in inspecting, testing, and if necessary replacing the temperature-pressure-relief valve (TPR valve).

Once those critical life-safety issues are resolved we can worry about

- temperature/pressure gauge accuracy (search InspectApedia.com for these phrases to read details)

- proper aquastat settings (article on this page)

- proper aquastat installation including of its temperature sensor that is inserted into a well on the boiler (maybe you're missing temperature-conductive grease)

On 2019-11-24 by Ed - reduce the thermostat settings but how do I stop heating domestic hot water at my boiler

Great site! I aImost understand many of your aquastat articles.

I maintain a property that has an oil fired boiler that supplies both heat, via old school radiators and domestic hot water.
The place is seldom occupied.

I know that I can reduce thermostat settings to minimize heating expense, but I also notice a lot of fuel is being used to heat domestic hot water unnecessarily. How do I reduce or eliminate the heating of domestic water, unless I want it?
Thank you for your help.

On 2019-11-24 by (mod) - If the domestic hot water is being produced by an indirect water heater

Ed

If the domestic hot water is being produced by an indirect water heater that's being heated by your boiler then there will be a separate aquastat control mounted on that hot water tank.

If the hot water for your building is being produced by a tankless coil then the aquastat on your boiler controls that setting. However for a tankless coil case, if no one is drawing hot water, the only thing you need to do would be to set the low limit down to its lowest number.

On 2019-03-23 by Dwain Mcmillion - same rule of thumb apply with a steam boiler, with a high/low limit aquatast

Does this same rule of thumb apply with a steam boiler, with a high/low limit aquatast, with a 5-30 degree differential?. when the heating cycles just reaches its end(the point where the water is the hottest) the temp out of the mixing valve is maxed at 140, for a short period of time. so for aquastat i’m thinking somewhere around 180 for high limit and 15 on the differential?

On 2019-03-24 by (mod) -

In my opinion yes

For other readers, a steam boiler may also use a tankless coil that's of course below the steam boiler water line.

Since it's making steam you can figure that the boiler temperature must be reaching 212 degF (at sea level).

On 2019-01-14 by Ray Ingram - mixing valve settings vs aquastat settings

My mixing valve was recently leaking so I had a buddy of mine replace it I have a two story home good hot water downstairs but upstairs water not as hot mixing valve is opened all the way up instead of in between and I have

The settings in the box are set at 1:60 low 180 I The settings in the box are set at 160 low 180 I did The differential is set at 20 Do you have any suggestions for me

On 2019-01-14 by (mod) -

I like the DIFF set to 25. You can also put the HI up as high as 200 (no higher or the TPR valve will spill)

But diagnostic questions might include

- was the upstairs water hotter before the new mixing valve was installed?

- has hot water pressure (actually flow rate) changed?

- is your water supply hard - high in mineral content ?

In the ARTICLE INDEX found near the end of the article AQUASTAT HI LO DIFF SETTINGS you'll find several articles on TANKLESS HOT WATER COILS and their limitations as well as articles on how to increase the amount of hot water available.

On 2019-01-12 by Anthony - how does the aquastat work if there is no call fdor heat

So I'm confused here. In this article on this other webpage on this site, https://inspectapedia.com/heat/Aquastats.php, it says that, when there's no call for heat, a furnace always will turn on at 10 degrees below the LO set point and turn off when the temp reaches the low point plus the differential MINUS 10 DEREES.

In other words, the differential is added to the temperature at which the furnace turns on (10 degrees below the LO set point), not to the LO set point, as it seems to suggest above.

So above it says: If the LO is 120 and the differential is 10, the furnace will shutoff at 130. But according to the other article, if I understand it correctly, if the LO is 20 and the differential is 10, the shutoff is 120. In other words, a 10 degree differential always shuts off a furnace at the LO set point and a max 25 differential always shuts off the furnace at the LO set point plus 15 degrees, not 25.

The second explanation seems to be supported by the website https://forum.heatinghelp.com/discussion/82927/aquastat-settings

But that third webpage raises another point of confusion. It says the circulator is allowed to run at 10 degrees below the LO set point (the furnace-on point). Both articles on this site suggest that the circulator is not allowed to turn on until the LO level plus differential is reached, (differential -10 for the other page).

Can anyone clarify? What is right? Or am I just reading all this incorrectly - I'm not that smart :O)

Many thanks

Anthony Giorgianni

On 2019-01-12 by (mod) -

THanks for asking Anthony, I'll review the two articles and the details to be sure we express these temperatures and limits clearly.

What can be confusing is failing to note that we're talking about the Differential OR DIFF control - on a call for heat, the DIFF control will lock out the circulator at low temperatures.

IF an aquastat is wired and set up to keep the boiler hot for making domestic hot water using a tankless coil, THEN the DIFF and LO become important as they keep heat in the boiler for that purpose, and also because the aquastat will prevent the heating zone circulator from running to deliver hot water to radiators or baseboards while you're in the shower consuming domestic hot water.

Clarifying further, the "130" degrees to which you refer is mentioned in our explanation of the DIFF control and what that does.

Quoting:

Our photo (left) shows the DIFF adjustment on a Honeywell aquastat. This DIFF is set to its lowest value: 10 °F.

At the setting shown, if LO were set to 120, when the burner is re-heating the boiler water and water temperature rises to 130 F the burner will turn off and the heating circulator pumps will be allowed to turn on (and they will actually turn on if the room thermostat is calling for heat).

[Click to enlarge any image]

On 2018-06-05 by tom.lawson@exeleadbiopharma.com

Should I shut off the pilot on my heating hot water boiler in the summer? It looks like it is set a little high and I thought it might save money to shut it off but I know old boilers (1960's model) don't like being cool. The boiler temp is around 130 degrees.

On 2018-06-05 by (mod) - rust damage from condensation when the burner and pilot or left off for long periods

You're correct that's some heavy equipment can suffer rust damage from condensation when the burner and pilot or left off for long periods of time. So it's not something I would recommend.

On 2018-06-06 by Anonymous

Thanks. I think there in a pilot flame adjustment I can make to turn it down a bit.

On 2018-01-26 by WILL Delito

New aquastat from HomeDepot, lo at 130, hi at 180, diff at 18. When calls for heat burner cuts out as in a hiccup on clinb to temp

On 2018-02-11 by (mod) - burner hiccups when climbing to the set temperature

That sounds like an oil delivery, electrode, or nozzle problem and merits a service call, Will.

On 2018-10-18 by Randy - raised aquastat but heat is no better

My boiler aquastat is raised to 160 Hi and 140 LO because part of my second floor is not getting heat but I have not seen any improvement.

While the front half of the house radiators are hot the back part especially the second floor there is no heat. What could be the problem with the hot water circulation?
Thank you for a response

On 2018-10-18 by (mod) -

if one of your zones is getting no heat at all then the problem is not the settings on the aquastat on the primary controller. That's simply controls the temperature operating range of the boiler.

More likely you have a zone Valve or circulator pump that is not working, or you might have an air bound heating Zone. If you can confirm that the zone is calling for Heat and the circulator is running and search this website for

air bound heating system

to see the diagnosis and repair needed.

Question: how to set the DIFF control on your Aquastat

(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) glgagne2@gmail.com 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?

Reply:

Joe, in AQUASTAT HI LO DIFF SETTINGS 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.

Question: do these aquastat settings make sense to you? HI 200, LO 110, DIFF 10 ?

(Feb 28, 2016) 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 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.

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:

(Feb 28, 2016) Bill said:

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:

(Feb 29, 2016) (mod) said:
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.

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.

Reply:

Colton,

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

Reply:

anon

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

AQUASTAT LO & DIFF DISABLED

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)

Reply:

Ryan,

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 ARTICLE INDEX 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) "

Reply:

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

AQUASTAT LO & DIFF DISABLED

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

Reply:

Dec 30, 2014) (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

AQUASTAT LO & DIFF DISABLED

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) (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: 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?

Reply:

Luis

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: 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

...

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-2 - more-recent Q&A about proper and best settings on an aquastat controller

Or see these

Recommended Articles

• AQUASTAT CONTROLS - home
  • AQUASTAT HI LO DIFF SETTINGS
  • AQUASTAT LO & DIFF DISABLED
  • AQUASTAT L6006A HONEYWELL GUIDE
  • AQUASTAT L7224U UNIVERSAL OPERATION & ERROR CODES
  • AQUASTAT L7224U UNIVERSAL DIAGNOSIS
  • AQUASTAT L8124 AQUASTAT INSTALL / REPAIR
  • AQUASTAT L8148 AQUASTAT INSTALLATION
  • AQUASTAT L8148 HONEYWELL DIAGNOSIS & REPAIR
  • AQUASTAT R8182-SERIES CONTROL
  • AQUASTAT RESET BUTTON
  • AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND
  • AQUASTAT SENSOR PROBE STUCK in the AQUASTAT WELL
  • AQUASTAT SETTINGS vs HEATING COST
  • AQUASTAT SELECTION GUIDE
  • OUTDOOR RESET MODULE AQUASTAT ADJUSTER
  • REMOTE THERMOSTAT CONTROLS for HVAC
  • TANKLESS COIL HOT WATER HEATER TEMPERATURE CONTROL
• MANUALS for HEATING & A/C SYSTEM CONTROLS
• SINGLE FUNCTION LIMIT SWITCH

Suggested citation for this web page

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

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING BOILERS

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Questions & answers or comments about the best settings for aquastat controls on heating boilers

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Citations & References

In addition to any citations in the article above, a full list is available on request.

---

ADVERTISEMENT

---