Q&A on how to choose the settings for a heating boiler aquastat control:

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

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

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

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

[Click to enlarge any image]

On 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.

On 2018-10-18 by Randy

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 2018-01-27 by (mod) - boiler doesn't come back on

Lucas

Possibly the aquastat's actual sensor is not in good contact with the side of the steel well into which it's inserted into the boiler - or the installer omitted the heat-conducting grease recommended by the manufacturer. Otherwise I suspect a defective sensor.

On 2018-01-27 by Lucas

My boiler is shutting off when the water reaches the aquastat hi setting but its not coming back on until after the water temp drops significantly and the pump is circulating cold water. I have a L8148 A. The temp is set at 200 as it came and the low temp is not adjustable.

On 2017-12-12 by (mod) -

normally the burner will turn off when the boiler temperature reaches the high limit if it's operating on a call for heat. Keep in mind that the low must be set at least 20 degrees below the high or the control will not work properly

On 2017-12-12 by Anonymous

when should y burner turn off? aquastat turns the burner off at the low temp (160) . The water temp never really reaches the hi temp (180) or near it

On 2016-12-28 by (mod) - problem with the temperature sensor of the aquastat control

Bruce,

I suspect a problem with the temperature sensor of the aquastat control - perhaps it's not in good contact in the well into which its inserted, or the thermal grease was omitted.

On 2016-12-28 2 by Bruce

My aquastat is set at 195 high, 175 low and 25 differential.

The low setting seems to work as designed, however when my thermostat calls for heat, the burner doesn't turn on until the temp falls to around 120. My understanding is that it should start the burner immediately. Don't know what the problem is. Any help would be appreciated. Thanks.

On 2016-10-26 by Jesse

I have a pellet boiler tied into my oil burner and a super stor water heater. What do you recommend to have aquastat settings H,L DIFF on the oil burner set at so the 2 systems operate efficiently. Right now the the oil burner kicks on and the pellet boiler doesn't ignite or still at a low burn rate because the unit is not calling for heat. Thanks for the help!

On 2016-01-24 by (mod) -

Jim the flow rate of water into a tankless heater will determine if it turns on or not. Too-slow an inlet rate will keep the burner off.
Search InspectApedia

for TANKLESS WATER HEATERS for details.

You can operate both at once but the output water temp may be too high and so risk scalding burns unless you also install a tempering valve.

Properly adjusted a demand or tankless water heater (not a tankless coil on a boiler) gives endless hot water at whatever temp you want. I would concentrate on solving the input pressure issue.

On 2016-01-24 by Jim

Question: Have tankless coil on oil boiler with aquastat Hi/Lo/Dif - Installed Rinnai propane tankless on demand hot water heater -

There is a long lag time and no hot water when faucet partially open - Plenty of hot water when faucet fully open - The oil tankless coil had shorter lag time and provided hot water when faucet partially open or full open - Can I operate both the propane tankless and the oil tankless at the same time -

They both are connected and have their own shutoff valves on the hot and cold water lines - The reason for doing this, is to have hot water when faucet partially open and cut down on the lag time

On 2015-04-23 by (mod) - WHY do we use the HI LO DIFF settings discussed here?

Karl:

In summer the thermostat never calls for heat. So the boiler would cool down room temperature. But the LO and DIFF, if enabled, will keep the boiler hot to make hot water, turning it on when the boiler temp falls into that range.

If a home is not using a tankless coil then there is no need to keep the boiler hot in summer as it's not being asked to make hot water through a tankless coil. So for a boiler with no tankless coil one could disable that feature.

Thanks for asking, Karl.

On 2015-04-23 by Karl R.

Another example to illustrate my confusion :

"*Earlier, in response to a query I had posted in which I described my system as a "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" --- I had asked what would be the best, most efficient summertime setting for Hi/Lo/Diff on my Honeywell aquastat for the above setup.....

You responded : "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. *"

Yet in response to a DIFFERENT query, you stated:

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

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

The LO and DIFF are almost irrelevant to this operation and use, and that may be why your tech disabled that circuit*"

If the LO and DIFF settings are irrelevant to my setup, then how...or why...."the boiler is running only off of the LO and DIFF " and I don't need to make any adjustments ? ? ? ?

Seriously confused.

I am really sorry if it seems I am beating a dead horse here, but I am very confused....

Earlier, in response to a query I had posted in which I described my system as a "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" --- I had asked what would be the best, most efficient summertime setting for Hi/Lo/Diff on my Honeywell aquastat for the above setup.....

You responded : "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.

---------------------------------

Yet in another, earlier query by another gentleman to the same scenario you responded :

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

AQUASTAT LO & DIFF DISABLED "

----------------

How do I reconcile the statement "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."

and

"you can abandon the tankless coil and disable the appropriate parts of the aquastat "

If I disable the appropriate parts of the Aquastat, how then is the boiler is running only off of the LO and DIFF

I'm sorry, but I'm really confused.

On 2018-01-26 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 2018-01-26 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 2018-01-01 by (mod) -

That's great, Jim.

Let me know if you continue to have trouble and we'll take it from there

On 2018-01-01 by Jim

Thank you for the quick response! You are correct on your assumptions and my incorrect description. Appreciate it very much and will use that guidance.

On 2018-01-01 by (mod) -

If your system uses an aquastat you have hydronic or hot water heat - properly called a boiler, though many people use the word "furnace" generically. (Furnaces are hot air systems) - just to be clear.

Now for a boiler, you'll get more efficient heat transfer by setting the aquastat HI as described at AQUASTAT HI LO DIFF SETTINGS - topic home.

On 2018-01-01 1 by Jim

Could use some guidance as my situation is very similar to this thread. Switch to heat pumps for both heating and a separate Heat pump HW system.

My oil furnace is now only for backup forced hot water backup heat on single digit and colder days. I've left my aquastat hi/low at the summer values, which I thought were efficient (110/140).

Now I notice it takes longer for the backup heat to assist, but I'm ok with that as long as my oil consumption is lower/efficient. Did I miss the mark with these settings?

On 2017-12-19 by Anonymous - Laars boiler with a Honeywell L8148 a controller there is no way to set the LLC

I have a laars boiler with a Honeywell L8148 a controller there is no way to set the LLC or the differential does it have a factory differential built-in

On 2017-11-25 by (mod) -

George, the HI, LO, and DIFF settings in the article AQUASTAT HI LO DIFF SETTINGS at "Rule of Thumb Settings" should be the best place for you to start.

180, 160, 125.

On 2017-11-25 by George

I just replaced the a quastat, on my Burnham V -74 bolier, what should the high limit be set at

On 2017-11-03 by (mod) -

Set HI 200 (as long as the relief valve doesn't open)

Set LO to 160

Set DIFF to as high as you can 25

usually will give the most hot water.

I used to set the DIFF as LOW as it would go - maybe to 10, thinking that would turn the boiler on soonest when we're using up hot water.
But Honeywell's explanation for the wide DIFF (25) is given inAQUASTAT HI LO DIFF SETTINGS at the section titled

Guide to Setting the "DIFF" (differential) on a Honeywell R8182D heating boiler aquastat & Similar Controls:

There are other factors:

1. a tankless coil is NEVER going to give endless hot water

2. incoming cold water entering the tankless coil in winter where you live may be very cold - more than the boiler can heat up very high

3. your boiler may be a physically small one - so there's not much thermal mass - so the incoming cold water draws the heat out of that little boiler quickly and then you've run out of heat
because

4. the rate that the oil or gas burner can put heat into the boiler is always less than the rate that the tankless coil can draw heat out of the boiler.

Please take a look at HOT WATER IMPROVEMENTS - at https://inspectapedia.com/plumbing/Hot_Water_Improvement.php

On 2017-11-03 by Anonymous

I gave that a short I kept the high set to 180
turned the low down to 160
and set the Diff to 20

I get less how water than I did before, I tried running a bath for my young children, and I got the tub about 1/8 full before that water got borderline cold, it ran cold for about 5+ minutes before it started getting warm again

On 2017-11-02 by (mod) - Your aquastat is set incorrectly.

Kyle,

Your aquastat is set incorrectly.

Take a look at the recommended settings in AQUASTAT HI LO DIFF SETTINGS and use those. Then make this final adjustment:

in order to get the most hot water from your system that uses a tankless coil in side of your heating boiler you will want to set the differential or DIFF dial to its highest number which on most aquastats is 25 degrees

On 2017-11-02 by Kyle

Hello I need some help / advice,
I have a pearless boiler with a Honeywell L7224 Electronic Aquastat in the basement

HL is set to 180

LL is set to 170

and LDF is set to 10

On the 2nd floor we have 2 full bathrooms, when we take a short or try to fill the tub, its hot at first then it cools off, to a luke warm (on the cold side) after 3-5 minutes, then will get hot again after another 5 minutes. sometimes this cycle repeats, taking a hot bath is almost impossible, because you start filling the tub with hotwater and it mixes in the colder luke warm water shortly after.

I feel like I don't have the most ideal settings for my aquastat. can anyone familiar with this maybe point me in the right direction,

On 2017-10-26 by Lance

At 195 degrees the safety valve lifts momentarily. I set it to 190 and I hoping that the safety won't lift. The temp/ pressure gauge went to 30 psi at 195 degrees the safety is rated for 30 psi. Does this seem right. We're in Chicago. If it still lifts I will check bladder for proper pressure. 2 story house with basement. Should pressure be 12 lbs on bladder? Thank you

On 2017-10-21 by albert hopfel

what will i set the diff. setting at 10 or 20,for my oil furnace honeywell aquastat

On 2017-09-24 by (mod) -

Jim,

As the LO it's only needed to work with a tankless coil for making domestic hot water, and as apparently you're not making that use of this boiler, you might want to disable the low function entirely.
this article series includes the description of how to do that.

Then you can set the high limit down as you suggest although there is some advantage to keeping a bit of heat in the boiler even when your house isn't calling for heat as it avoids rust corrosion and his damaged.

The fact is that if your house isn't calling for heat when they boiler cools off enough to turn on the burner the on cycle will be rather short because you're only heating the water in the boiler itself. The total cost of those Cycles would be quite small.

keep in mind that it's the inside thermostat in the heated portion of the house that's going to be responsible for turning the boiler on for a longer on cycle.

I actually prefer leaving my boiler set to 180 or 200 because that's more efficient way to operate a boiler when I'm running for heat and when my boiler doesn't include an automatic outdoor temperature compensating device.

On 2017-09-23 by JIM

I have two boilers for my home heating system. The main system is an electric boiler that runs on a Dual Fuel system. When the Power company decides to turn off the electricity to the main boiler, the system switches over to my backup Oil boiler (Burnham).

The back up boiler is installed in my garage where the temperature is approx. 40 degrees in the winter.

I have tried to set the HI and LOW settings to not run if the house is not calling for heat. Sometimes the cooler temps in my garage will cool off the boiler water temps and want to come on, wasting oil consumption.

What would be the best HI/LOW temps for this situation. Knowing on occasion the Electrical power will be turned off for a few hours and the house might need the back up oil boiler to take over the task. Can I set the LOW on 100 and leave the HI on 180?? I'm thinking the lower limit may not be reached in a few hours of the Main boiler being shut off. Your thoughts?

On 2017-05-22 by (mod) -

Thanks, Brian, for the comment.

On 2017-05-22 by Brian

I just reset my two boilers. The pumps run continuously. We have Honeywell L4006E, I think. I set the LO dial to 150* F, with a 20* DIFF, the boiler cuts on at 140* and cuts off at 160*. Which is all I( need to keep 140* water for dishwasher, and 120* at the mixing valve

. I'll change it back to 160/180 in the fall. Thanks for the info. No need to mess with the high limit stat, another device on the same circuit, Which I suppose cuts off the boiler if temps get too high.

On 2017-04-26 by (mod) -

Glad to assist, your questions help us see where we need to research, clarify, add information, so thanks, Steve.

On 2017-04-26by Steve

Yes, that was me that wrote before. I couldn't find your answer initially, but now I see your full response. Thanks! It seems as though the boiler settings are still 180/160 despite the addition of the ORM. Should I reduce the high setting and disable the low setting as I think one article suggested?

As I understand it, the ORM should cut some of the heating coils (6 of them) off as the outside temp increases. (Yes?) Also, I do have a check valve as your pictures show. How do I verify that it is not on the "forced open" position? I/m certain I will have further questions, so thanks again.

On 2017-04-26 by (mod) -

Steve

Typically aquastats come factory set assuming that there is a tankless coil on a boiler; the boiler will keep some heat in the boiler year-round (in case the tankless coil needs it) even if there is no call for heat.

If heating water is actually circulating out of the boiler even when your thermostats are not calling for heat, that too will cause the boiler temperature to drop and the burner to run to re-heat the system. That's an error: I suspect a zone valve that's not closing or a system check valve that's not closing when it should - permitting hot water to circulate by convection or as you put it, by gravity.

More detailed diagnosis of this problem is at HEAT WON'T TURN OFF - - https://inspectapedia.com/heat/Heat_Stays_On.php

The ORM, as I understand them, changes the operating temperature range of the boiler - it should not be disabling it; rather its intent is to give longer boiler-on cycles on a call for heat when the outdoor temperature is not too low. It does that by changing the effective HI setting.

I addressed this question where you (I think it was you) asked it previously: if the room temperature is below the set temperature on the room thermostat then hot heating water should circulate out of the boiler through the room's radiating devices. Unless your home is in Canada where installers set up the system differently, it is the temperature of the water in the boiler itself that turns the burner on and off.

Review the ORM settings with the manufacturer of your module - or its instructions. It may not be working properly.
Details are at OUTDOOR RESET MODULE AQUASTAT ADJUSTER https://inspectapedia.com/heat/Boiler-Energy-Savings-Devices.php

On 2017-04-26 by Steve - Monitron Slant Fin electric boiler settings

I have just recently installed a new Monitron Slant Fin electric boiler to my 20 year old system. I have 5 zones...two downstairs in floor heat and 3 upstairs in baseboard heat. I have the L7248L Aquastat and I have added the I also installed the Honeywell C7089 sensor and Outdoor Reset Module to this system.

My boiler settings are 180 High and 160 Low with 10 differential. The outdoor upper temperature setting is 40 degrees. In northern Minnesota when the temperature gets below 20 degrees the system works very well. have 2 questions:

1) When the the temp is ~35 degrees for a long period of time, the boiler stays between 180 and 160 degrees even if the thermostats are not calling for heat. This results in hot water circulating upstairs by simple gravity and it gets too warm. Shouldn't the boiler shut down with no calls for heat?

2) If the outside temp is 42 and cloudy for several days, the ORM shuts down the boiler and the house gets cold because the boiler does not kick in even if the thermostats ARE calling for heat. Shouldn't the boiler kick in if the thermostats are calling?

On 2017-01-21 by (mod) -

Try the settings discussed in the article AQUASTAT HI LO DIFF SETTINGS

And try adding an anti-scald valve as well so the hotter water coming out of your tankless coil doesn't scald someone in the shower.

On 2017-01-21 by Dan

How do I increase the water temperature?

On 2017-01-02 by (mod) -

MIke:

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

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

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

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

MIke:

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

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

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

...

...

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

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Suggested citation for this web page

AQUASTAT HI LO DIFF SETTINGS FAQs-4 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

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