Blue jumper disables low limit controlHow to Disable the Aquastat's LO/DIFF function
How & why to turn off the LOW portion of an aquastat heating boiler control when not using the tankless coil to make domestic hot water

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How to disable the LO and DIFF feature on a heating boiler aquastat combination control:

Here we explain how and why you might want to turn off the LO Limit on a typical heating boiler aquastat. This step might make sense if your boiler does not use a tankless coil at all or if you want to disable using the tankless coil for making domestic hot water for washing & bathing. We include some do's and don'ts and some warnings about what else happens when the heating boiler is left shut down for extended periods.

In this article series on heating boiler aquastat settings we explain how to choose the best settings for a heating boiler aquastat - the combination control that sets boiler temperature and may also control hot water production via a tankless coil on the heating boiler. We also give advice on how to set the aquastat controls if heating with a woodstove.

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Guide to Disabling the LO and DIFF settings on a Boiler that Does Not Use a Tankless Coil for Domestic Hot Water

If a tankless coil is not installed on a boiler where this combination aquastat control is installed, this combination control may still be in use. And that's not always desirable. It depends.

In this case, the "LO" has almost no use whatsoever and, if you read the instructions provided by the control manufacturer you'll probably see that the manufacturer calls for the "LO" to be electrically disconnected entirely - it's simply a matter of removing a jumper wire (usually the blue wire pointed-to by our pen in the photo at below left, next to the DIFF control.

Blue jumper disables low limit control

Here is how we get the blue wire out to effectively turn off the LO limit (or "turn off" the boiler feature that would otherwise keep heat in the boiler to serve a tankless coil when the boiler is not otherwise calling for heat).

Honeywell R8182D low limit disconnected (C) Daniel Friedman

Summer Operation With the LO limit Enabled: the Boiler Runs for Short Intervals

Short heating boiler "on" cycles? If you do not disconnect the "LO" limit switch on a combination control on a heating boiler where no tankless coil is installed, during the summer you may wonder why once in a while you hear your heating boiler running.

Why is the heating boiler mysteriously turning itself on and off? Now you can solve this mystery.

Incidentally another cause of oil burner short-cycling on might be poor contact between the boiler temperature sensing bulb (part of this control) and the well into the boiler water into which the sensor is placed. Or a defective sensor bulb might also cause this problem, in which case the control along with its temperature sensor will need to be replaced.

Cast iron boiler leaks? In a few cases, certain cast iron boilers may cool off and begin to leak between their cast iron sections during the summer. This may be a reason to keep the "LO" wired and active on just these heating boilers. Below we include additional warnings about stuff to watch for when disabling the LO limit on an aquastat.

Warnings When Disabling the LO Limit on an Aquastat

Watch out: if you are going to leave an oil fired heating boiler shut down for the warm months of the year, we suggest that it should be carefully and thoroughly cleaned at the end of the heating season before you turn it off. Cleaning is easiest then, and the heating techs are less busy.

If you leave a sooty boiler sitting in a damp cool area such as most home basements, soot in the boiler may become a gooey mess, may be much harder to clean off later, and it may also form corrosive acids that damage the boiler, particularly a steel one; also as we warn below, a few models of older push-nipple type cast iron boilers may leak if left cold.

Question: What's the DIFF? Here is a DIFF twist question for setting the Differential on an Aquastat when Running a Woodstove

I've read all the informative articles about adjusting the aquastat control on a heating boiler, however I have a situation that was not addressed.

I've just installed a woodstove in my house. I am running it 24/7 because of the price of oil. I have been turning my boiler off for 23 hours each day and just turning it on to clean dinner dishes and take showers - for one hour. Is this okay? To shut the boiler down for 23 hours each day? The temp in the basement is about 46 degrees.

I have 4 zones on my hydronic system and a tankless coil inside the Utica Starfire II oil boiler. 82% efficiency. None of the 4 heating zones need to call for heat when the woodstove is operating.

Should I leave the boiler off as I have been the last few weeks? OR, should I turn the boiler on 24/7 and leave the LO set at 120 (lowest point) and set the high at 180-160-140 ? What should the DIFF be set at? We only need hot water about one hour each day - from about 9pm to 10pm. - Damian

Reply: Watch out for Freezing Pipes and Boiler Leaks

At similar woodstove-heated homes where a hydronic boiler heating system was installed, we found that the woodstove could keep the room containing the heating thermostat warm enough that the boiler would virtually never run. And everyone felt smug about saving on heating cost. But you're right that there are a few things to worry about besides having to keep the boiler hot for the tankless coil used to produce domestic hot water for washing and bathing.

Keeping the Tankless Coil Hot:

For readers not familiar with the HI LO and DIFF settings on aquastats, see AQUASTAT CONTROL FUNCTIONS as well as this article.

Your note indicates that you are using a tankless coil for producing domestic hot water. You could leave the diff set quite low, say down to 120, but the problem is likely to be that you won't have much hot water when you do need it, unless your boiler were an older, physically larger or cast iron unit that had plenty of thermal mass. So one option would be to leave the Hi and Lo set at their normal positions, say HI at 180 or even 200, and the LO at least 20 degrees below the HI, at 160 or 180 respectively.

The HI will actually be irrelevant in your as you're never calling for heat - just leave it alone so that if you're away from home and the boiler needs to kick in to prevent pipe freezing, the settings will be at their normal position.

On modern heating boilers we like to keep the DIFF set to its lowest setting because that turns on the boiler soonest when you are running out of hot water and helps prevent having to get the soap out of your hair using cold water.

Our OPINION is that the standby losses for the boiler are not bad with this setup because your oil or gas burner is not having to heat up all of that water that's in the heating distribution piping (or baseboards or radiators), it's just keeping water right there in the boiler hot.

If your hot water usage volume is very small and/or if you don't have a mixing valve to avoid scalding, you can set the LO to a still lower number if you prefer.

Tradeoffs on Turning Off the Boiler to Save Money

Turning off the heating boiler in order to save money during most of the day when you are not using hot water, sounds appealing. A much debated question is just where is the break-even point between the standby losses from keeping the boiler warm, versus the extra fuel you have to burn to heat up an ice cold boiler when you need hot water from the tankless coil.

Our OPINION is that the cost to heat up a very cold boiler in a basement or crawl area just to make hot water is worse if it's a larger boiler or a cast iron unit that will have more (cold) thermal mass. If it's a modern, physically smaller steel boiler, your cost to heat it up from cold will be less - but then your tankless coil is going to suck that heat out quickly when you're running hot water through the tankless coil regardless.

An exactly correct answer to this question requires either some theoretical calculations for your particular boiler, burner, and the gph or BTUh rate of the burner along with the thermal mass of the boiler. Easier might be to keep an eye on the total boiler run time under the two circumstances: left on and an occasional intermittent burn cycle versus left off and a longer heat-up cycle. It's slightly more complex: heating up an ice cold boiler may force the burner to run at less than optimum efficiency for a longer part of its burn cycle than otherwise.

Warnings about Turning off the Boiler Completely

Bottom line on Woodstoves and Hydronic Heating

Reader Question: how to disable the LO & DIFF on an Aquastat when Using a Separate Tankless Water Heater

Thank you for a great web site. It's very well done and informative. I have a question regarding the temporary disabling of the domestic hot water of my boiler tankless coil using a L8124A Honeywell Aquastat.

I have an oil fired boiler with a tankless coil installed. I just installed a whole house electric tankless instant domestic hot water heater for summer use and I turn off the boiler to save on fuel cost (much better savings). I connected this unit to the outgoing domestic hot water pipe of the tankless coil.

I have decided to not use the boiler to heat the domestic hot water. So that when the boiler is in use in the winter for heating the house, the idea now is to allow the pre-heated water from the coil to flow thru the electric hot water system thus the electric system will be functioning only when the water temp. gets below it's minimum settings.

I want to disable the boiler/coil via the Aquastat in order for the incoming water to run thru the coil but prevent the boiler from firing when the temperature of the domestic hot water falls from the boiler at which time the electric system will take over. I just want the boiler to start when the house is calling for heat or the boiler heating water temp. gets below a certain Low setting.

So the questions are:

1) How do I disable the domestic hot water at the Aquastat and maintain the proper temp for the heating system?

2) If I wanted to turn the coil function on at some point, should the electric hot water heater fail, is it possible to just attach a switch/toggle in between the relay? and wire that I guess I disconnect on the Aquastat?

Thank you for any input you can give me. J.L. 10/19/2013


Our main article on setting the heating boiler aquastat is at AQUASTAT HI LO DIFF SETTINGS. Hoping it'll make the answer to your question easier for you and others to find, I have moved out of that article into a new separate article an explanation of  how & when to disable the LOW & DIFF controls. Basically, as you'll see in the photo, you pull the blue wire and cap it and the LO and DIFF will go to sleep;

How do I disable the domestic hot water at the Aquastat and maintain the proper temp for the heating system?

The aquastat LO/DIFF disabling guide is now at AQUASTAT LO & DIFF DISABLED - above on this page. Following those instructions (remove and cap the blue jumper wire or equivalent), if you disable the LO/DIFF features of a typical heating boiler aquastat, the boiler will not run at all until there is a call for heat. It's basically running off of the HI only; (The differential for heat is hard-wired into the control).

The proper settings for heating the building will remain undisturbed. That's because the building heating functions of your aquastat are controlled principally by the HI limit control setting.

Chart of function of HI LO and DIFF on an aquastat (C) Honeywell

[Click to enlarge any image]

On a call for heat the burner heats the boiler until it reaches the HI limit. That's the "cut-off" temperature for the aquastat. On a heating call, the "cut-on" temperature for building heating cycles is hard-wired in the control.

At AQUASTAT CONTROLS, our home page for this control, we explain in more detail, from which I excerpt:

The boiler Cut-on temperature or burner turn-on temperature for the boiler, AS LONG AS THE THERMOSTAT IS CALLING FOR HEAT is fixed at 10 deg.F. below wherever the HI is set. (Green in our edited version of Honeywell's drawing.) On some controls this hard-wired fixed gap may be 15 °F.

On some controls this hard-wired fixed gap may be 15 °F.

So if HI is set to 200 °F that's the cutout temperature, and the cut-on temperature for the boiler, as long as the thermostat is asking for heat, will be (200 - 10) = 190 deg.F. (or 185 °F on controls with a 15 degree fixed gap.)

[Click to see an enlarged, detailed version of this or any illustration at

If I wanted to turn the coil function on at some point, should the electric hot water heater fail, is it possible to just attach a switch/toggle in between the relay? and wire that I guess I disconnect on the Aquastat?

In this case one would not pull the wire out of its connecting terminals on the aquastat control. Instead of just pulling out and capping the blue wire in our illustration, you would, WITH ELECTRICAL POWER TURNED OFF, you would have to cut the wire to insert a suitable 120VAC toggle switch right in line with the blue wire we show in the illustration above. You'll need to be sure that your on-off switch is electrically safe, secure against jiggling around or short circuits.

Watch out: modifying any heating equipment control risks making a mistake, damaging the control, voiding a warranty, or creating an unsafe condition. Below I list some additional considerations.

Your plan is an interesting one and I look forward to hearing how it works out for you. Here are some considerations:

Additional Considerations When Leaving the LO/DIFF Disabled

Above at Warnings When Disabling the LO Limit on an Aquastat I list some more caveats. Here are some additional considerations that pertain to your case of disabling the LO and tankless coil completely when using a separate electric tankless water heater:

You may not see much benefit running cold first thru the coil and then into the tankless heater when not in the heating season; the boiler will be cold anyway so the heat gain in the incoming water will be close to nil.

During the heating season, when the boiler is hot due to a call for heat, you'll steal some of that heat for the tankless coil and thus will heat up water coming into the tankless water heater;

Watch out: tankless water heaters simply raise the incoming water temperature as much as the heater can (depending on its settings), sending the hotter water on to the calling fixture(s).  If you pre-heat the incoming water I'm not sure what your particular water heater will do - it may send dangerously scalding hot water over to the fixtures, unless there is a mixing valve or anti-scald valve or equivalent control installed. Therefore I'd give a call to the manufacturer of your system to ask their opinion.

Watch out also: if your heating boiler is oil fired in particular, I'd be sure to have it cleaned at the end of the heating season; when you disable the LO on the aquastat the boiler never runs all summer (which I like) but if the boiler was left sooty, condensation over the summer can glaze and thus petrify the soot inside the boiler making it tough to clean; if you have the boiler cleaned at the end of the heating season, while the soot is still soft and fresh, cleaning is easier.

Also, this is a bit more obscure, a few cast iron boiler models may leak at their push-nipples if left totally cold for a long period.

Alternative to Disabling the LO Limit: use plumbing to bypass the tankless coil or shut off water to the coil

If you decide you'd prefer to leave the boiler on when outside of the heating season, but want to skip making it run to make domestic hot water,  just leave the LO enabled, but have your plumber install a bypass valve and piping so you can in essence turn off cold water that would enter the coil, routing it instead right to the new tankless water heater. In that arrangement the boiler will run for short intervals and not often, but will keep some heat in the boiler all the time.

Watch out: don't confuse turning off water into the tankless coil with turning off water supply to the boiler's pressure-reducing automatic water feed valve. The later, aka a water feeder, functions as a safety device to assure that water is kept inside the boiler. IF the boiler does not also have a low water cutoff safety control (recommended), allowing a boiler to run dry would be dangerous, not to mention it's likely to damage or destroy the boiler.

Let me know if the article leaves you confounded or if you have further questions.


Continue reading at AQUASTAT HI LO DIFF SETTINGS that explains the best settings to use & how & when to disable the LOW & DIFF controls or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see MANUALS for HEATING SYSTEM CONTROLS for a list of aquastat installation & repair guides

Or see these

Article Series Contents

Suggested citation for this web page

AQUASTAT LO & DIFF DISABLED at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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