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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.
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).
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.
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:
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.
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 CONTROL, our home page for this control, we explain in more detail, from which I excerpt:
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?
My hydrogic boiler don’t supplied domestic hot water I’ve a separate hot water tank for that. - N.P. 11/26/2013
The article above on this page discusses disabling the low limit. I like to do this on a boiler to reduce unnecessary fuel use during the off-season when heat is not being produced to heat the building. IF the boiler is not being used for heating you could also simply turn it off during warm weather. But there are some caveats:
1. If we do not clean the boiler before shutting it down for the summer ( or setting it to never run because we disable the lo-limit on an aquastat) the soot and crud in the boiler can become petrified, goopy and much harder to clean out later.
2. In some boilers, particularly in a high humidity and cool area such as many basements, condensation in the boiler makes problem #1 above much worse and can add corrosive effects to the boiler interior. Corrosion of steel boilers is a particular concern.
3. SOME (not all) models of cast iron boilers that are assembled with cast push-nipples between boiler sections can leak when they cool down completely.
So if you want to safe a buck by turning off a boiler when it's not needed for heat, if it's going to be off all summer, have it cleaned first. And if it's an old cast-iron boiler ask the heating service company if it is a brand and model known for push-nipple leakage when cold. I think that most heating service techs automatically say "leave it alone" and "leave the boiler on" - because that's the safe answer, and besides, their employer usually is a company that sells heating oil. But they've got a point - it's the safe answer though not the most economical one.
Questions & answers or comments about the best settings for aquastat controls on heating boilers
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