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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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).
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.
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
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.
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.
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.
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;
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.
[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.)
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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:
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.
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.
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Please also see AQUASTAT HI LO DIFF SETTINGS FAQs
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.
(Nov 13, 2012) Radhames Delacruz said:
When disabling the lo and diff settings, I noticed where it says jumper there are two wires, one is yellow and the other is red. Do I just remove the one wire underneath the "lo" control?
(Nov 13, 2012) Buddy said:
Radhames, I have a Honeywell 8124C Aquastat and successfully disabled the lo and diff functions. I removed only the blue wire that is closest to the diff adjustment. Please refer to the two lower photos on this page and you will see a the tip of a writing pen on the upper of the two photos pointing at the blue wire. The lowest photo shows the blue wire with an orange wire nut on it. Be sure to cover the bare end of the blue wire after you remove it. The yellow and red wires should be left in place.
(Nov 13, 2012) Radhames said:
I went ahead and removed that blue wire from my aquastat and its working. I cant thank you enough for your help. The info that I've found here is info the guys that came to service my boiler didnt know. Thanks again. :-)
(Nov 27, 2012) (mod) said:
Rather than raising the HI LO to make the water hotter, I'd consider partly closing the valves on the radiators that are early in the loop to slow the flow rate into them and thus give more heat to the later units.
Also make sure your heating lines are insulated.
(Nov 27, 2012) Radhames said:
Thanks for your advice, I always thought insulating the heater lines would be a good idea, but never did it. But I will now. and one last thing, Dan...when you say heater lines, those are the ones in the basement, the ones that feed the pipes that go to the radiators. Right?
(Nov 28, 2012) (mod) said:
Yes. You don't need to do anything heroic like ripping open inaccessible walls or ceilings. There will be a benefit from insulating those heat distribution pipes that you can get to.
Generally we like to insulate heat distribution pipes to try to be sure the heat we are paying to make is delivered mostly to occupied spaces in the building.
The only case in which I might leave insulation off is where we need heat in a non-living area space like a damp crawl area or below a cold floor.
(Sept 13, 2014) Steve said:
Hi How can I prevent the circulator to my indirect tank from turning on if the water temp in the boiler is below the tank water temp. Controls are Honeywell L8124A C and taco SR 501.
The Low Limit on a common Honeywell aquastat like yours is designed to do just what you want - it will "lock out" heating zone circulators if water temperature is below the LO setting. Wire the circulator for the indirect tank to that circuit.
(Oct 13, 2014) email@example.com said:
Yes, I have read most all of your "stuff" and oh my could it be any more difficult. I have to explain the system first to people before they can understand what is written. Just feed back
Now, my experience is that there is lots of residual heat left in the furnace after the flame turns off and as such the pump should remain on until the temperature/residual heat is circulated out of the furnace and into the home.. toasty and warm for a long time without the need for more fuel. Then when the room thermostat indicates that finally three or four hours later that it needs more heat the furnace comes on and when the temperature gets high enough the pump comes on and stays on till the temperature inside the furnace is low enough to turn it off.
The same way that fans on AC units stay on till all the residual cool is pulled out of it to the temperature desired. In this case the temperature will have to be set which makes sense that the low cut on thermostatically controlled switch could be used in reverse as a cut off at low and ON when reaches say 100 or 110... it is my experience that the amount of energy needed to heat anything increases exponentially as the temperature goes up just as it does in cooling below 32.... so to conserve on fuel the faster you remove the heat from the coils the less energy will be needed. No more heating up to 500 and then turning on the pump when all you need is to pull off a temperature of say 90 degrees. Make sense?
Just look at a chart of the number of BTU's needed to heat a gallon of water from 60 to 70 and the increase from 70 to 80, 80 to 90 and the huge spike to go past boiling.. all a waist since we only need to go to a few degrees hotter than the room temperature.. unless of course you have tenants that like to walk around in their underwear in the winter time and sweat pants in the summer.
How can I use the existing LOW temperature relay in reverse to turn on the pump at a high temp and off at a low temp and use the Hi setting as is to shut off FLAME when too hot?? Thank you.
Oh, forgot, the "idiots" plumbers did not know that the system was designed to provide hot water and heat to the radiators and charge $800 to re plumb the system and another 800 to install a gas hot water heater... so the only thing needed now is the heating and I want to set it up to come on at 100 and cut off at around 85 or even 95 and 85 perhaps the DIFF thingy could be used? Im gonna look at schematic while awaiting your response.
(Oct 20, 2014) Ray said:
I have a tankless Peerless WB-3 boiler with a Honeywell L4081b 1047 aquastat. I don't need the boiler for domestic hot water because I have a separate electric water tank, so I only use the boiler for heat. Right now the aquastat is set to H 180 L 160 Diff 10. Should I change the settings? Can I disable the aquastat and how do I do it on that specific model?
I installed a shutoff valve on the cold water pipe going into the coil, so right now there's no water going to the coil.
(Feb 18, 2015) Bill said:
When you disconnect the blue wire on the aquastat in order to disable the lo/differential, how does the circulator turn on and off.
Bill, when the thermostat calls for heat the cieculator relay in the aquastat or a separate circulator relay if you have more than one, turns on the circulator pump. The HI limit will turn on the burner as needed.
The wire you are disconnecting is only disabling the LO
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