Photograph of a multi function combination control on a heating boilerHeater Aquastat Diagnostic FAQs
Aquastat problem diagnostic questions help replace, install, set-up, & wire heating system aquastat controls

  • AQUASTAT DIAGNOSTIC FAQs - CONTENTS: diagnostic questions & answers help operate, choose settings, & troubleshoot a typical heating boiler combination control aquastat like the Honeywell R8182D, the Honeywell L8124A, and the Honeywell L8151A.
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Diagnostic questions for aquastat controls on heating equipment:

These questions & answers help you to understand how to set an aquastat's HI, LO, DIFF dials & how to troubleshoot & repair this heating boiler control.

This article series explains how aquastats work and what the different aquastat controls are, what they do, and how they are set. We define the HI LO and DIFF controls on heating boiler aquastats and explains what they do and how they work. We explain the location and use of the heating boiler reset button found on aquastats.

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Aquastat Diagnostic & Setting Questions & Answers

Aquastat hi limit controlRecent questions and answers about aquastats posted here or at AQUASTAT CONTROLS or at AQUASTAT TROUBLESHOOTING

On 2017-04-20 by Hylton - Our aquastat sends 24v to the electrical ignition but the boiler doesn't fire up.

Strange gas boiler problem. Our aquastat sends 24v to the electrical ignition but the boiler doesn't fire up. Tried replacing the electrical ignition board (thought the board was bad) but it still doesn't start up. Thoughts?

On 2017-04-22 by (mod)


We'd need to know more.
With a new control board and voltage to it, does the gas valve open or not? Does the igniter spark or not?
There could be a bad gas valve or a bad connection to it.

On 2017-02-02 by AEG - splitting one radiator into two, piping alternatives

I had a contractor divide one room into two. We removed one large and long radiator and replaced with two smaller radiators on each side of the divide. Can you split a hot water supply line to supply two radiators? Or should you go into one radiator, then supply the second one out of the return from the first radiator? They did the first.



You can, with proper plumbing run the existing long radiator with two separated ones - you just need access to run the pipe to the second rad and then its return back to the original point of return or to another one that works. That is a "daisy chain" or sequential piping arrangement.

But working from the floor or ceiling below, if the plumber found it more convenient - that is it depends on heating pipe routing - she might feed radiator 1 and 2 separately off of an existing hot water heat supply line (and of course returns) - either is possible, it's a matter of which gives easiest plumbing.

On 2017-01-29 by Steve S - Honeywell L4006E aquastat troubleshooting

Hi. Your site is fantastic.

I believe I looked everywhere on your site and have not been able to find an answer.

I have a gas hydronic heating boiler that is used for baseboard heating, as well as for our hot water. The hearing is for one zone only, and there is a circulator as necessary. There is a combination aquastat that controls the high and low limits. In addition, there's a manual reset high limit aquastat (Honeywell L4006e) set for 200 degrees). Recently, when there was no heating going to the baseboard, we noticed that the manual reset button was tripped.

We reset it and it worked ok for a few hours. Each time we reset it, it seems to work ok, but only for a short period. I believe the boiler shuts off on high limit as set in the combination aquastat, BUT the temperature still rises substantially and therefore power gets tripped by the high limit manual reset aquastat (set for around 205 degrees).

I figured I can adjust the combination aquastat settings so that it would not trip. I set the low limit at 155, the high at about 185, the differential at 15, and the manual reset at 205. This should have helped (in my opinion) but it still tripped.

One final point, there are three other identical boilers in the room next to ours, and the high, low, differential, and manual reset settings are all just about the same. Their low is about 165, the high about 190, the differential at 15, and the manual reset at 200. The other three are working fine.

What can be causing this? What can I test or look out for? I appreciate your response.



When the aquastat reset button is tripped off then the control found an unsafe condition (or the control is defective which is less likely).

The instructions for the Honeywell L4006E (the "E" means it has a high limit reset switch), include this advice for testing the aquastat limit switch:

In checking out the system, adjust the limit setting low enough so that the temperature of the controlled medium (water) reaches the high limit setting. When the limit setting is reached the Aquastat controller locks out and the burner shuts down.

When temperature of the controlled medium drops to the high limit setting, minus [the] differential [setting], push the manual reset button and the system should again be operative. Reset control to proper high limit setting.

To me this points out that if the control is not damaged, your system is either set to too-high a temperature or its temperature control is not working properly.

There might be a small temperature rise after a boiler reaches its limit due to residual heat in the steel or cast iron of the boiler but that ought not be enough to be causing trouble.

Your High (185) minus Diff (15) = 160. Reset the aquastat when the temp is below 160. Then if the problem recurs I suspect a defective temperature sensor or a defective control.

In the article above I'll be sure we include a link to the instructions for the AQUASTAT L4006, L4006E, L4007 HONEYWELL GUIDE [PDF]

On 2017-01-15 by (mod) Before replacing a control I'd want to run some diagnostics (

Before replacing a control I'd want to run some diagnostics (such as are given in the control's installation and repair manual) and maybe change the flame rod.

On 2017-01-15 by Louis hood Honeywell C99-100 with flame sensor boiler control having to reset boiler gits to 160 and trips out this control cost a lot what is my problem is their another control that I can use?????

On 2017-01-15 by (mod) Honeywell C99 controller


I can't say from just your brief e-text if the problem is with the Honeywell C99 controller, a flame sensor, or another control or burner operating safety problem.

I think by "flame rod" you refer to an igniter system for gas fired heating equipment. IF that's the problem, for example if the igniter is sparking but not igniting the flame, I would replace it using a compatible replacement, perhaps the Honeywell Q3400A1024 30-Inch Igniter Flame Rod Assembly. (About $40. U.S.D.)

On 2017-01-15 by Louis hood

I have a Honeywell c99-100 with flame rod keep shutting down boiler gets to about 160 have to reset

On 2017-01-15 by Joiry

Hi, I have a wiring/installation question regarding triple aquastats. I am interested in converting my Honeywell dual aquastat to a triple aquastat heat manger, perhaps the Hydrolevel Hydrostat or the like. I have a separate domestic hot water source.

The dual aquastat presently operates a two zone hydronic oil fired boiler running at 82% eff. Unfortunately both zones are controlled by simple line voltage thermostats. After reading the schematics on triple aquastats, it seems apparent that low voltage wiring is necessary, especially for the heat manager to work properly. Is this true?

Is there no way to operate a triple aquastat with two zone line voltage thermostat wiring?

It is possible for me to convert the thermostat wiring to low voltage fairly easily in the house as well as wire in a switching relay to allow the second zone to communicate with the triple aquastat. I'd appreciate any advice on whatever options will prevent the boiler from running to high limit 24hrs a day during the heating season. I know that was a lot of questions. Thank you for your time!

On 2017-01-05 by Anonymous

The thermostats were off, so with them being off, is it still possible for them to trigger the interval firing of the boiler?

I have a gas water heater that is not heated by the boiler. I was thinking about disabling the LO limit but the regular interval firing is the confusing issue. Thanks.

On 2017-01-05 by (mod) heater is firing at a very regular interval


When a heater is firing at a very regular interval I suspect that some other timed component or feature is causing it.

Look for a thermostat with a clock timer or schedule that is improperly-set or is malfunctioning (Nest heat schedule?) or even something indirect such as a leak in a hot water pipe spilling hot water from a plumbing system whose hot water is heated by the heating boiler either via tankless coil or an indirect fired water heater.

On 2017-01-05 by John

Why is my hot water baseboard boiler firing every 24 minutes for about 2 minutes and 40 seconds each time even though the thermostats are off? The boilers seems to be on a timer but the thermostats are off. I have a honeywell triple relay aquatstat on the boiler. Thanks.

On 2016-11-20 by bill

Wood fired boiler we are using auquastat to run a selinoid to open a damper. Its a Honeywell 7006a. Problem is its a 3 wire setup and we need only two. Running off a switch for main power how do we wire it to work properly

On 2016-11-18 by Glenn

I have an oil fired boiler. I'm having this issue: I have to press the red reset button to start the burner. It then runs fine until it reaches the high limit temperature.

Then the boiler shuts off and trips the red reset button. The red light illuminates. If I press it once the burner will start again until it reaches temperature then it trips the circuit once again.

I've replaced the CAD sensor, electrodes and set the electrodes with the Beckett gauge, replaced the fuel nozzle, all the filters and the Honeywell R8184 really. The aqua stat is a very old Honeywell. Not sure of the model. Could it be the aqua stat causing the issue?

On 2016-11-10 by (mod) - what is the difference is between the L4148e 1257, L4148E 1265 and the L4148e 1299 model aquastats?

Honeywell offers aquastat controllers for both oil and gas fired heating equipment such as hydronic (hot water) heating boilers and water heaters. Both single function (high limit or low limit are separate controls) and multi-function aquastats are provided.
The contemporary Honeywell aquastat line includes the

I have not found an L4148 series such as in your question. Guessing you meant L8148 I note:

Often an alternative case is available to convert vertical to horizontal mount for these controllers.

A link to the Installation and Operating guide and cross reference manual from Honeywell is give in the article above.

On 2016-11-09 by Gary

Would like to know what the difference is between the L4148e 1257, L4148E 1265 and the L4148e 1299 model aqua stats. Thanks

On 2016-10-26 by richard

hi there, looking fro some wiring advice on hooking up a dual aquastat honeywell L6062a for a wood boiler only with one circulator and dc draft controller with one thermostat
any thing you could provide would be great.

On 2016-09-07 by (mod)


First you can rule out both a bad thermostat AND bad thermostat wires by disconnecting the thermostat at the boiler rather than at the thermostat

If heat continues to circulate then you could have a bad circulator relay that is stuck ON (that's unusual)
heat may be circulating by convection even if the relay isn't trying to run the circulator and even if the circulator itself is not running. In that case a check valve at the boiler hot water exit piping may be stuck open.

Search for HEAT WON'T TURN OFF to see how to diagnose and fix this problem in more detail

On 2016-09-07 by Ryan

Could a defective aquastat cause the heat in one zone to keep pumping even if the thermostat is not calling for heat. Note thermostat has been ruled out. purchased a new one and also swapped with other zone.


On 2015-10-29 by Anonymous

i do not get hot water my settings are low please help

On 2015-10-26 by kathy coleman

i need to to know hi and lo settings on honeywell triple aquastat relay l8124ac and dif

On 2015-10-17 2 by steve

my control is not calibrated right. My low is at 180 and my hi is at 150 but it works perfect. hot water is 200 and kicks on at 160 for lo.

what gives?

Question: What are the Optimal LO & DIFF settings for moderate hot water use in summer?

What would be the optimal setting for the LO & DIFF settings for fuel savings with low and with moderate hot water usage in the summer months when the boiler functioning for heat does not come into play. If you would please identify the low usage from the moderate usage settings it would be greatly appreciated. - Mark

This is an excellent article on the DIFF setting as it pertains to using a Tankless Coil. During the summer in North Carolina when outside temperatures are in the 90's and my system never calls for heat, I am only running my Burnham boiler to provide hot water. It is not unusual to burn 1 gal of oil/day (30 gals / month) just for residential hot water. With oil prices well above $3.50/gal, $100/ month for hot water is a little extreme.

I am curious if some combination of LO Limit / DIFF might cause the burner to run less while still maintaining adequate heat to provide hot water. For example, if my current settings are LO 160/ HI 180/ DIFF 10, might adjusting the LO/DIFF settings either up or down lower oil consumption? I suppose another option could be to use an electric hot water heater with a tank during the summer months. - Tony

I have a VERY old system. Have two separate aquastats, one used for LO and one used for High. No Diff. I have a separate hot water heater. Because of the old system the oil company technician feels that I should keep the furnace on during the summer so that there isn't a "shock" to the system when it is turned on in the fall. Question: What are the best HI-LO settings in the summer and in the heating season.? - Steve

Reply: Summer Aquastat Setting Recommendations

Mark and Tony and Steve: For summer use, if you set the LO to its lowest temperature setting (typically 102 F) you will be keeping minimal heat in the boiler to support the tankless coil in summer and will lower your heating fuel consumption by as much as 30%.

For summer use, if you set the DIFF to its lowest setting you will also lower your fuel consumption.

The result of these settings will also be a reduction in the quantity and also the maximum temperature of hot water supplied to the home - which may be OK for cases when hot water use is only moderate.

If you have an automatic tempering valve or mixing valve installed on your hot water supply that will increase the total hot water quantity available by drawing heated hot water from the boiler only at the rate needed.


You can leave the HI setting on your Aquastat alone in summer; since the system will never be calling for heat, the HI will be asleep all summer.

If you set the LO down to a lower number in summer you will be keeping less heat in the boiler, and in that sense burning less oil; but if your mixing valve is a MANUAL one you may need to adjust it so that water at the tap is hot enough.

If you keep the DIFF to its lowest number in summer you're also minimizing the oil burner on-time. Or conversely, if you set the DIFF to a higher number (say in summer), quoting from the article above:

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.


Given that you are not using your heating boiler for making domestic hot water, the reason a service tech would recommend keeping it on in summer is most likely because some older cast iron boilers can develop leaks between the boiler sections if the boiler is allowed to cool off to stone cold. Such a leak may not "heal" on its own when heat is restored, and it can lead to worse boiler damage.

For summer use on such a system, keeping just about any heat at all in the system, say 100 °F should be sufficient. If your LO is currently set above that number, try setting it down for the summer - but don't forget to restore it at the start of the heating season.

I am inferring that the HI on your system is an upper limit on a call for heat and won't be operating in summer. If I'm mistaken, just keep the HI 20 °F above the LO.

Question: What are the best Hi Lo DIFF settings on an aquastat?

Right now I have baseboard heat and I have the boiler set at HI-170 LO-150 and DIFF at 10. Is that a good Setting. I'm trying to save money on oil. thank you - Matt

I'm still not 100% sure on what my settings should be. I have a Weil Mclain boiler that's about 6 years old with a DHW coil installed. I had a 30 gallon electric hot water heater installed after the coil, and during the summer I would shut off the boiler and use only the hot water heater. During heating season the water would be pre-warmed by the coil and the electric heater wouldn't be doing much.

Problem is, we seem to be using a lot of oil. The boiler is constantly running, every time we use the shower, run the dishwasher, wash our hands, etc. It's also constantly kicking on to keep hot water in standby. During the early heating season, it's a waster, since the boiler isn't on very much to provide heat to the house. What should my settings be? Will I save more money by just not using the coil and telling the aquastat there's no coil? - Matthew

I live in Maine and the winter season is here. What would be a good Hi and Low setting for my aquastat? Its is starting to dip below 10 degrees at night. Thanks. - Nick


Nick and Matt:

Under AQUASTAT CONTROL FUNCTIONS (the article above on this page) you'll find good aquastat HI LO and DIFF setting recommendations in the article titled Optimal LO/DIFF settings save fuel

Matt: you can significantly save on heating costs by lots of other measures including using a thermostat setback to lower temperatures at night or when the home is unoccupied. See HEATING COST SAVINGS for our suggestions of a variety of ways to save on heating costs, and importantly some priorities: what heat cost savings steps should we address first at a building?

Question: What are Normal Boiler Temperatures When the Aquastat has Turned off the System?

Our aquastat shuts off burner at 190 degrees but old and new temp gauge continues to rise to 215 degrees. is this normal? - Warren G.

Reply: It is normal for boiler temperatures to sometimes rise a bit after the burner has shut down

Warren: it is normal for temperatures inside a heating boiler to continue to rise a bit after the Aquastat has turned off the oil burner at the HI or when the thermostat has stopped calling for heat.

This temperature rise may occur because:


As long as boiler temperature does not rise high enough to cause the pressure/temperature relief valve to spill you should be fine.

Question: When Abandoning a Tankless Coil: Should We Change the Aquastat Primary Boiler Control Settings

I am currently installing a hot water heater to replace a tankless coil in my boiler. Do I need a different aquastat to keep my boiler from trying to heat the coil? Can I replace my present aquastat #l8148a with #l8124a??? - John Haines

We installed an electric hot water heater, do we have to change anything to make the boiler function just for heat and not hot water (for bathing, dishes etc) ? - Ernie

I have replaced the tankless coil in the boiler with a gas water heater thus eliminating the need for the boiler to heat water for bathing,washing etc. question does the aquastat still needs to be connected for my boiler to run to heat the house? - jorge gonzalez

Reply: Disable the LO & DIFF settings OR set them to their lowest setting when abandoning a tankless coil


Take a look at the instructions that come with the aquastat and you'll see a list of devices it can replace. But most likely you do NOT have to replace your aquastat with a different model just because you are ceasing to use the tankless coil for domestic hot water.

On the aquastats discussed here, the instructions (link given below) describe a simple process for disconnecting a wire that will disable the low-limit control, thus eliminating that function when a tankless coil is no longer in use.

Ernie, When you abandon or stop using a tankless coil on a heating boiler, you can save on heating fuel consumption by disabling the LO / DIFF feature on your aquastat. The effect is that the aquastat (primary boiler controller) will stop keeping the boiler hot when there is no call for heating the home itself.

If you leave the LO/DIFF working, the boiler will keep heating itself even in summer when there is no call for building heat - because it thinks it's staying hot to support use of the (now abandoned) tankless coil.

Jorge, you still need the aquastat control, since it has to manage the boiler on and off temperatures when the wall thermostat(s) call for heat in the home itself. But as we explain above, the aquastat controls can be changed to stop keeping heat in the boiler to support the tankless coil that is no longer in use.

The procedure for disabling the tankless coil heating system is in our Aquastat Settings article at DISABLING LO & DIFF on an aquastat.

Question: How Much Will I Save on Heating Cost by Lowering the LO/DIFF Settings on the Aquastat?

Thanks for answering my questions on my recent hot water heater installation. I disconnected the differential wire (see DISABLING LO & DIFF) on my aquastat and now my system works perfectly. - Tony

Thanks for answering my question from June 28, 2011. I did, in fact, reduce the LO limit setting this summer from 160 to 120 and it reduced my oil consumption for heating hot water by about 30 percent. I left the DIFF setting unchanged at 10. To be honest, I was a little unclear from your response about the effect raising/lowering the DIFF setting would have on overall oil consumption during the summer months.

However, adjusting the LO setting definitely had the desired effect. It was not necessary to adjust my manual mixing valve because the shower water was at a comfortable level. Thanks for taking the time to answer my question and for the valuable information contained on this website! - Tony

Reply: Oil Consumption Savings Reported to be 30% When LO set Back

Thanks for the feedback that lowering the LO setting will save heating fuel during summer months. We have edited the HI, LO, and DIFF functions above to make that text more clear.

Basically, by lowering the LO to its lowest point (120) you reduced that amount of heat kept in the boiler during the summer (when the only function of the boiler is to stay hot in order to heat the tankless coil used to make domestic hot water). Like you I'd have also set the DIFF to 10 as well.

If you are continuing to use a tankless coil for your home hot water supply, you might want to set the LO back up to 20 degrees below the HI in winter because in cold weather the tankless coil is receiving colder incoming water from the building public or private water supply.

Reader Comment: heating cost savings from aquastat settings

Hi Dan. ..I just wanted to clear up that the 30% savings you mentioned in the article were those that I reported in my post as a follow up to the June 28, 2011 post. ...

I can confirm the excellent point you made about setting the LO back up to 20 degrees below the HI in winter because of colder incoming water from the public water supply. It is now late October in NC and I still have the LO limit setting from this summer at 120 with the DIFF setting unchanged at 10. I have not begun to use the system for heat yet and am only running the boiler for hot water.

I have noticed a definite decrease in shower water temperature and will shortly need to reset the LO back up to 20 degrees below the HI. Again, thanks for all the valuable information you provide on your website! - Tony [October 2011]

Question: sorting out HI LO and DIFF - what setting does what?

There seems to be some conflicting info on the site. In some articles, you say that the DIFF is the temperature rise over the LO setting. For example, DIFF 15 and LO 120 would mean you'd have a range of 120-135. In this article, it's explained that there always a 10 degree drop below LO and a rise above LO of DIFF-10, meaning DIFF 15 and LO 120 would give you a range of 110-125. Which is true? And what about aquastats that have a 5 degree DIFF setting? - Brian

Reply: summary of HI LO DIFF definitions and functions on an aquastat

Thank you for the clarification request, Brian. We've reviewed our HI LO DIFF setting articles to be sure that the text is now self-consistent.

Summarizing the definitions of HI, LO, & DIFF controls on an aquastat control

HI Limit Definition: The HI is the cut off point on a call for heat. You can think of the HI dial as the "heat control" in this regard: as long as the room thermostat is calling for heat, when the boiler temperature falls 10 degrees below the HI the burner will (normally) turn on. That 10 degrees is hard-wired into the control - it's not adjustable.

LO Limit Definition: the LO is a control function added to keep heat in the boiler when the room thermostat is not calling for heat. This feature is important on heating boilers that use a tankless coil immersed in the boiler to produce domestic hot water.

The DIFF Definition: the DIFF is an added control that defines the cut-off temperature (above the LO) during a burner-on cycle initiated by the LO control - that is, we are not calling for heat, and we're keeping heat in the boiler to make domestic hot water. It works as you described.

Circulator lockout: There is an interaction between the LO and DIFF settings and the delivery of heat to the building however. That is that the LO operating range on an aquastat also will lock out the heating circulator pump - stopping delivery of hot water out of the boiler and into baseboards or radiators - if the boiler temperature is too low. (That's why aquastat instructions specify that the LO should always be at least 20 °F below the HI setting).

The intent is to give priority to the tankless coil and thus the person in the shower. For heating boilers at homes that do not use a tankless coil to make domestic hot water, it is sometimes (not always) reasonable (and a savings on fuel during summer months) to disable the LO/DIFF controls - as we explain at Disabling LO & DIFF

See Aquastat control HI LO settings for details about how the HI and LO settings work.

See Aquastat control DIFF settings for details about the DIFF setting and how it works

See Optimal LO/DIFF settings save fuel for some recommended settings to save on heating fuel cost

Question/Comment: Rusty crud found on the bottom of the heating boiler firebox

As Tony said in his comment above, I also was using about 1 gal of oil per day during the non-heating months. In Connecticut, we are paying close to $4.00 per gal now. I installed an electric water heater in parallel (NOT series) with the boiler's tankless heater. Either the tankless or the electric can be used since the water does not go through one and then the other.

(Glad I still had the tankless piped up since I could still have hot water during the recent 10 day power outage in Connecticut. The 4000 watt generator could run the boiler but not the 4500 watt electric heater.) I found that we are using about 11 kwh/day for domestic hot water by timing how long the 4500 watt heater elements were on each day - a 240 volt relay activating a battery powered clock.

The cost at $0.17/kwh is about $1.90/day - better than 1 gal/day at $4.00. I turned off the boiler when I turned on the electric water heater.

My concern is that when I went to tune-up the boiler for the 2012 heating season, I found about one cup of rust (some of the material was magnetic) on the bottom of the firebox below the cast iron boiler sections. I open the firebox for cleaning each year and have not seen this much rust before. I have brush cleaned the flue passageways twice since new and don't recall seeing this type of material.

The boiler is a Dunkirk that was new in 2001. Water leakage does not appear to be a problem. Should I be worried about damaging the boiler (corrosion) by letting it go stone cold during the non-heating season? - Buddy 1/12/12


Buddy, thanks for the important comments & question. Indeed on many boilers, if you leave the unit shut down for months there can be problems with rust and also with solidification of soot and crud deposits that make seasonal cleaning more difficult (and perhaps costly).

If the boiler and tankless coil is left "on" but not in use, it will run only very little, as it runs just to keep itself hot - it's never cooled down to a longer on-cycle by having moved its hot water out into heating zones.

Alternatively at least it's a good idea to have the annual service/cleanout done in the spring at the end of heating season - don't let it sit over the summer.

Question: retrofitting a 15 year old boiler with a new Honeywell Electronic Aquastat

I am thinking about retrofitting my 15 years old oil burner with new Honeywell electronic aquastat which includes Outside Temperature Reset module. Has anybody done it ? This would allow me to adjust HI water temperature as a function of outside air temperature. They say this would result in 10-20% fuel savings. Has anybody done it ? Is it worth doing it ? Thank you. - Pavel 2/13/12


The retrofit aquastat control you describe will function properly if properly connected. the actual heating fuel savings ... well that depends on quite a few variables. "Up to" claims, in advertising law, need be true only about 10% of the time.

Question: trying to understand how aquastats work on heating boilers or water heaters

(Feb 10, 2014) Tom said:

Hi - I am trying to better understand the operation of an Aquastat. If have hot water coming out of my gas water heater (actually cold for a while cause it is a long run) does it hold off delivering water to the output connection (to sink) until the water is of the correct temperature?

While that is happening, is it correct that the cold water in the feed pipe from the water heater to the Aquastat goes back around into the home's cold water supply?

(Nov 24, 2012) Radhames said:

I have a room that does not get hot enough, the radiators in this room are the furthest in by the time these start to get warm the boiler shutts off. Should I raise the temperature on the Hi and Lo settings, raise the temperature on the thermostat or relocate the thermostat. Thanks

(Jan 3, 2013) Brian said:

I have a unique setup with a boiler and aquastat that heats (3) zones - one is the coil air exchanger to heat the indoor pool room, the next a line for the hot tub and lastly a line for heating the pool. Just this week we noticed the hot tub was to the point of overflowing a few days in a row, and then this morning was pretty much drained, and the pool level higher. Is this a failure of the aquastat or the boiler itself?! Thanks in advance.

(Jan 9, 2013) Cathy said:

I have a 3 zone boiler heating (hydronic)system. Upstair work fine but lower level only works when we bleed the valves.after closing the valves downstairs the system there gets cold again even though the circulating pump is still running. The aquastat is set at 180 degrees and shuts off at exactly 180 degrees when thermostat temperature is reached but on lower level which is on a separate thermostat,it doe not shut off like upper level does, the temperature goes up to 230 degrees.We have been bleeding the system continuously and have had 4 plumbers come and no one can find the problem.

(Jan 15, 2013) PHILIP said:

I juat replaced the aquastat, the furnace is lit NO HOT WATER circulating... OUCH

(Jan 23, 2013) Greg Macpherson said:

Ihave a heat exchanger hooked up to a coal boiler. Iwant an aqua stat to turn the circulating pump on and off, which one do i need?

(Feb 26, 2013) Joe B said:

I am running a Honeywell triple aquastat L8151a with (1) circulator pump and (3) zones. My thermostat is calling for heat, and the zone valve is open. The circ pump doesn't come on line and therefore the boiler does not go online either. I have replaced the pump and have been able to make it run by forcing the circ pump relay closed. Of course as the cooler water enters the boiler, the boiler lights off. I have checked all Hi and Low and Diff settings. All look within the parameters described above. Is it safe to assume that the aquastat needs to be replaced? Is this aquastat suitable for this kind of (3) zone, (1) circ pump operation? This boiler does include the domestic hot water service.


Tom, the aquastats used on a heating boiler, because they have more jobs to do, are the devices shown on this page;

a domestic hot water for washing and bathing, if made in a dedicated water heater, perhaps in your case a hot water tank heated by a gas burner, will also have a limit control, but a simpler one - search InspectApedia for Water Heaters to see those controls.

In any case, no aquastat holds off on delivering water - these are temperature controls that turn a burner (oil or gas) on and off.

For your second question: most residential hot water supply systems (we're not talking about heating water in baseboards, we're talking about hot water at the sink) are one-way systems: cold water input pressure at the water heater pushes hot water out of the water heater on to the building fixtures when someone opens a hot water valve - say at a sink.

There are hot water circulating systems, used e.g. in apartments and other large buildings, that keep the domestic hot water supply circulating through a loop of piping all the time so that there are not long waits for hot water at the fixture; those systems include an additional circulator pump and heater control.

From just your question I can't say what you've got installed. You're welcome to use our CONTACT link to send photos if youy like.

Question: boiler turns on and off too rapidly

(Feb 19, 2014) Mark said:

My boiler is firing on and off rapidly. It seems that the contactor inside the aquastat that controls the burner is opening and closing when heat or hot water is called for. Can this be replaced individually or does the entire aquastat need replacing?


Mark I am not sure what's going on, but one would think that the problem is one of proper control wiring and adjustment, not a need to replace the equipment. Take a look at our advice on aquastat control settings to see if there is an obvious mistake with the HI LO and DIFF settings on your aquastat

(Feb 20, 2014) Mark Thomas said:

Thanks for the reply, Dan. I guess I should have been more clear. My boiler is 14 years old and this problem just started.


OK Mark. So IF nobody has changed the HI LO DIFF settings and they're at typical numbers, say 180, 160, (we'll excuse the DIFF number) and the control is behaving erratically, it still could be a flame sensor problem, service problem, or indeed there could be a failing relay in the control.

If you hear buzzing or clicking at a relay that'd be a clue.

In a multi zone hot water heating system typically a relay in the primary control operates one circulator and additional circulator controls (and relays) operate each additional circulator. So most likely for an indirect fired water heater there is a separate circulator relay.

One would think that the problem then would lie in the

- the water heater's sensor monitoring hot water heater temperature
- the control wiring for the hot water circulator control
- the circulator control itself
- a loose wire somewhere in that system

BUT if your system is wired as they usually are in the U.S., it is the heating boiler temperature that is sensed by the aquastat on the boiler (not on the indirect fired water heater tank) that turns the burner on and off. If that equipment is working properly the boiler will turn on when boiler temperature (not your indirect water heater temp) drops to the LO or cut-in and the boiler would keep running until boiler temp reaches the HI or cut out temp.

So I'm back to looking for a bad temp sensor on the boiler, bad burner relay, or a loose wire in that circuit. Of course there could be some other issue like a bad relay in the flame sensor safety control.

BUT again, and thinking as typing: if the problem were in the boiler controls it ought to show up on a call for heat not just on a call for hot water.


If the problem is in the hot water control wiring (maybe it's not wired as I'm guessing) then the problem would only show up on a call for hot water.

By turning DOWN the thermostats in the home (so to be sure you're not calling for heat) and running hot water until your water heater is cold and you're calling for hot water you ought to be able to sort out the two.

And this is an example of flying blind: I assume from your comment your "hot water" is an indirect fired water heater; if your system is using a tankless coil then everything is right at the boiler and all the diagnostics are in its aquastat.

Question: aquastat changes when eliminate a tankless coil

(Apr 4, 2014) Anonymous said:

if I eliminate tankless coil and put in electric water heater, what happens with aquastat? I have forced hot water for heat


You don't have to do anything, Anon, but if you want the boiler to stop maintaining temperature when there is no call for heat (which it will do for the tankless coil), then you want to read



(Apr 12, 2014) Jimmy said:

How do I correct the the temputure control with electrical settings



Question: hot water heating circulator won't come on

(Apr 21, 2014) Chanthy said:

My boiler won't circulate the hot water. I have changed the pump and the aqua stat has a broken pin where the switch would click. Does that make a difference? The switch is clicking/pulling in but hot water won't circulate. Any suggestion what could be wrong.



First check that the thermostat is calling for heat and the thermostat wires are pulling in the circulator relay.

In addition to checking and repairing the aquastat if its relay is not pulling in to turn on the circulator, check that any zone valves or manual control valves are open (if present).

Question: Peerless WB-3 boiler with a Honeywell L4081b 1047 aquastat. I don't need the boiler for domestic hot water

(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?


You don't have to do anything, Ray, but as we noted earlier in this FAQs series, if you want the boiler to stop maintaining temperature when there is no call for heat (which it will do for the tankless coil), then you want to read



(Nov 9, 2014) Blacklab said:

What Honeywell control could be used with a coal fired ,hot water system to CUTOFF power in the event of a No-Fire or Fire -Went Out condition?


Ask your heating service tech about cad cell relays.

Question: symptoms / causes of a bad aquastat

(Nov 21, 2014) Don said:

what happens to a l4081b aqua stat when it gos bad


Don there are several failure modes in aquastats:

- a failed relay no longer switches on or off as it should

- circuit board components can burn-up

- a temperature sensor may fail

- wiring connections to the device may be loose or broken or shorted

Question: boiler won't turn on

(Nov 28, 2014) Anonymous said:

my boiler will only go on when i put thermostat to 72 degrees or higher boiler and circulator turn on together


Anon the thermostat has to be set above room temperature for heat to run.

Also, check for dust clogging in the thermostat.

Question: zone valve won't run - incompatible?

(Nov 29, 2014) Ken keller - said:

I have a 3- zone hot water baseboard system. Oil fired boiler, separate electric hot water heater. The other day I had to rplace two old and leaking zone valves.

The Plumber and the electrician wired up the new zone valves and said they won't be able to turn on the furnace with the thermostat because the new zone valves don't work like the old valves. The only way the furnace will heat the water is for the upstairs zone to be set on high. ?? Please help me !!



A separate electric hot water heater - providing domestic hot water for washing and bathing - has nothing to do with an oil fired heating boiler nor its controls. It is a completely separate appliance.

In typical heating boiler operation (for heating your hot water baseboards) when there are three zones, each thermostat is wired to a zone valve. When the thermostat calls for heat the zone valve opens. When the zone valve is fully open, an end-switch in the valve closes a second switch that connects by wires to the boiler's primary control to turn on the boiler.

Traditionally in a 3 zone system there is a slight difference from what I just said: one zone, probably your upstairs zone, is wired directly to thermostat contacts on the boiler's primary control - that thermostat turns on a circulator and when needed, the boiler. The second two zones work as I described above.

So I suspect that the new zones installed are either improperly wired or were not properly chosen. It makes no sense to me that you'd have to call for heat where you may not want it (upstairs) just to get heat where you do want it in a separate zone (say downstairs).

Call the heating service manager at your service company and ask for help from an experienced heating service tech.

Keep us posted.

Question: said:

what would be a good range for the two sensors (Dials) on the honey well L4010B aquastat oil fired boiler w/ coil and circulator 1 zone heat Thanks

forgot to mention ..a lot of info for a layman to absorb


Kris you can see


but indeed a layman should not have to understand heating system controls - you should be able to ask your heating service company for this help at the time of annual service or any time your heating system is not working.


(Jan 26, 2015) Joe Dlouhy said:
I have a home boiler used for radiator heat only. I have a Honeywell aquastat controller for the boiler(I am not at home so I cant look at any model numbers or brands) which is about 3 yrs old and the boiler and pump about 7 years old. My boiler is currently functioning like this; the thermostat is on and calling for heat, the boiler turns on and starts to heat and the electronic flue opens. The boiler runs for 30 sec or a min. with no circulation pump turn on and the boiler shuts down and the flue closes.

Last time I had a problem with the pump it was just loose wires under the protective cap where the aquastat wires tied into the pump wires but I checked the wires and they are tight. I think the aquatstat is not telling the pump to turn on (I think the pump is spinning free but I will have to double check as there is not much to grip or spin to be able to check). I tried turning on and offthe main power switch to see if it would reset but no dice. I have my heating guy coming over later when I get home. Any suggestions?


Joe if the circulator does not run the boiler should still keep firing until it has reached its upper limit as set on the aquastat. Start by confirming that.

Then feel pipes at the outlet and inlet of the circulator for heat.


(Feb 19, 2015) ken said:

oil burner runs for a day and stops. replaced filter, electrodes, bleed system. Runs all night stops in morning. Bleed system again, runs another night and stops again. What is it?>

Question: honeywell r8182f aquastat, not sure if the relay switch is bad, but boiler won't fire up.

(Feb 21, 2015) Tom said:

I have an oil fired boiler, it seems that I am having problems with it starting, thermostat seems ok, power to the aquastat, etc,

but what I am experiencing is when I turn the power on to the unit, all I get is a clicking sound inside the aquastat, take the cover off and look,

I see there is a reset button and next to it a single relay switch, and that is where the clicking sound is happening, turn power on and the switch clicks but will not fire up the boiler, it has a honeywell r8182f aquastat, not sure if the relay switch is bad, but it won't fire up.


From your note it sounds like a bad relay. You tried the reset button ONCE, right?
Start diagnosis at


Reader follow-up

(Feb 21, 2015) Tom said:

relay is not tripped, when I turn the thermostat to call for heat the relay switch pulls in and makes a clicking sound but the boiler won't fire, does the same if I leave the thermostat up and turn on the power switch to the boiler, its like the switch pulls in and clicks but not firing up the boiler, bad aquastat or maybe the transformer on the boiler itself, any help would be appreciated.


If the burner is NOT off on RESET and the electric motor doesn't start up on the burner when the relay pulls in, I'd look for a bad relay or bad wiring connections. Double check that the boiler is not off on safety.

(Feb 21, 2015) Tom said:

well the relay pulls in like I think its supposed to, maybe I'm wrong, but the motor does not turn and it does not fire, so with a call for heat the relay shouldn't pull in or click in, is this correct.


If the burner relay on the aquastat does not stay pulled in it may be a bad relay. Check for voltage at the motor terminals.

(Feb 21, 2015) Tom said:

no, it seems to stay in as long as the thermostat calls for heat, it will infact pull in and out with the thermostat being turned up or down simulating a call or cancel for heat, don't think i have voltmeter that is working properly, do have a continuity tester, but probably not the right tester for the job.

(Feb 21, 2015) Tom said:

ok, an update, when I turn the thermostat to call for heat, and leave it on for a couple of minutes, the reset button does pop, not sure if this helps.


So the burner is going off on reset. This will happen any time the cad cell relay never sees flame after about 20 seconds. If the motor never even tries to run and if you have voltage at the motor then

If you're not seeing voltage at the motor then there is a disconnected wire or a failed aquastat.

In sum:

The reset switch on the cad cell relay or in an aquastat will trip in a short time, perhaps 30 seconds after the aquastqt tries to start the oil burner if no flame is sensed.m

If at startup the motor does not run at all, a thermal protective relay may have tripped on or in the motor, or the motor or any of the devices it tries to rotate may be seized.

Check for voltage to the burner at the attempted start.

(Feb 21, 2015) Tom said:

picking up a volt meter on my home tomorrow, will let you know what I find, thanks for all your help so far..

Question: PEI Aquastast giving me grief

1 March 2015 Raj Mathews said:
The oil fired hot water heater, installed 2011 and serviced annually, in a rental property in distant PEI is giving me grief. In winters, the heater shuts off, and I have to call for service.

Technicians have not been able to find the root cause, but often the issue is resolved by pressing the reset button. It is 60 gallon unit, servicing the six 1 bedroom apartments with regular tubs. What could be the likely cause - faulty aquastat? Does the aquastat have any electrical components that is impacted by moisture or electrical surge? I need some direction to be able to ask the right questions. Thank ypu



Typically if the heater is shutting off on reset it's because of a combustion or fuel or ignition problem at the burner. I don't assume it's the aquastat but it might also be a loose wire at the aquastat.

Ask the heating techs to look for signs of a combustion or ignition or fuel problem that's tripping the safety switch, such as sooting, smells, etc.


Continue reading at AQUASTAT TROUBLESHOOTING 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 this

Article Series Contents

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