Honeywell R8182D internal (C) Honeywell Troubleshooting Heating System Boiler Aquastats
How to diagnose problems with heating system aquastats

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Heating aquastat repair:

Guide to diagnostic troubleshooting for heating boiler aquastat controls: this article describes common operating problems with heating system aquastat controls and how these troubles are diagnosed and fixed.

We discuss aquastat problems such as improper adjustment, improper wiring, and relay buzzing or failures.

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Aquastat Boiler Control Diagnostic Troubleshooting Guide

Circulator Relay (C) Daniel Friedman\

Shown above, a hydronic heating system circulator pump relay control with its cover removed. The basic components and connections in the circulator relay include:

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How to Diagnose & Fix an Aquastat Control on a Heating Boiler

If you have not already done so, we suggest reviewing the basic operation of a heating boiler aquastat described at AQUASTAT CONTROLS

Then see AQUASTAT HI LO DIFF SETTINGS to be sure your control has been set properly.

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Question: what basic tests can I do to diagnos a clicking aquastat relay or other issues?

Just install a new aquastat to my gas boier it continues to click. so turn it off. - Wayne, 2017/11/29

This Q&A were posted originally at AQUASTAT CONTROLS - home

Reply: try these basic Aquastat troubleshooting steps to check for power


Check first for a loose wire or bad electrical connection.

If clicking continues let's look more closely at what is clicking - probably a relay switch in the aquastat. If so, check the connections to the circulator or burner that the aquastat is trying to operate. If those were working before, and apologizing that I am shooting from the hip, let's do these basic aquastat tests. If none of those sort this out I'm left suspecting a bad relay and thus a bad control.

Tell me the control brand and model and I'll check further.

How to Conduct Basic Boiler Aquastat Controller Tests:

1. Turn thermostat down - Turn the wall thermostat all the way down so it is not calling for heat.

2. Check incoming voltage - Using a DMM or VOM check for line voltage (120VAC) at the power terminals to the aquastat (i.e. power is on) by finding 120VAc at the two Line-IN terminals, L1 and L3 are typical markings.

Watch out: opening the cover and / or touching electrical connections while power is on to any electrical device risks shock or death by electrocution. If you know do not know how to do this safely STOP and ask for help from a trained service technician or licensed electrician.

3. Check the low voltage transformer output: Check for 24VAC at the T and TV terminals - you will find 24VAC at one of these and 0 VAC at the other. If both terminals show 0 volts then the transformer is bad.

4. Check thermostat switch terminals: Now turn the Thermostat all the way UP so that it is calling for heat (remember to set it back down later). You should see 24VAC on BOTH of the thermostat or TT terminals inside the aquastat. If you don't then the thermostat is not calling for heat and most-likely you've got an open thermostat wire or a bad thermostat itself.

5. Check burner power terminals: Now while still calling for heat, check that the aquastat is sending 120VAC voltage to the burner (the two B terminals B1 and B2) confirming that the aquastat is trying to turn on the burner (oil burner) or trying to open the gas valve if it's a gas boiler. This should be turning on the burner.

6. Check circulator terminals: Now while still calling for heat, check that the aquastat is also sending voltage to the circulator pump - look for 120VAC at the C1 and C2 terminals. This should be turning on the circulator.

If voltage is everywhere it should be and a relay is clicking constantly OR a transformer is buzzing, those component(s) are probably bad.

Separately at RELAY CONTROL SWITCHES using a water pump pressure control switch as an example we discuss clicking and chattering relay switches. While it is possible to replace a bad relay switch on an aquastat few technicians do so. More often the tech replaces the entire control.


Question: are there re-built primary controls for boilers?

i had an issue with my boiler , i figured it out it was my aquastats l8124,any I called my local place and was told a used one was 190.00 rebuilt?? i was told by another place there are no rebuilt!!!!!! - Anon 10/3/12


Some heating equipment suppliers can provide re-built as well as new primary controls. The folks who told you there are no rebuilt aquastats - well I'm not sure if that's because they didn't know or they didn't sell.

Question: Buzzing sounds from heating controls

When my oil burner is starting up I hear a loud buzzing from the boiler room.

Reply: check for a bad relay or a relay that is being impeded by the control cover

Buzzing noisy aquastats, also any other heating controls that use relays, such as some cad cells, stack relays, and circulator relays: if you hear a buzzing sound coming from your aquastat check to see if the control cover is pressing down on one of its relays. Details, photographs, and an explanation are

Also check for a bad relay in the control. Often a failing relay unit will buzz in a heating system control.

Question: Boiler won't run: when one of my three thermostats calls for heat the circulator runs but the boiler does not

I have a three zone hot water baseboard heating system. I have three zone valves and only one circulator pump. My zone valves are made by honeywell and I believe they are the newer version. When I raise the thermostat (upstairs zone) I can hear the circulator pump kick on, the switch on top of the zone valve is in open position and I can hear a click come from the boiler.

The problem is that the boiler never comes on to heat the water to a sufficient temperature. When I put my hand on the pipe directly coming out of this zone valve the it is warm at best. My baseboards upstairs never really get that hot and are unable to raise the temperature in the upstairs of the house.

When I operate the other thermostats in the other zones of the house, the furnace kicks on and heats the the water to temperature. What could be causing the furnace to not kick on in the in the one zone? - Joe.

Reply: from reader Joe P.

If it is only in the one zone you describe: it may be that the boiler is up to temperature and does not need to "kick on". Observe the temperature on the gauge. Small loops/zones do not take the heat away as fast. Also, there could be blockage. I have also seen in homes that set the thermostats to low, that freezing occurs in the loop, and thus, poor heat output. You also may have air in the lines which will have to be purged. Turn down t-stat to lower temperature so that all heating goes to the troubled loop and see what happens.

Clarification: Turn down the t-stats to the two other loops (that are working) so that all heat goes to the troubled loop that you have activated (via t-stat for troubled loop).


Joe, from your description we can exclude the case that the boiler does not run because it's already up to temperature - since you say that you feel no heat in the problem circulator piping and baseboards.

We like Joe P's advice to see what happens when you turn down or up thermostats to the other zones. Here are more diagnostics:

Heating zone air bound or stuck zone valve: Joe P. refers to a possible blockage in the problem heating zone. Indeed if the zone is air-bound.


Or if the zone valve motor is itself stuck then even though the system circulator runs, it never pushes hot boiler water out into the problem heating zone and therefore it never pushes cold water back from the problem heating zone into the boiler where that cooler water would cause the boiler to turn on.


Question: Boiler sometimes won't run": some of my zone thermostats call for heat, the circulator runs, but the boiler won't come

When one or sometimes two of my three heating zone thermostats call for heat, the circulator runs, but for two of the zones, the boiler won't come on until the third zone thermostat is also calling for heat. - DF

Reply: check for a bad control, switch, or relay

This problem could be due to a wiring error or a control problem, as we detail below.

Burn-up on Honeywell Type L8182A, C L8151A Aquastat control board leaves one or two heating zones inoperative

Honeywell L8124A control board burnup (C) D Friedman

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We recently observed the symptoms, debugging, and solution to a similar no-heat in one zone problem. The zone was not air bound. Three thermostats control three individual zone circulators (Taco brand).

The thermostat for zone 3 in a house would call for heat, trip on the circulator, but the boiler would not turn on unless one of the other zones was calling for heat.

The second service tech (from Bottini Fuel, Poughkeepsie, the first fellow said he couldn't see anything wrong), traced the problem to a burned-out circuit on the aquastat control board. We replaced the aquastat and indeed now any thermostat that calls for heat can both turn on the circulator and if appropriate (based on temperatures at the boiler) turn on the boiler's oil or gas burner when it should.

Our photo of this very Honeywell L8124A aquastat control circuit board, with the wires removed, shows brown burn marks around the ZC terminal on the device. (Photo above left). Aside from more sophisticated circuit testing, you might spot this kind of trouble by simple visual inspection of electrical controls and components. Nothing should look "burned-up".

Watch out: a miswired multiple zone or multiple circulator pump system can also prevent one or more of the zones from operating correctly. So can a mix of different brands of zone valves that require different wiring.

Question: temperature difference between on and off too great

I have a 17 year old Rheem boiler which works just fine. However the temperature differential between on and off has become too great despite my setting the thermostat to 5 degrees. It is time for a new thermostat, but I don't know what to buy.

The cover on my thermostat says honeywell aquastat type l8100. eco thermostat. The body says L4005A2080. another part of the body says C0704. I have a blue and white wire to connect to the gas control. There is one bare copper wire going from the center of the thermostat screw on to the left side. That's all the description I have. - Alan 8/21/12


Take a look at our room thermostat troubleshooting information at THERMOSTATS, HEATING / COOLING - it would be a shame to replace equipment when all that was needed was to blow or vacuum dust off of the existing unit.

Question: Boiler will run but the circulator pump(s) do not ever start.

Aquastat with improper control settings HI and LO (C) D Friedman

When any of my three heating zone thermostats call for heat, the boiler will run, but it seems to take forever for the baseboards to get warm, especially on the longer runs.

I finally figured that none of the circulators is running. Because there are three circulator pumps and circulator relays, each controlled by an individual wall thermostat, it seems unlikely that all of them are bad. What should I check? - Charlie G., Calais, Maine.

Reply: check the aquastat control settings for crossed or reversed HI LO controls, then check primary control relays

The photo (above left) illustrates Charlie's aquastat with improper settings for the HI and LO. The HI is set to 180 and the LO is set to just under 200!. Sorry Charlie G. but that's not going to work right.

Photograph of a multi function combination control on a heating boiler

We have found this problem lots of times when a homeowner or someone else messed around with the HI, LO, and DIFF settings on the aquastat without having a good idea of what those dials do. Aquastat manufacturers such as Honeywell make clear that

The HI should always be set at least 20 °F. above the LO.

If you "cross the controls" - that is, if you set the LO to a temperature that is less than 20 degrees lower than the HI setting, you have basically locked out the circulators.

The oil burner runs, but the LO control is preventing the circulator from coming on. Take a look at the definitions of HI and LO above and this will be more obvious.

Our photo immediately above shows settings that OK - the HI is set to a little over 180 and the LO is set to 160.

Question: my heating boiler won't start

this morning the boiler was not heating. It worked very well during the night. No light in the furnace. I turn off and turn on the gas ignition and nothing happened. What can be the problem? has the damper related to this issue? I change the button from one side to another side and nothing. Also, how can I switch on the spill gas? I think that the problem is the sensor, because it was smoke near the boiler area and maybe the sensor shut off the gas but I do not know where is it. My boiler is UTICA model. - Anon 11/7/12


Anon, with just the information you state, I'd start at the beginning of our no-heat for boilers,
see DIAGNOSE & FIX HEATING PROBLEMS-BOILER - troubleshooting. Indeed there could be a bad flue gas spill sensor, but I wouldn't start there.

Question: Indirect fired water heater operation when a tankless coil is also available

I have a strange configuraton. My DHW tank is a zone (tank was added after the fact), but only triggers the LO setting, so it doesn't get the the hotter water that my heating zones get. To complicate things, the water still passes through the tankless and the "Temp-A-Rator" mixing valve.

So, in effect, I pre-heat my hot water. Bypassing the tankless would be expensive and I guess I'd need to then have the tank wired so that it triggers a higher water temp (additional aquastat?). I also don't understand this [apparently obsolete] mixing valve. Is it manual? Should I replace it? Ok, I'm rambling. Would appreciate any and all thoughts. - Chris - Chris 5/10/12



When domestic hot water is made by a separate tank heated by a zone on the heating boiler, we call that an indirect-fired water heater system. The LO and DIFF settings have nothing to do with that water heating method - the LO and DIFF make sense and are used when there is a tankless coil in use.

The heating zone, control, and circulator pump (or zone valve) that heats your separate hot water tank just looks like another heating zone as far as the heating boiler and its controls are concerned. Hot water from inside the boiler itself circulates through a heating coil in the bottom of the hot water tank in response to a thermostat on the hot water tank.

But if a heating boiler uses a tankless coil as a pre-heater or post-heater for the water entering the hot water tank, then indeed you've got two water heating methods in parallel - which can be confusing. In fact, you can think of them separately, and controlled separately.

Take a look at the water piping that brings cold water from the building water supply into your hot water heating tank. If the cold water runs first through the tankless coil, then yes you are pre-heating water entering the hot water heater tank. Some plumbers pipe in the opposite direction, that is, allowing hot water leaving the hot water tank to pass through the tankless coil for a boost that takes effect when the hot water in the separate tank is nearly used up or is cool.

See INDIRECT FIRED WATER HEATERS for details of how these systems work.

Question: L4081B aquastat doesn't seem to be behaving like it should

Great information here. I had to read a few times to put it all together, but I think I finally got it. I have an L4081B aquastat and it doesn't seem to be behaving like it should.

From a cold start with no call for heat (I turned down the thermostats) the burner stayed on all the way to the HI setting (in my case 180°). Shouldn't it cut off at the upper limit of the LO setting?

My Hi is set to 180° and LO at 130 with a 20° DIFF. I have a tankless coil. - Steve 11/7/2012


If there is no call for heat but the boiler cycles on, I'd expect that ON cycle to be driven by the LO and DIFF controls just as you describe.

Question: my heater won't turn off; replacing the thermostat did no good, what do I try next?

my heater just don't stop heating my house i tried a new thermostat what should i try next please - Frank Carr 11/12/2012

Reply: things to check when the heat won't turn off, & some Safety Warnings

Buddy said: Frank, You may have a problem with your boiler controls or a check valve in the hot water piping system.

Take a look at "Guide to Heating System Boiler Check Valves & Flow Control Valves" [to see if this information helps you determine if a check valve is sticking open. In any case, you will probably need the services of a service technician to check out the controls or a plumber to replace a check valve if it is sticking open. - Buddy 11/12/2012


Daniel (Moderator) said:
Frank: in addition to Buddy's suggestion just above, some other reasons that heat may not turn off in a building include:

Question: how do I find a replacement Aquastat for a Honeywell L8024B1048 (triple aquastat) ?

Honeywell L8024B,D triple aquastat heater controller (C) InspectAPedia

I have a Gas Hydrotherm 86000 Model R108 furnace. I cannot find a replacement Aquastat. I have called Hydrotherm they say call Honeywell. Honeywell say its discontinued they have no information for a universal replacement.The hot water tankless is disconnected as I have a gas hot water tank.

I heat most of the time with a coal stove so the only time the furnace runs is if I go away for a few days. I'm 73 retired firefighter (36 yrs) . The furnace is in good shape hate to replace just because cant find aquastat. Do you have any Ideas for a replacement universal aquastat relay. attached is all I have on unit.

Any suggestion is welcome. I usually have to clean contacts to get it running when I use it. - J.M. 12/28/2012

Photo (left) of a Honeywell L8024B & the wiring diagram below were provided by reader J.M.

Reply: parts substitution & replacements for the Honeywell L80241048B,D triple aquastat, Wiring Diagram for Hydrotherm Duo-Service


Contact your local heating supplier with the part number (inside the cover check that you correctly identified your Honeywell Aquastat as the Honeywell L8024B1048 (triple aquastat, also the L8024D) - that's probably all they need) from the original aquastat; there are tables of substitution and changeout part numbers and names for just about every control ever made.

I'm sure they'll be glad to help you select the correct one. In fact I'm confused that you report not obtaining help from Honeywell themselves; we have usually found that the company's technical support personnel to be informed and helpful.

Honeywell L8024B aquastat wiring instructions

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I'm not sure about the L8024B series information you've received, and I understand that there is not necessarily "universal" replacement for all applications, but in fact the Honeywell L8124 series (AQS8124B1039 or the Honeywell Multi Function Aquastat L8124B1039) can substitute for many of the controllers in the L8124 and L8024 aquastat line.

We regularly see that controller advertised as replacing the Aquastat models with part numbers L8124B1039, L8124B1021, L8024B, and L8024D. indeed it might work for you as well. Also check out the Honeywell, the L7224U Universal Aquastat controller.

Be sure to review your control and how it was used with your heating service supplier;

I would be quite surprised if they couldn't immediately give you the part you need, but if you don't have success let us know and we will research further.

The wiring schematic for the Honeywell L8024B triple aquastat (thanks to your photos) is reproduced at left.

Your aquastat, the L8024B controller, also sold in the L8024D model, is being used in your case to control a Hydro-Therm natural gas-fired heater and possibly a circulator pump; often a review of exactly how a controller is being used in a given application can make more clear just what replacement aquastat will work best.

If you cannot find a suitable replacement control such as the 8124B 1039, we may find a "new old stock" (NOS) L8024B,D, etc. series control available from a reseller.

Reader question: Honeywell R8182D replaced three times

My Honeywell R8182d 1111 keeps tripping the red safe button. This is the 3rd unit installed, they seem to last about 5 years and quit. I installed a new aquastat control about two weeks ago. It worked fine for 2 days, then went away for 4 days and when we returned the Beckett oil burner afg was off.


If i knew what to replace I would. I'm concerned about the heating season. We use the furnace for heating our hot water ,Utica external 40 gal tank, (not tankless). can you help with your expert advice? thank you, J.& J. 8/24/2013


The reader ultimately let us know that the new control had been defective. Details about this case are now at HEATING SERVICE SNAGS

Reader Question: my bad heating system aquastat only runs after we give it a good smack

Thank you for a wonderful and informative website! We have an boiler for our radiator system in our home but it's not starting up unless we smack the cover of the aquastat repeatedly ... and then it clicks a few times the burner flames eventually starts up - after a few smacks. If you can provide any insight to our problem, we'd greatly appreciate it. Thank you, J.D. 11/27/2013

Reply: smacking the aquastat - things to check: loose wires, buzzing, overheating, burning, damaged aquastat control

Buzzing relay in a heating aquastat control (C) Daniel FriedmanNice question. Where is the aquastat mounted? On the boiler itself or remotely say, on or next to the oil burner? I ask because part of diagnosing why smacking or tapping on an electrical control does something involves figuring out what you may be jiggling in the control when you rap, tap, or smack it.

I suspect a loose wire, loose connection, failing aquastat relay, or possibly a bad connection between the aquastat's temperature sensor and the sides of the well into which the sensor is inserted.

Of course there may be some other issue we haven't thought-of - something that would be immediately apparent to an astute heating service technician.

Start by turning power off (watch out for fatal electrocution hazards) and checking that all of the connections are tight. Don't forget to check the thermostat wires too. Also listen for buzzing relays in the aquastat.

In the aquastat photo above left I'm pointing to a relay that may be buzzing if it's failing.

While the cover is off of the control, look with care for evidence of overheating or burning around any of the wire connectors or contacts At below left are burn marks around the lower-left power-connection screw.

Burn & overheating marks in a heating aquastat control (C) Daniel Friedman Honeywell aquastat circuit board showing overheating & burn damage (C) Daniel Friedman

In my photo at above right I'm pointing to burn marks on the printed circuit board of a Honeywell L8124A aquastat on the reverse side of the overheated wire connection terminal shown above-left. This aquastat was behaving erratically for a year or more before a sharp heating service tech from Bottini Oil (Poughkeepsie) spotted the damage. We replaced the control.

Oil Burner Short Cycling Traced to Poor Aquastat Temperature Sensor Contact: use the heat conductive grease

We have observed occasional short or sporadic oil burner (and some gas burner) "on" cycles that were hard to track down. Technicians even replaced aquastats on a test system we've been monitoring, only to have the problem continue.

Way back we traced the burner on cycle problem to poor contact of the aquastat's temperature sensor in the sensor well. After assuring that the problem was not in the room thermostat, that there were no loose wires, that the aquastat relay was working normally and in fact that the whole aquastat was new, it was following the manufacturer's installation instructions to use a heat-conductive grease in the aquastat sensor well that fixed the problem.

Why we need that heat-conductive grease in the aquastat sensor probe well

To communicate the temperature of the water in the boiler to the controlling aquastat heat must flow from the boiler water to the surface of the sensor well - basically a steel tube mounted through an opening in the boiler body. The water heats the sensor well steel.

Heat flows from the water through the steel of the sensor well to the surface of the aquastat's temperature sensing probe.

If this probe is not in very good thermal contact with the interior surface of the sensor well the aquastat may not always control the boiler on-off cycles properly.

The thermal or heat-conductive grease assures good thermal contact between the aquastat temperature sensing probe and the inner surface of the sensor well into which the probe is inserted.

Without this conductive grease or "paste" in the boiler's aquastat temperature sensing probe well, even bending the aquastat sensor line to try to assure that it will have good thermal contact with the sensor well sides, that contact may vary over time, causing the sensor to flip the boiler on and off in too-short on-cycles.

Details are in this reader Q&A:

6/1/2914 Heath M said:

I am curious about aquastats using immersion wells. I have 2 of them and i find that in general they are more accurate than the strap on type. However I find that they are still not as accurate as i would like to see them.

The 2 that i have there is nothing inside the well itself to transfer heat from the well to the copper bulb inside the well it just sits there making loose contact with the sides in some spots. It isn't even a semi snug fit they just dangle in there. Is there normally some sort of oil or heat sink compound that should fill the well for better heat transfer? I can't help but think this would help increase accuracy.

Reply: use of a thermal conducting grease with aquastat sensors


No question that strap-on units are less accurate and IMO less reliable so less safe.

I too am troubled by that contact-constancy problem and suspect it explains why there are sometimes short on-cycles of the equipment.

Aquastat immersion well  sensor details, adapted from Honeywell's L7224U universal aquastat installation instructions cited in this article series (C) InspectApedia 2015

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If we read the installation instructions we're told to use a thermal conductive paste inside the well, as illustrated at above left in our adaptation from the installation instructions for the AQUASTAT L7224U UNIVERSAL Universal programmable aquastat.

The use of the manufacturer's recommended thermal-conductive paste would go a long way towards overcoming the contact constancy question that you raise.

In discussing this with heating boiler service techs who do not bother with the thermal paste when installing the aquastat, I heard their complaints about the paste (thermal-conductive grease) asserting that using the grease makes it difficult to remove the device in the future as the grease "cements" the sensor into the well.

This may have been true for some products in the past but currently (2014 & recent years prior to that year) the thermal grease is not acting like a glue.

The manufacturer knows what makes their control work properly. The thermal-grease assures reliable and continuous thermal contact between interior surface of the immersion well on the boiler and the temperature sensing tip of the aquastat's heat probe. Without this grease, thermal movement and possibly even slight corrosion that occur between the surface of the probe and the side of the immersion well can cause the temperature sensor to behave erratically.

The temperature-conductive grease is needed and should be used.

Honeywell feels so strongly on this point that a packet of heat conductive grease is packaged with new aquastats. I've read that the formula for the conductive grease was amended to remove a drying agent, preventing the heat transfer grease from gluing the sensor into the sensor well.

Worse, once the aquastat control has been installed without the recommended grease, if the boiler is cycling erratically and you want to add the conductive grease, and if it's a close-mount aquastat whose sensor probe extends off the back of the aquastat directly into the temperature sensor well on the boiler, taking the aquastat back off to then add the conductive paste or grease is a fair amount of trouble that the tech certainly doesn't want to tackle.

Get some heat conductive grease and use it when installing the heating boiler aquastat sensor.



Continue reading at AQUASTAT DIAGNOSTIC FAQs 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

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