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Troubleshoot L8148 aquastats

Guide to troubleshooting & repairing the Honeywell (now Resideo) L8148-series aquastat controls on heating equipment:

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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Honeywell L8148 Series Aquastat Diagnostic Steps

Overheating marks on the circuit board show that this pirmary control has been damaged beyond repair (C) Daniel Friedman at InspectApedia.com

Excerpting from Honeywell's manual for this controller,

The L8148 Aquastat® Relays are immersion type hydronic controllers that combine high limit protection with switching relay control of burner and circulator motors.

Details about the features and installation of this aquastat are in our sister article found

at AQUASTAT L8148 AQUASTAT INSTALLATION

Question: I shorted some wires and now the aquastat doesn't work - do I need a new one?

2019/11/15 Anon2 said

Hi, I am a homeowner. My Utica boiler has L8148 aquastat.

I was changing the Taco circulator pump and I made a mistake while power is on and I caused a short with line going to taco pump.

Now the aquastat has stopped working after the electric short.

So do I have to replace the aquastat?

Reply: Steps in Troubleshooting the Honeywell L8148 Aquastat & Similar

If you shorted the power wires to any heating control including the Honeywell L8148 Aquastat then it’s quite possible that the control has been damaged or destroyed and will need replacement.

Basics of What the Aquastat is Doing

In the U.S. the way heating boilers are wired, the thermostat turns on the circulator pump and it is the temperature in the boiler that separately, by cooling down, tells the Aquastat to Turn on the oil or gas burner to re-heat the boiler.

So our general approach in diagnosing an aquastat is to see if first the thermostat successfully causes the aquastat to turn on the circulator(s) and then to see if second when the boiler is cool or cold whether the aquastat turns on the oil or gas burner.

Watch out: in Canada many heating systems are wired so that the circulator runs all the time during the heating season and the thermostat (and thence the aquastat) turn the boiler on and off. (The reason for this is that in a colder climate house temperature will be kept more even and the risk of frozen heating zone or other pipes is reduced.)

In any event if the L8148 Isn’t turning on the circulator (s) and thus is never causing the boiler to run, here are some diagnostic steps, starting with what a homeowner can do and continuing with what a service technician might do.

Homeowner Checks at the Aquastat

1 .Look at the Aquastat for Obvious Damage

Remove the gray cover on the L8148 Aquastat at the boiler. You may need to loosen two screws at two of the cover edges to remove it from the aquastat body. Don’t fully remove the screws, just loosen them.

Now DO NOT TOUCH anything inside the aquastat; there is live voltage here including 120VAC that could shock or kill you.

JUST LOOK at the aquastat internals; if you see brown or black burn marks on the circuit board or on any other components the aquastat is probably damaged beyond economical repair.

Above we show brown and overheating marks on the green circuit board of a Honeywell heating system control. This controller was shot and needed to be replaced. If you see burn marks like this don't bother with costly diagnostics.

If there is no obvious visual evidence of a burn-up nor smell of burned electrical components, the heating system and possibly even the control still needs diagnostic service and repair.

2. Check that the thermostats are calling for heat

While you listen at and look at the aquastat, have an accomplice turn all of your thermostats all the way up.

You should hear a "click" at the aquastat and if you were watching closely, the aquastat relay switch may have "closed" by clicking "down" to turn on the circulator pump(s).

If not the call for heat isn't being relayed to the circulator(s).

3. Check that the Aquastat electrical circuit power is on

Check at your electrical panel for the fuse or circuit breaker powering your heating system - be sure that electrical power is on.

If in doubt, switch the breaker fully to OFF position then turn it back on, or replace the fuse (if it's a fuse pane) with a new one.

Watch out: do not remove the panel or fuse box cover and don't stick a finger or any tool inside the electrical panel as you could be shocked or killed.

Heating Service Technician Aquastat Diagnosis & Repair Steps

Call your heating service company. Most heating service techs will simply replace a bad aquastat even though a new Honeywell L8148 is costly - about $175. U.S. - because their time to run detailed diagnostics is expensive too.

However a service technician might elect to try some of the following steps to be sure that her diagnosis of a bad Aquastat L8148 is the right guess.

In the text below “you” refers to “the service technician, not the homeowner.

At the L8148 Aquastat the technician may check for supply voltage and may check the thermostat circuit voltage using a DMM or VOM as follows:

Watch out: Safety details are

at DMMs VOMs SAFE USE OF 

1. Check that the thermostat(s) are calling for heat

And check that and that room temperature is below the set temp in the thermostat. (I turn the TTs all the way up - and leave my truck keys atop one of them so I don't forget to set them back to the homeowner setting before I leave.)

Honeywell L8148A Wiring Connections & Hi Limit Controller (C) Cited & discussed at InspectApedia.com adapted from Honeywell2. Check for line voltage at the power terminals in the Aquastat

There should be 120VAC at terminals L1 and L2 where by convention L1 is the “hot” or “black” wire and L2 the neutral or “white” wire.

If there is no voltage supplied to the aquastat then the problem begins at the electrical panel - blown fuse, tripped breaker, or in a wiring short or disconnect between the electrical panel and the electrical box powering the aquastat and heating boiler or furnace.

[Click to enlarge any image]

If there is voltage then let's be sure the thermostat's call for heat shows up at the Aquastat

3. Check the thermostat wires

On the Honeywell L8148A Aquastat relay

these two terminals to which the two thermostat wires connect are simply marked “THERMOSTAT” and are found next to the transformer in the control.

On the Honeywell L8148E

Aquastat (used to control separate, multiple zone valves or zone circulator relay switches) the two thermostat wires will be connected to terminals marked T and TV.

If you need to rule out the thermostat itself as the problem, simply remove the wall thermostat and jump or tie together the two heat wires - usually red and white - to call for heat. This is the same as turning the thermostat all the way up.

Details of troubleshooting thermostats and thermostat wiring are at

Watch out: Before leaving the job don't forget to restore the thermostat wires to the terminals wherever you've disconnected them.

If you've ruled out a problem at the thermostat itself and at the thermostat wires, continue below.

4. Check the Aquastat Relay Switch & Voltage at Circulator Terminals C1 - C2

On a call for heat the relay switch should click and should "close" or be in the "down" position on the circuit board.

At the Aquastat, at terminals C1 and C2 check for line voltage (120VAC) supplied to the circulator pump.

Watch out: I have seen techs just push down the relay to confirm that voltage is then produced and the circulator pump runs, but if the thermostat is calling for heat and the relay doesn't close on its own then either the control board or the relay is bad.

Watch out: there is live voltage here. You can be shocked or killed.

If the relay closes as it should and there is voltage at the circulator terminals then the problem is downstream from the aquastat.

5. Check thermostat wires at zone valves or zone circulator relays

If the heating system uses multiple zones each will be controlled by a zone valve (that opens or closes a loop of heating water pipe and that turns a common circulator pump on)

OR each zone may be controlled by individual heating zone circulator pumps that are in turn switched on or off by a separate heating zone circulator relay control.

When a thermostat controlling an individual heating zone calls for heat in this design either an end switch on the zone valve OR a relay switch in the circulator relay turns on the circulator for that zone.

At the zone valve or at the circulator relay control check the thermostat terminals for 24VAC at that control and check line voltage at the common circulator or at the line voltage terminals in the circulator relay if one is used.

On a zone valve system if the zone valve doesn't operate when there is voltage across the thermostat wire terminals (24VAC) at the zone valve, then the problem is at the valve - the control head itself, or the actual hot water line valve (could be jammed).

If there's no voltage at the TT terminals on the zone valve then we're back to diagnosing the thermostat itself or the thermostat wires or the low voltage transformer - topics we listed earlier.

On a multiple circulator system if the circulator relay doesn't close when there is voltage across the thermostat wire terminals (24VAC) at the circulator relay, then the problem is a bad relay or relay control board.

If the circulator relay closes and produces line voltage at the circulator terminals C1 C2 in the relay, but the circulator doesn't run then the problem is in the circulator or circulator power circuit.

If the relay has closed and there is voltage on the C1 C2 terminals but the circulator(s) won’t run then

see CIRCULATOR PUMP WON'T START 

6. Canadian systems: circulators always on:

At the L8184 Aquastat check for line voltage to the burner - Terminals B1 B2 (or between L1 and B2 on some models.)

When the thermostat is calling for heat the burner will nevertheless remain OFF until the boiler temperature falls to or below the CUT-IN temperature on the limit control on the Aquastat (typically 15 deg. below the HI LIMIT) so you may need to run heat in the building long enough to cool down the boiler to see the burner turn on.

Once the boiler temperature drops below the cut-in the burner should run.

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

[Click to enlarge any image]

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.

IF voltage is being supplied to the burner (to turn it on) and the burner never runs, the the problem is probably not the aquastat but the burner itself, such as a burner that has tripped off a cad cell relay or other safety control.

See details

at OIL BURNER WONT RUN 

or on an LP or natural gas burner system the burner is turned on by a low voltage circuit

Gas burners on a heating boiler typically used a Honeywell L814B like the L8148E1257/B that has an isolated burner circuit typically used with powerpile systems. That aquastat is replaced by the L8148J1009/U Aquastat Relay, a universal replacement offering 120 – 240 F range with 8 degree F fixed differential, low volt/millivolt

See gas fired boiler troubleshooting details starting at

DIAGNOSE & FIX HEATING PROBLEMS-BOILER  at https://inspectapedia.com/heat/Heating-Boiler-Diagnosis.php

Same diagnostics and same electrocution shock warnings as above.

Manuals for the Honeywell L8148 series Aquastat Relays

Honeywell L8148E Aquastat connections & interior (C) Adapted from Honeywell cited & discussed at InspectApedia.com

Note: As their numbers can be confusing and are sometimes transposed, don't confuse the Honeywell L8148-series Aquastats with the Honeywell R8184 G,N or R4194 D Protectorelay oil burner controls (cad cell flame sensor control).

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-02-04 by InspectApedia Publisher - be nervous about forcing ANY electrical device to close

@Tom T.,

That's a clever if risky band-aid, Tom.

It sounds as if the spongy-band-aid is confirming that either the relay is losing its closing-strength or it has burned contact points. What you did "works" - and I recall when one of the astronauts aboard a space shuttle "fixed" their on-board computer by giving a good whack while the experts on the ground were still paralyzed in meetings arguing about what to do.

But I am a little nervous about forcing ANY electrical device to close - that there could be an overheat and electrical short or fire. So do keep a close eye on your sponge-bob repair of a bad aquastat relay.

To be candid, I found bad relays, I tested them by pushing them shut (as does your sponge) but i never replaced one on the circuit board, I just replaced the aquastat entirely.

If the relay itself is bad, you probably don't have to remove the thermal sensor signal end from the aquastat if you can de-solder off and re-solder in an equivalent relay.

If you proceed with this advanced Junior Engineering project, do take photos that we might share.

Thanks.

DF

On 2023-02-04 by Tom T.

@InspectApedia Publisher, Thanks. Yes I'm familiar with that diagram. My question was about the connection inside the box, which isn't shown. Also, I was asking about buying the individual switching relay, not a complete Aquastat. I doubt any local HVAC suppler will sell me just a switching relay.

Anyway, I wanted to let you and your readers know about the bandaid I've come up with for my stubborn relay. I've taped a small piece of spongy foam to the flappy thing on top of the relay and placed the Aquastat cover back on the box. I'm holding the lid in a position with some zip ties to apply just enough pressure to the top of the spongy foam so that when there's a Call for Heat the relay actuates.

Thanks again for everything.

On 2023-02-03 by InspectApedia Publisher - two very different installation details for the thermal sensor for aquastats

@Tom T,

Keep in mind that there are two very different installation details for the thermal sensor for aquastats.

Some aquastats have the sensor mounted projecting right off of the back of the control that is in turn mounted right onto the boiler.

Other aquastats use a "remote" sensor whose sensor bulb is connected by a thin copper tube between the body of the aquastat and the sensor mounted in a well on the boiler (or as a strap-on sensor in some installations).

About buying parts from us, no.

We do not sell anything. No products, no services.

InspectAPedia.com provides building and environmental diagnostic and repair information.

In order to absolutely assure our readers that we write and report without bias we do not sell any products nor do we have any business or financial relationships that could create such conflicts of interest.

However aquastat controls are widely available from your local plumbing and heating suppliers as well as from online vendors. I recommend going to your local supplier as they can offer direct help and advice for your situation - if the vendor will talk to you at all. Some local HVAC suppliers are reluctant to even sell parts to consumers as they don't want to offend their main customers: heating service companies and technicians.

On 2023-02-03 by InspectApedia Publisher

@Tom T,

A quick search of InspectApedia using the on-page search box finds our article that addresses your thermal sensor and sensor well question in considerable detail:

AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND

illustrates the components at the boiler and explains about thermal grease.

In my OPINION on recent boilers there is not a shred of issue about glop, mess or stickiness of thermal-conductive grease. There was a gripe among technicians who said that they encountered, on much older boilers, thermal grease that hardened, making removal of the sensor from the well more difficult.

Aquastat sensor bulb (C) InspectApedia.com

On 2023-02-03 by Tom T

@InspectApedia Publisher, Thanks for your quick reply. Since it seems I will have to disconnect and de-mount the whole L8148E, it seems I need to know more about the immersion bulb connection at the Aquastat sensor port.

During the de-mounting process, would I disconnect the immersion bulb from the Aquastat sensor port ( if so, how?) or, would I have to pull the other end out of the well on boiler and have a delicate, messy (covered in thermal grease) tail extending from the Aquastat?

If I disconnect the immersion bulb from the Aquastat sensor port, is there a risk of spilling the liquid that is inside the bulb? I know its a lot to ask, but pictures of the sensor port and/or solder side of the switching relay would be helpful.

Thanks again for any reply and for this great page (do you sell the parts in question?)

On 2023-02-02 by InspectApedia Publisher - circulator relay replacement is not simple repair

@Tom T,

Thank you for a helpful question about circulator relay replacement.

I should be more clear that while a relay can be replaced on a circuit board, typically that requires de-soldering the old and re-soldering the new relay, steps that very few heating service technicians are prepared and willing to perform.

Instead they'll typically replace the control itself.

Unfortunately it's not a simple plugin repair.

On 2023-02-02 by Tom T

My boiler is controlled in part by a Honeywell L8148E. Everything seems to work except that intermittently the circulator pump does not run. During these times, if I press on the flap on the top of the circulator relay, the relay pulls in and the pump runs for the remainder of the call for heat.

In many spots on this page you indicate that the relay can be replaced. However, it is not clear if the relay simple unplugs from a socket or if the the whole L8148E must be disassembled so that one can un-solder the old relay and re-solder in the new relay.

Please clarify. Also, if you can provide a part number and/or supplier for the relay, I would be grateful. Thanks.

To be clear, this is a very old (at least 35year) L8148E. That appears to be the complete model number. I see no addition digits. Also, what I called the circulation relay I note now your diagrams call the "Switching relay."

On 2023-02-01 by InspectApedia Publisher - Where is the lK2 Thermistor /Switch on the L8148E control

@Bill 186765,

The bulb that inserts into the sensor well in the boiler is a temperature sensor and copper tube that communicates to the control itself.

The immersion bulb attached to copper tubing that connects to a sensor port in the aquastat where it operates a switch.

Both bulb and tube are filled with a fill liquid that expands or contracts in response to temperature. (Just how well it does that depends on the quality of thermal contact in the sensor well - which is why the manufacturer wants you to use a thermal conductive paste).

As boiler temperature increases, pressure in the bulb and cap tube increase, causing the end of the cap tube in the aquastat to operate the actual temperature response switch.

There is some design variation in just how these temperature sensors work on various aquastats, but this excerpt from Honeywell is helpful:

Sensing element

The liquid filled sensing element, including well or sensor assembly, responds to temperature changes of water in a heating system.

The sensor element has a diaphragm type power head connected to a tube type capsule, through a small bore capillary.

Expansion of the liquid, which completely fills the sensing element, is the means of motion of the diaphragm.

If you can tell me where you read about a IK2 Thermistor/Switch on aquastats I'll be glad to study that further to get us both on the same page here.

Thanks

DF

See

AQUASTATS THERMOSTAT PRODUCT HANDBOOK [PDF] Resideo, retrieved 2023/01/31 original source: https://docs.rs-online.com/a61b/0900766b80691404.pdf

On 2023-01-30 by Bill 186765

Where is the lK2 Thermistor /Switch on the L8148E control. Is it part of the Bulb that goes into the boiler?

On 2022-11-10 by InspectApedia (Editor) - What is causing my monitron electric boiler to overheat and produce steam

@Don,

If the aquastat is properly set, then I suspect there's a poor contact between the thermal sensor and the surface of the sensor well on the boiler, or the sensor itself could be defective.

On 2022-11-09 by Don

What is causing my monitron electric boiler to overheat and produce steam

On 2022-12-01 by InspectApedia (Editor) - replace the relay on an aquastat with a sister replacement

@K Carter,

Thank you for the question and photo - it led us to find some errors in our PDFs and in the article above

In sum, you'd use an L8148L to replace an (unavailable) L8148G. The difference?

The L8148G was not designed to support multiple circulator zones while the L8148L can support multi-circulator zones (but of course does not HAVE to)

We're still looking for an L8148G manual, though the G will be very similar to the other L8148 controls for which we provide manuals above on this page.

It's possible to replace the relay on an aquastat - it won't be unique to the L8148G - but considering the age of your control, few service technicians would make the attempt. Instead, I think a technician would replace your L8148G with one of its sister replacements

On 2022-11-28 by K Carter - L8148G relay won't pull in

I have a L8148G board that the relay doesn't pull in on. I can only find L8148A,E, and J online. Can someone provide any info on the G version.

L8148G Aquastat Relay board (C) InspectApedia.com KCarter

On 2022-11-21 by InspectApedia (Editor) - primary circulator loses power

@Steve S.,

Let's start with a check of the aqua stats settings to be sure that you're not accidentally locking out the circulator. The low limit needs to be at least 20° below the high.

On 2022-11-17 by Steve S.

I have an L8148A1017 and on a call for heat the burner and primary circulator come on. When the aquastat senses desired temperature the burner shuts off and there is still a call for heat, but the primary circulator loses power. What is causing this? I checked the amp draw on the circulator and it was around 2 and specs on the grundfos 3 speed pump says .7 amps is normal.

On 2022-06-27 by InspectApedia-911 (mod) - L8148B that occasionally fails to work when the thermostat is calling for heat

@George B,

Intermittent failures are hard to track down.

The next time your system won't heat, before turning power off, see if pressing on the relay in the control turns on the circulator.

Watch out: there's live voltage - watch what you touch, or don't go there if you're uncertain: you could be shocked or worse.

Other errors are a bad sensor or bad sensor contact or a failing control board or simply a loose wire.

On 2022-06-27 by George B

I have a L8148B that occasionally fails to work when the thermostat is calling for heat. If I shut off power, by the power switch, the aquastat will work and will do so for a number of days and then stop working again. I power on and off and it starts working again.

Sometimes I need to turn the power on/off two or three times before the aquastat works and the boiler ignites. Any suggestions as to the problem?

On 2022-04-16 by Inspectapedia Com Moderator - aquastat relay hangs up - needs replacement

@Ken,

There are OEM and generic replacement 24VAC relays like the one shown below, but most service techs prefer to replace thee entire aquastat control.

On 2022-04-15 by Ken

Gas fired Olsen boiler has an L8148E 1265 aquastat controlling the igniter and circulating pump.

Twice this season the relay has hung up, the igniter does not engage but the pump does. Can you identify this relay? I assume that it will be cheaper to switch out rather than replace the whole unit.

On 2022-04-14 by Inspectapedia Com Moderator

@Rune,

You will see in this article series that there is a universal replacement aquastat but there's no reason not to replace with a new copy of the same one that you have.

See AQUASTAT L7224U UNIVERSAL OPERATION

On 2022-04-14 by Rune

@Inspectapedia Com Moderator,

I like how you think - and had already checked the relay with an insulated screwdriver. I can push it in, it'll kill the boiler and click right back on. After this I was able to intermittently read 70-80V or 117 V on the Circulator connectors (indicates a weak connection in the relay to my understanding).

The circulator is brand new and installed a few days ago. I am Macgyvering it to run via an external powersource while the valves are open to get heat dispersed in the house.

My trouble shooting (as newbie as it might be) actually started since I saw the boiler running and no circulator, so bought and installed a new circulator. It seems like it would only run when the boiler was on which seemed wrong - why generate all that nice heat, turn off the boiler and then not distribute it around the house using the pump.

Should I upgrade to a better, newer Aquastat? Will they all allow the circulator to run while the valve(s) is open? Or do I need a more fancy version?

Thank you

On 2022-04-14 by Inspectapedia Com Moderator

@Rune,

Few techs will replace the relay, though that's cheaper than a new control.

So you may need to replace the whole aquastat.

But before doing that,

and I say this with extreme nervousness because you're a stranger and I don't know how skilled you are and if you touch live electrical contacts you could be shocked or killed,

but all that said, sometimes I will simply press on the relay to see if that turns on the circulator.

That may tell that there is a bad relay.

And of course check for things that would be embarrassing to have missed like a loose or broken wire.

I check the circulator Itself by making sure that it will run when given direct power, but you already said that the aquastat was not sending power to the circulator.

On 2022-04-14 by Rune

@Inspectapedia Com Moderator,

Thank you. What are the best options going forward?

-Buying a relay only?
-Buying the same 8148A and exchanging it?
-Upgrading it?

Thank you,

Rune

On 2022-04-14 by Inspectapedia Com Moderator

@Rune,

OK so there may be a bad circulator relay

On 2022-04-14 by Rune

@Inspectapedia Com Moderator,

It is a heating system in the US - Colorado. And it does just what you wrote. I have 123-4 V on the system. When the boiler is running it is getting power, however there is no voltage on the circulator.

I don't know how to check the end switch in the zone valve. Any guidance?

I truly appreciate this page and your reply - I'm a sucker for competence.

Thank you, Rune

Honeywell 8148A in Colorado (C) InspectApedia.com Rune

On 2022-04-14 by Inspectapedia Com Moderator - circulator not turning on when it should

@Rune,

If yours is a heating system in the US, then the usual way that it's wired is that the thermostat calls for heat and that opens the zone valve.

When the zone valve is in the fully open position, an end switch in the zone valve turns on the circulator.

Then the boiler temperature is what actually turns the burner on and off.

So if that's how your system is wired and the end switch is not turning on the zone valve there is a problem with the zone valve and switch, or the circulator relay, or the circulator wiring, or the circulator pump itself. That's something that are on site heating service technician can sort out with a few simple tests.

It makes sense to diagnose the problem accurately before we try adding on devices or switches or features that otherwise someone may think solve the problem that you describe.

On 2022-04-14 by Rune

Gentlemen,

I have the following setup:

Dunkirk XEB-5-HA5
Honeywell 8148A
2x Honeywell 40004850-001
Grundfos UPS15-58FRC

It is running two zones heat.

The thermostats are able to trigger heat in both zones resulting in the valves opening and the boiler firing up. The upper limit seems to consistently kick off at 180F and fire back up when temp gets low.

The circulator however will at best only run a bit while the boiler is running.

What are my options?

I'd like the circulator to run when one or both of the valves are open, but that doesn't seem to be the case...?

New Aquastat? Upgraded model?

Thank you, Rune

On 2022-02-14 by Inspectapedia Com Moderator - use of reset button may need a call for service

@greg,

I'm a little confused about the aquastat RESET turning on the circulator pump, since usually when the aquastat turns off and requires a re-set there is a burner or flame or combustion problem - needing inspection, repair, or cleaning and adjustment by your heating service tech.

But if really what's turning on is the circulator, not the burner, when you RESET the aquastat, there could be a loose electrical connection or bad relay or bad transformer in the control.

I'd call for service before you lose heat more solidly.

On 2022-02-14 by greg

My boiler system runs great, cuts on and off like it suppose to. Only problem is that a couple times a week first thing in the morning I have to hit reset so circulating pump will come on. runs great rest of day. It is a Honeywell Aquastat L8148A. Any suggestions ...thanks Greg

On 2022-01-17 by Inspectapedia Com Moderator

@Mnorce,

if you are referring to C1 and C2 that looks dark, that's just a shadow.

You mentioned that what looks like a burn in the photo may be a shadow. Sorry it's pretty blurry.

If you are absolutely confident that your wiring connections are correct them it sounds as if the control has internal damage.

(Re-posted by Moderator after message was lost)

On 2022-01-17 by Mnorce

I apologize, I took some pics to help me explain it better. The picture shows my power coming in and then to my transformer, from there it goes to my Aquastat, the second picture.

The only way I can get the furnace to fire up is to keep the relay open with a piece of cardboard (marked with the red arrow in the second picture), then hit the burner button on my Blower. Once it's fired up everything works fine unless that cardboard is removed.

Thank you in advance for any suggestions or opinions you may have.

(Re-posted by moderator after message was lost)

Aquastat Control (C) Inspectapedia Mnorce

On 2022-01-17 by Inspectapedia Com Moderator

@Matt,

Continuing

it's possible that your control has internal damage.

It could be a subtle clue. For example looking at the bottom of your photo it looks to me as if one of the burner terminal is burned.

On 2022-01-16 by Inspectapedia Com Moderator - aquastat not working right

@Matt,

I hope you at least feel that it's companionable for me to tell you that I'm confused by this too and I can't figure it out from just the information in your text.

I don't know whether there's a defective control or wiring error. It sounds to me as if we need an experienced service technician on site.

I wish I could say more, but with so little information, anything else would be so speculative as to almost certainly be wrong.

On 2022-01-13 by Matt

My issue is that the relay only sends power to the ignitor when it's in the open position.

Two days ago i lost heat, thought it was the circulatory pump but it wasn't. I replaced my Aquastat and still nothing. then when I was verifying my wiring, I opened the relay and the ignitor kicked in.

It's 17 degrees today outside and 52 inside so I have a pice of cardboard keeping the relay open until it get's warm enough where I don't have to wear 5 layers of clothes in my house :)

Any thoughts on why this would be just the opposite?

On 2022-01-11 by Danjoefriedman (mod) - Buzzing from the control switch

@Mike,

I would change out the control (as most techs are not about to go to the trouble and risk of replacing just the relay and its transformer on the circuit board)

I'd tell the customer that in our opinion it's better to replace the control now than to wait for the relay or its transformer to fail hard and leave them with no heat and maybe an emergency heat call.

Details are at

CHATTERING RELAY CONTROL SWITCHES

and

CHATTERING RELAYS, HVAC EQUIPMENT

and

HEATING SYSTEM BUZZING NOISE

On 2022-01-10 by Mike

I have a customer who called with a loud buzzing noise coming from the relay in the L8148A1017. The circulator and burner are running normally. Do I need to change out the control?

On 2021-04-30 - by (mod) - buzzing noise cured by replacing the aquastat

Don

Having replaced the aquastat and noting that the noise has stopped sounds reasonable and might well confirm our guess at a bad aquastat relay. Better to fix a problem like this before it fails hard and leaves your building with no heat in freezing weather.

While it's technically possible to do so, we will virtually never find a heating service tech trying to replace just the relay on the control board of a failed HVAC control; the time and trouble and risks just don't add up for the service company; it's quicker and easier to replace the whole control.

On 2021-04-30 by Don Hardcastle

@Don Hardcastle, they replaced the Aquastat, even though it was new, and the noise seems to have disappeared.

On 2021-04-17 - by (mod) -

@Don Hardcastle,

A buzzing aquastat is usually a failing relay or on occasion a failing low voltage transformer. That's disappointing on a new aquastat - so check for a bad transformer first.

See details about buzzing aquastats and relays and our suggestions at

AQUASTAT TROUBLESHOOTING https://inspectapedia.com/heat/Aquastat_Diagnosis_Repair.php

and at

HEATING SYSTEM BUZZING NOISE https://inspectapedia.com/heat/Heater-Buzzing-Noise.php

and don't hesitate to ask follow-up questions as needed;

please also keep me posted on your repair.

Also see WHITE RODGERS CONTROLS & MANUALS - all

On 2021-04-17 by Don Hardcastle - buzzing aquastat cured

My Aquastat L8148A sometimes makes a loud humming type sound. It appears to be coming from the switch relay. When I push the relay the noise stops.

The plumbing company wants to replace the White Rodgers zone valves with Honeywell zone valves.

They think that is the solution to the noise. The noise occurs when any of the 4 zone valves comes on. One of the zone valves is only a couple of months old. The Aquastat is only a couple months old also.

On 2021-03-22 - by (mod) - gas pilot light keeps going out

@MICHAEL, sometimes when you have a known good gas valve but the pilot won't stay lit, AND if you're sure the pilot assembly is also new and good,

And the problem is that the sensor is not placed properly and the flame or That the other end of the thermocouple is not fully seated in the gas valve.

I've been fooled on this myself a couple of times. I found if I didn't screw the little fitting sufficiently tightly into the bottom of the gas valve the pilot never opened the valve.

On 2021-03-22 by MICHAEL

I have a slant fin hot water furnace. I have had continued issues with my pilot going out. I replaced the honeywell thermocouple on a number of occasions with an additional honeywell brand thermocouple. When the burner does run the gas flame height is normal and the gas pressure appears good.

I then assumed it was the pilot mechanism in a defective gas valve that was the problem. I replaced the gas valve with a new valve. The pilot light continues to go out periodically. It is a mystery. Can I rule out any connection between the pilot failing and the aquastat control for the burner? Thank

On 2021-03-09 - by (mod) -

@Ken, Nice; I'd like to see the diagram; you can use the "Add Image" button if you want to try posting that.

On 2021-03-08 by Ken

1k ad 2k are relays. The expanded circuit diagram was in my boiler instruction manual

On 2021-02-17 - by (mod) -

Lc

I can only guess with just a tiny bit of information in your note but it's possible that your aquastat relay has burned contact super that the relay itself has failed. If after checking all of the electrical connections to be sure there's nothing simple like a loose wire, or a bad thermostat, you may have to replace the relay or the control itself.

On 2021-02-17 by Lc

I have an honeywell aquastat relay l8148e and the relay switch won't turn on the ignitor. I have to click it and then it turns the ignitor on. What can I do to fix the problem?

On 2020-12-29 by Anonymous

I have a resideo L8148j1009 which is replacing a Honeywell aquastat. I have wired it in and can not get the contactor switch to pull in or burner to light. what am I doing wrong ?

On 2020-12-05 - by (mod) - can I replace just the bad relay or do I need a whole new aquastat control?

Most technicians just replace the control entirely rather than attempting a field repair. I think they figured that they had a labor and the risk of an unsuccessful repair calculate out to that conclusion.

On 2020-12-05 by Jeff

Is it better to replace the whole circular relay or can i do it piece by piece and wich would be cheaper

On 2020-10-26 - by (mod) -

OK still I'm concerned about those overheated wires; And there should not be more than 24VAC across those terminals.

Watch out: if you are not familiar with safe and proper electrical wiring you could get shocked or start a fire. - as I warn everyone.

On 2020-10-26 by Anonymous

I just wired it based on the one I removed. Same red wires with black spots on the one that had been running for years.

On 2020-10-26 by (mod) - signs of overheated wires in Aquastat

Crystal:

Watch out: I see burned wires at the top of your controls, those two red wires

- I suspect there is a wiring error or a voltage error at this aquastat, and it's possible that the control has been damaged as a result.

Those wires are connected to what are supposed to be the Thermostat terminals, normally low-voltage 24VAC - so I think something's wrong here.

Who wired this up for you?

See the HONEYWELL L8148 AQUASTAT WIRING INSTRUCTIONS given as a PDF found linked-to in our section title

Manuals for the Honeywell L8148 series Aquastat Relays

near the end of the article above, and also note this excerpt - below

On 2020-10-26 by Crystal - new aquastat, no heat

Hi everyone,

I just installed a new AquatatL8418J on my boiler (along with a new expansion tank, pressure relieve valve nut and automatic pressure vent) and I can't get heat.

Aquatat L8418J wiring problem (C) InspectApedia.com CrystalThe boiler turns on, the water heats up but I can't get it to send out the water when the thermostats call for it.

I have 2 zones with separate thermostats and it isn't working for either so I am fairly sure it is not the thermostats or the zone valves causing the problem.

I labeled all my wires and rewired it based on the labels but I am wondering if I messed up.

I am thinking it is the T and TV connections that would be causing this particular issue but I don't know enough about it to be sure.

For fun both wires are red so its difficult to tell which is which and they both lead out to red wires on the zone valve motor boxes.

The relay doesn't engage when the thermostat is turned on, and if I manually press the relay the pump seems to come on so I think that rules out the pump as the issue either.

Side note, the replacement was technically L8148J1009 and my original one did not have 1009 on the model number but they looked exactly alike so I was guessing they updated the model number in the decades since the original was installed.
Thanks for any help you can give, its getting cold tonight :)

On 2020-10-25 by (mod) - troubleshooting L8148E Aquastat

Ed

First check the settings in your control to be sure that your high limit is set below 200 degrees. Make sure that the low limit is at least 20 degrees lower than that.

If the settings are correct then I suspect either that the sensor in the sensor well is not correctly sensing boiler temperature or has failed, or the control itself has failed. Those are repairs that can be done by your heating service tech

On 2020-10-25 by Ed F

I have an aquastat L8148E and when I call for heat it fires up and when it reaches set temperature the circulator pumps keep running and the temp just keeps rising until I have to turn the power switch off because it gets like a sauna? Please help

On 2020-10-18 by danjoefriedman (mod)

Billy

Go to the boiler itself to see if it is hot or cold and if it is cold than we know it's not starting in response to a call for heat.

If that is the case look for the reset button on the equity side and you can press that 1 time to see if you can start your boiler

On 2020-10-18 by Billy

Put a call in for heat and can hear water running through the pipes of the baseboard radiators but no heat at all. Thermostat on, water runs; thermostat off, water stops running, as it should. Aquastat is a L8124A or C, I think. Or it may be L8148, and I think the relay is R845A. What gives?

On 2020-03-09 - by (mod) - The relay is not pulling in?

If the relay won't pull in when voltage is applied to the terminals then it is probably defective (unusual in new equipment)

Check first for a wiring error.

Don't forget to confirm that the problem isn't an open or broken thermostat wire

On 2020-03-09 by Ken

Bought a new honeywell aqua stat L814J1009. The relay is not pulling in?

On 2020-01-14 - by (mod) - 90 VAC reading between B1 and B2 on the aquastat

Travis

I suspect the problem is in a connection or wire right on the aquastat control board or relay.

I'm assuming that you've checked or can check that
- the supply voltage to your building is around 120VAC at the same time that you are seeing 90VAC at the aquastat B1 B2 (burner) terminals

- there is no problem with under-sized wiring

- the circuit itself is not overloaded

- there are no obvious loose wiring connections

On 2020-01-14 by Travis Z

Largely, my aquastat seems to be operating in accordance with what is described above, but for the fact that I get a 90 VAC reading between B1 and B2.

I have 117 VAC between L1 & L2, and interestingly enough that same 117 VAC between L2 (L2 as common) and B1 (boiler line output).

The conclusion is there is some voltage drop... having 90 VAC on the boiler supply output (B1/B2)

I assume is not enough to fire the boiler... not sure on that. Can anyone confirm? Any ideas what would be causing the voltage drop?

On 2020-01-01 - by (mod) - Hitting the side of the box, or sometimes carefully pushing on areas of the board, would then cause the burner to fire.

Thank you for your generous words Fred and for the posting which will certainly help other readers.

On 2020-01-01 by Fred

First off, THANK YOU for this awesome website! It was very helpful in diagnosing an intermittent problem with my L8148 1090 Aquastat. The Aquastat was not always firing the burner correctly.

I'll describe it here, in case anyone in the future has a similar issue: It would initially fire correctly, but sometimes cut out at a mid-range temperature say 120F or so, or sometimes not re-fire after reaching the high-temp limit and then cooling past the normal low "limit".

Hitting the side of the box, or sometimes carefully pushing on areas of the board, would then cause the burner to fire. There are videos on YouTube stating that on these models, some solder joints on the backside might go bad. I believe that was the case with mine.

Replacing the Aquastat fixed the issue. I inspected the back of the board of the old one, and there were no obvious bad solders, but I'm still pretty confident that was the culprit. Thanks for this page. It gave me a much better understanding of the function and proper operation of the Aquastat.

On 2019-11-15 by (mod) - I shorted a wire to the circulator

Steps in Troubleshooting the Honeywell L8148 Aquastat

If you shorted the power wires to any heating control including the Honeywell L8148 Aquastat then it’s quite possible that the control has been damaged or destroyed and will need replacement.

In any event if the L8148 Isn’t turning on the circulator (s) and thus is never causing the boiler to run, here are some diagnostic steps starting with what a homeowner can do and continuing with what a service technician might do.

Please now find your question and my very detailed reply giving some step by step suggestions in the article above on this page.

On 2019-11-15 by Anon2

Hi,
I am a homeowner. My Utica boiler has L8148 aquastat. I was changing the taco pump and I made a mistake while power is on and I caused a short with line going to taco pump. And aquastat stop working after electric short. So do I have to replace the aquastat?

On 2019-03-02 by (mod) - my heating boiler won't come on

Anon

I'd start diagnosis at

NO HEAT - BOILER

or see

CIRCULATOR PUMP WON'T START

On 2019-03-01 by Anonymous

I had a problem with my heat not working on 1 st floor I replaced my circulator pump thinking that was the problem however it was not I hooked up the new circulator pump and it did not work but it works when i plug it into an extension cord so I believe it is something wrong with my aquastat relay differential. can you help me mine is an oil boiler

On 2018-12-27 by (mod) - gas burner won't ignite

Louis

If a gas burner won't ignite

- confirm that there's fuel

- check for a tripped flue gas spill switch sensor - some have to be manually re-set

- With electronic ignition there's no standing pilot but you could have a bad igniter or bad igniter wiring. If the igniter doesn't spark on a call for heat that's what I suspect.

- if those are not the issue the eliminate the thermostat from the circuit by disconnecting it at the aquastat and try jumping the two

Thermostat terminals - If the valve opens and burners ignite we have isolated the issue to the thermostat wiring or thermostat itself

On 2018-12-27 by Louis

Heater will not fireup has electric startcan you helpgot new sensor and pilot


...

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