Here we explain how to diagnose and fix problems with the Honeywell Resideo L7224U Universal Oil Heating Boiler Electronic Aquastat Controller and we include a guide to the L7224U LED Error Codes.

Honeywell L7224U Universal Aquastat Diagnostics

Here we describe specific problem troubleshooting for the Honeywell L7224U universal aquastat that replaces many of the older aquastat models used on heating boilers.

In case you missed it, In 2018 Honeywell announced that Resideo would be the corporate name of the Honeywell Homes product portfolio and ADI global distribution businesses when they became a stand-alone, publicly traded company following completion of a spin that was expected by the end of 2018.

The L7224U1002 provides status and diagnostic information through an LED display to enhance the diagnostic process.

First, for a guide to the Honeywell L7224U diagnostic buttons, controls and LED readout codes,

see AQUASTAT L7224U UNIVERSAL OPERATION & ERROR CODES

Then you may want to return to this page by using the "back" button in your browser.

If you're having trouble unlocking that pesky anti-tamper setting on the Resideo / Honeywell L7224U

see AQUASTAT L7224U UNLOCK & ADJUST SETTINGS

Troubleshooting the Honeywell L7224U Aquastat

Where is the aquastat located on heating equipment?

Because the aquastat is a control used on hot water heating systems it will be located most often right on the heating boiler or water heater.

That's because the aquastat includes a temperature sensor that has to be inserted into the water being heated so that the water temperature is monitored.

In some installations the aquastat may be wall-mounted next to the heating unit and connected to it by a flexible probe line whose end is inserted into a temperature sensing well right on the heater unit.

Our photo of a modern hydronic (hot water) heating boiler (above left) includes a red circle around the aquastat or primary controller on this heater.

Some older heating systems used a "strap-on" aquastat that is clamped to a hot water pipe just above the heating appliance - shown at left, courtesy of Carson Dunlap Associates .

See STRAP ON LIMIT CONTROLS

Readers who are not familiar with this control should review the definitions and functions of aquastats and the aquastat HI, LO, and DIFF settings found on older boiler controls, starting

at AQUASTAT CONTROL FUNCTIONS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Check the temperature sensor installation if the aquastat behavior is erratic.

[Click to enlarge any image]

Illustrated above is an adapted drawing from the installation instructions for the aquastat discussed in this article, the Honeywell L7224U.

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.

Check that the Aquastat HI, HI-DIFF, LO, LO-DIFF settings are correct and not crossed or in conflict

In the FAQs of this article you'll read that several readers commented about great difficulty getting the Honeywell L7224U aquastat to run the boiler at a sufficiently high temperature and some reported that the HI and LO control functions seem reversed.

We suspect that part of the trouble is in pressing buttons to get the right settings past the control's anti-tampering feature.

I include these L7224U adjustment procedure notes from the company's control quick guide linked-to in the citations just below.

To discourage unauthorized changing of Aquastat settings, a procedure to enter the adjustment mode is required.

To enter the adjustment mode, press the UP, DOWN, and I buttons simultaneously for three seconds.

Press the I button until the feature requiring adjustment is displayed:

• HL - High Limit
• LL - Low Limit
• Ldf - Low Limit Differential
• F - C - °F or °C

Then press the UP and/or DOWN buttons to move the set point to the desired value.

After 60 seconds without any button inputs, the control will automatically return to the READ mode.

[For systems that are not using a tankless coil]

To use the L7224U in a cold start boiler application, disable the Low Limit function by pressing the UP arrow button, DOWN arrow button and I buttons simultaneously for three seconds.
Then push the I button until LL is displayed. Then press the down arrow button until OFF is displayed. 

Before contacting Honeywell to bug them about how to operate this aquastat, try taking another look at the first 3 PDF files linked-to

at HONEYWELL L7224U AQUASTAT MANUALS

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-05-28 by InspectApedia DF (mod) - Error code is bt (not 6t) on the L7224U AQUASTAT

@Mr. H,

You're quite right; for other readers we explain that

AQUASTAT L7224U UNIVERSAL OPERATION & ERROR CODES includes a table of all of the codes expected to appear in the display of the L7224U.

There is no 6t display code on the L7224U.

But there is a bt code that means the display is referring to "boiler temperature" - it's not an error code.

This is a normal display read-out that gives the current temperature of the boiler.

In RUN mode the display flashes alternatively to show temperature followed by (°F or °C)

When you have no hot water at all we can't know, from just your text, if the problem is that the boiler simply is not running at all or if its settings are incorrect or if there is a problem with the hot water controls. Does water come out of the faucet when "Hot" is turned on but that water never gets even warm, or does no water flow at all.

Is your hot water made by a tankless coil on the heating boiler, by an indirect water heater, or by separate oil or other-fueled water heater.

Hot Water Troubleshooting if your boiler uses the L7224U

• AQUASTAT L7224U UNIVERSAL DIAGNOSIS - you are on this page - and in the right place (start at the top of the page) if your hot water is made by your heating boiler that, as you've indicated, uses this aquastat -
  
  Your aquastat may be set improperly or something else may be keeping your boiler's oil burner from running at all.
• HOT WATER PRESSURE LOSS - if no hot water flows at all
• OIL BURNER WONT RUN - you have an oil fired water heater or a tankless coil on an oil fired heating boiler and the burner won't turn on

On 2023-05-28 by Mr. H. - Error code is 6t

I read the manual again (thanks for the link on your site) and this is not an error code but instead the boiler temp.

73 F is what follows the Bt reading.

I am not too good at this - any advice as to how to increase the temp?

I do not have hot water. Error code is 6t. Turned off power and replaced fuse (original looked ok) then turned on power but same message and no hot water.

Cannot see any loose wires. Any suggestions please? - @Mr. H,

On 2023-02-27 by InspectApedia Editor - how can the boiler be anywhere between 175 and 190s but the water is coming out only 120ish?

@Jerry,

I'm glad to try to help but please realize that just about 100% of the time that I, or better, a trained service tech, goes to your home, s/he's going to see something important that simply wasn't obvious to a normal homeowner, so too-often we're flying too-blind here.

If you can plod through the PDFs that we provide describing your aquastat to see exactly how it works, you'll see a neat graph that explains how the HI, LO, and DIFF interact to turn the boiler on or off or the circulator on or off. There are several different conditions and thus different operating patterns.

The burner may come on under control of either the HI (on a call for heat) or LO (to keep the boiler hot for a tankless coil) setting. If the thermostat never calls for heat then the LO will maintain heat in the boiler, but it's more complex than that, as you'll read in the PDF.

On 2023-02-26 by Jerry

@InspectApedia Editor , thanks so much for answering my questions.. i love to learn and thinker (if I'm becoming a pest, let me know). I had a service contract and altho was supposed to auto renew and did not.

It renews again in June so I figured a waste to see if they would give it to me after i started trying to deal with the issue this late in the contract; i will renew in June and if not solved call then...

So here's the update... with the hot water running, at the boiler... It kicked on at ~175 (LL is 185) and shut off at 185 (Is that correct...? I thought it kicked on at LL) The hot pipe coming out of the boiler went as low as in t he 80s and max out at 120s before the mixer valve (cold water in pipe read at 67).

So i'm baffled as to how the boiler is anywhere between 175 and 190s and the water is coming out only 120ish. Is there some kind of damper internal..?

On 2023-02-18 by InspectApedia Editor

@ Jerry,

You say this has been happening since installation in 2011 and at that time it was a new phenomenon for you. It could be that the water temperature control was not adjusted properly at the time of installation and that's why the issue began.

Now that it's getting worse, it could be a matter of a failing water temp regulating device, be it a mixing valve or anti-scald device or whatever you have. Anti-scald valves typically are warranted for just 12 months from the date of installation. Plumbers report a typical mixing valve life of five years.

Depending on your fuel source - electric, gas, oil - it may also be time for a service call if none has been done since the 2011 installation. An on-site plumber will be able to give a more accurate and thorough assessment than we can.

We just don't have enough information to give you a definite answer.

First, we need to be clear what we're talking about: Type of water temperature control device and fuel source

We discuss the various type of these devices here:

MIXING VALVES & ANTI SCALD VALVES

And we discuss the safety & functional inspection of anti scald valves & mixing valves at

MIXING VALVE & SCALD PROTECTION VALVE INSPECTIONS

Do take a look and let us know if you have additional questions.

On 2023-02-17 by Jerry

The boiler is in the laundry room and only 12 feet from the shower; pipes are insulated. I have lived here almost 30 yrs (25 as a renter) Boiler was replaced in 2011 and since have had this issue but seems to have gotten worse.

There is a mixing valve. I start a shower with the single handle fixture at 12:00 (even mix of hot and cold)... i have to keep adjusting before finish washing my hair to 9:00.

I hear the boiler turn on and reach temp and shit down and never have to move the handle from the 9:00 position again.

The aquastat sensor is centered in the coils; see image

On 2023-02-08 by InspectApedia Publisher - temperature of "hot" water that arrives at a plumbing fixture is affected by a number of factors

@Jerry,

I don't know where you're measuring water temperature.

But certainly if the temperature/pressure gauge on your heating boiler says the boiler itself is at say 170F, then water leaving the boiler into a heating loop (or into a loop of pipe heating an indirect water heater) would be close to that same temperature.

The temperature of "hot" water that arrives at a plumbing fixture is affected by

- the type of pipe material, run length, run routing through cool or cold areas, absence of insulation on piping, etc.

- the presence/absence of an anti-scald or mixing valve at the plumbing fixture

- occasionally: a mix-up in pipe connections: someone runs "hot" supply piping to a the "cold" faucet, and vice versa

I also see brief and unexpected water temperatures caused by heating or cooling of water standing in water pipes that are at rest in a hot or cold area.

For example the cold water line feeding an upper-floor bathroom is routed through a first floor boiler room (where it splits off to also run through a water heater).

When water in a cold water line has sat in piping, not flowing, for hours, and when the boiler has been operating, the boiler room itself is quite warm and water in the cold water supply pipe can be heated up quite warm where it passes close to the operating boiler.

When the cold water tap is first opened in the affected bathroom the first water coming out of the faucet is cool, then it becomes very warm, almost hot. But there is no piping error. Once the cold water has run long enough to flush that dose of "hot" water resting in the cold water line near the boiler, then the cold water in fact drops to almost as cold as the temperature of water that's entering the building.

On 2023-02-08 by Jerry

@InspectApedia Publisher, Hey, just realized you replied... Thanks so much for the input...

So here's the update... After replacing the sensor and no results; instead of just adding the grease I replaced the aquastat and then added the grease... No change... I put the settings at 185/205 figuring it would keep it a constant hotter.

Then i increased the hot with the mixing valve which i figured shouldn't matter because, whats the difference if the cold mixes at the boiler or the shower... the cold water is still gonna come thru; and that is the results...

I don't understand that if the aqustat kicks on even at 170 (which it did w/o any demand) how can that water remain cool..? 170 is hot water... I'm baffled... I think it may be what you said, the new smaller boilers just don't cut it...

The sensor point is different than the actual water coils so they must have a bigger temp differential due to the cold water flow and I can't (don't want to) set the boiler any hotter. Before i give up, any other thoughts...?

On 2023-01-11 by InspectApedia Publisher - heat conductive grease or paste may be recommended

@Jerry,

Yeah some installing techs don't like the conductive paste - griping that it petrifies and makes future sensor replacement more work. That used to be true, 30 or 40 years ago but I've not heard of any such problems with the contemporary products.

And it makes sense to follow the manufacturer's instructions that come with the aquastat - most likely you'll see that such a heat conductive grease or paste is recommended.

Still when your hot water is hot enough but goes cold quickly I suspect that the heat transfer into the tankless coil (that IS what you have, right?) is working ok but that the boiler itself doesn't contain a very large volume of hot water, OR your heating system is sending most of the heat to your radiators or baseboards not leaving the boiler very hot (the aquastat tries to give priority to the tankless coil but might not always succeed).

Newer steel boilers are smaller and have much less total thermal mass than the big old cast-iron monsters, making tankless coils provide less hot water than back then.

Please do look at the articles I recommended - there's more useful information there than I can re-create off-the-cuff de-novo here.

On 2023-01-11 by Jerry

@InspectApedia Publisher, Thanks so much...
Just to touch a few topics mentioned... the pipes are insulted and the mixing valve/scalding valve has been adjusted to test and it does work... but the water just goes cold after a short period of time.

As far as the sensor paste, I did not know it existed until reading this yesterday so i will look into it and add it... there was also none on the one I pulled out. This will be my next test....

As far as the temp of the boiler, the aquastat shows it go over 200 and I haven't checked yet to see where it goes down to but I do not believe it is below the low limit. the water pressure is fine/equal as the cold.

I will check out the other links you provided; i appreciate this because i did not know what else to look into.

Thanks for your time and I'll keep posted.

On 2023-01-11 by InspectApedia Publisher - not enough hot water from my L7224U Aquastat-Controlled Boiler

@Jerry,

I'm sorry that you missed my brief answer to this question above where we said:

Is the LO limit set above the HI or even close to it? The LO needs to be 20 degrees below the HI or it will lock out the upper limit control on many aquastats.

I'll add to that:

Your settings are reasonable. You'd know if your aquastat settings were too close to one another because that'd lock out the heating zone circulator and your heat in the building itself would be either inadequate or very slow.

But there are lots of other problems that can cause not enough hot water, some related to the aquastat, others not.

When you replaced the aquastat sensor did you use the manufacturer-recommended heat-conducting paste? And did you take care to bend the sensor tube to be sure that there was good contact between the sensor itself and the surface of the cleaned-out sensor well in the boiler?

You reiterated other reader questions about hot water supply that focus on the L7225U Aquastat controller.

And your sensor suspicion and replacement was a reasonable step since it sounds, from your description as if the boiler is simply not hot enough or heat is not being transferred from the boiler into the tankless coil.

But there are lots of other reasons that water at the shower is not hot-enough or that the hot water flow rate or "pressure" is inadequate so that mixing in any cold water makes the water too cool.

Example diagnostic questions:

Is there a mixing valve or tempering valve at the boiler that could be mis-adjusted or malfunctioning?

Is there an anti-scald valve anywhere in the hot water system that could be mis-adjusted or malfunctioning?

Are there long hot water piping runs, uninsulated, or routed near or through cold areas in the building?

For a thorough attack on "not enough hot water from my L7224U Aquastat-Controlled Boiler" when we don't have a strong suspicion that it's an adjustment or sensor problem at the aquastat itself, it's time to look closely at the advice given starting at

TANKLESS COIL HOT WATER INCREASE

You might also want to read

HOT WATER IMPROVEMENTS

where we have a number of diagnostic and improvement suggestions.

Watch out: if the flow rate of your hot water is noticeably less than that of the cold water at the same fixture, that's quite diagnostic (of a clogged tankless coil)

also see

HOT WATER PRESSURE IMPROVEMENT

Please read through those articles, let me know if that gives you a solution, and if not we'll continue from there.

On 2023-01-10 by Jerry

The question i have is the one that was never answered.... 'not enough hot water from my L7224U Aquastat-Controlled Boiler"

The boiler works fine but the demand for hot water does not kick in until the shower is 100% set to hot water and the temp is probably 100 degrees [chilly in the winter] coming out of the shower head... then i hear the boiler fire up and by the time it warms up, i'm finished with the shower.

I replaced the sensor and nothing changed... The high limit is 190, low 170, diff 10... What else could it possibly be,...??

On 2021-07-16 by inspectapedia.com.moderator - circulator pump runs continuously

@sudhakar kandikatla,

Sure: take a look at the diagnosis and repair advice found at

CIRCULATOR PUMP WON'T STOP RUNNING

found by searching InspectApedia.com for "circulator pump runs continuously"

and let me know how that works for you by posting on that page.

On 2021-07-16 by sudhakar kandikatla

replaced old Honeywell Aquastat with this L7224U Aquastat. Everything is working. I did not make any any changes to the factory settings. I have 2 zones for baseboard heating and 1 zone for indirect heating water heater.

I have only water heater zone is on, other 2 zones are off(for summer). I am seeing my circulator pump is always on. Can you please tell me what is wrong and how to fix it.
Thanks
Sudhakar

On 2021-02-10 by danjoefriedman (mod) - shouldn't have to constantly be replacing a primary control

I agree with your service technician that there's something rotten in the state of Denmark.

There's no reason that you would expect to constantly be replacing a primary control like this one. I can't see enough of your installation with the kinds of things I would be looking for would be improper wiring, for voltage delivery, or a bad location that's exposing the control to water or unusually high heat etc.

It might be worth a read through the page above where we list the number of things to check when people are having aquastat trouble.

On 2021-02-09 by Err 1 fault code

I have a Honeywell 7224U aqua stat. I have replaced the unit 4 times in the last 5 years, usually in the dead of winter when I switch from heat pump to infloor radiant heat. I had it runing once (for about a week) this year then turned it off.

Now it comes up with an Err 1 code again. The last repair guy said it maybe a voltage regulator and he said the voltage was a little low.

He said he has 500 plus customers and only replaced 3 in the last 5 years. What will cause the stat to do this once a year?

We use air to air heat pump most of the year and the oil boiler is about 20 years old.

Question: Pennsotti boiler's Riello burner runs just or 10 seconds, same for circulator; short cycling.

(Dec 8, 2014) John said:

My burner turns on for 10 seconds and runs then shuts off the circulator pump kicks on 10 seconds later it fires up again and will run for about 2 minutes max then shuts off, it's 25 degrees out now and this runs every 10 to 12 minutes like this and my thermos stat is set at 70, I've had techs clean and service it and they tell me there's nothing wrong with it,

if it's been off all day then it will run for 5 minutes and shut off once temp is met. I'm tired of paying oil companies money for nothing when it buns more oil running this way. Please help, this is a Pennsotti boiler with a Riello burner. Thanks

Reply:

John this sounds like a loose wire or a control problem. We don't want short burner on-cycles as it soots-up the system leading to malfunction.

Call the heating service company you use, speak politely with the service manager, ask them to send an experienced technician to your home. I'm afraid that just changing control settings will ignore a soot or operating problem and could leave the system unsafe.

Honeywell L7224U Aquastat Manuals & Guides

See HONEYWELL L7224U AQUASTAT MANUALS

...

Continue reading at HONEYWELL RESIDEO L7224U UNIVERSAL & L7248 AQUASTATS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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AQUASTAT L7224U UNIVERSAL DIAGNOSIS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING BOILERS

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