Here we explain the Honeywell L7224U Universal Oil Heating Boiler Electronic Aquastat Controller
This updated Honeywell® combination control that sets boiler temperature and may also control hot water production via a tankless coil on the heating boiler. The photo above shows the interior of a Honeywell L7224U combination heating control, also called an "aquastat".
This aquastat model includes the "universal" term in its name because it can replace more than 40 older heating boiler controls. This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
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The manual dials that we described at AQUASTAT CONTROL FUNCTIONS and at Aquastat control HI LO settings and for Hi, Lo, and at Aquastat control DIFF settings for DIFF disappear on the newer L7224U1002 Universal Oil Heating Boiler Electronic Aquastat Controller from Honeywell.
However the concepts and the actual settings of High limit, Low Limit, and differentials remain about the same for this and most other aquastat controls used on hydronic boilers, except that this new control adds additional levels of control including the ability to replace the "fixed" high limit DIFF on older aquastats with a set-variable High Limit DIFF on the new control.
This new control also adds both a two-character error code display and a collection of LEDs to assist the service technician in problem diagnosis and control setting.
And the basic operation of this control in operating the oil burner and/or zone circulators is about the same as well.
On a call for heat from the thermostat, the zone circulator starts when water temperature is above Low Limit setting (if applicable - that is, if this feature is enabled - the default). The heating boiler temperature is checked. The burner starts when the water temperature is below High Limit setting.
When the boiler temperature reaches or exceeds the High Limit, the burner is turned off. The burner restarts when the
water temperature drops back below the High Limit setting minus the differential. As long as the circulator is continuing to call for heat and provided the control has not "locked out" the circulator (as we discussed above) the zone circulator will continue to circulate hot water through the heating zone.
When the thermostat is satisfied - that is room temperature has reached the thermostat setting, the circulator and burner are turned off.
The Honeywell L7224U1002 Universal Oil Heating Boiler Electronic Aquastat Controller is a universal electronic aquastat (replacing over 40 other aquastat models) that provides circulator, burner and boiler temperature control with electronic temperature sensing.
This is a "triple aquastat" providing High, Low, and circulator controls, as well as advanced features permitting electronic configuration and an LED display for diagnostics. This aquastat can provide multiple zone control as well.
The L7224U1002 supports Honeywell's EnviraCOM™ communications that permits remote diagnosis as well as OnWatch diagnostics.
The low-limit on this control can be disabled for "cold-start" boiler applications such as where no tankless coil is in use.
The L7224U1002 provides status and diagnostic information through an LED display to enhance the diagnostic process.
In "Run" mode the LED displays the boiler temperature ("bt"...temperature... °F/degC). Three control buttons inside the control permit the service tech to scan through the control's settings as well as to change them, much like the functions on a programmable room thermostat.
Watch out: homeowners should not mess with the control settings. Doing so can lead to loss of heat, and/or costly and embarrassing service calls.
Using the "I" button, the LED display can be cycled through the individual status displays listed below. Cycling to each topic, the display shows in succession two or three readings: the identity of the item being displayed, the reading, and a F or C display for Fahrenheit of Centigrade (where applicable). The service technician uses a quick reference guide to decode these readings.
The aquastat LED provides will show "err" followed by these eight error codes used by the service technician for diagnostics:
If error conditions 6 or 7 is detected, the control has reset the High Limit, Low Limit and Differential setting to a default value and will continue to run at those settings. Performance of the system will be degraded, but every effort is being made to keep heat safely on in the building pending further diagnosis and repair.
If error condition 8 is detected, all outputs
except ZC (the circulator control) are shut down. The burner is off.The control
continues to function and restarts when all three user keys have been pressed longer than 60 seconds.
Watch out: homeowners should not mess with the control settings. Doing so can lead to loss of heat, or in this case possibly an unsafe condition, and/or costly and embarrassing service calls.
COMMENT: it was instructive to review the Honeywell service literature on the control as it directs the oil heat service technician to check wiring and voltages, but leaves to the technician the observation of an oil burner shut down due to dirty or sooty operation, fuel problems, or similar issues. The control monitors electrical conditions and temperature. Separate safety controls such as the flame-sensing cad cell (CAD CELL RELAY SWITCH) perform additional functions such as flame detection and dirty or sooty operation.
Five diagnostic LEDs inside the control provide additional diagnostic features, indicating (counterclockwise from upper left
[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.
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:
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 just below.
If you're still having trouble, Honeywell has also provided a powerpoint training session on this aquastat at http://customer.honeywell.com/NR/rdonlyres/5C67D10A-631C-4404-9149-25BA61F5D8BB/4041/L7224UUNIVERSALAQUASTATTrainingModule.ppt
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.
n 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 Dunlop Associates.
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.
(Feb 18, 2012) MaineMark said:
I had an old controller go bad on an oil furnace & had it replaced with a Honeywell L7224U. Now our domestic hot water rarely runs hot during a shower. Any ideas or recommendations.
(Jan 16, 2013) VinceB said:
Same issue as MaineMark. If you read how the L7224 controller operates in the Honeywell manual, it is totally different than how the older controllers are described as working on your website. Your website says that the L7224 operates the same (Hi limit, Lo limit, Diff) as, for example an L8124. I don't see it. Anybody have any suggestions?
(Feb 12, 2013) Lonmon said:
Same issue as MaineMark. I set the HI to 210 and the Lo to 190 according to the Burnham Boiler recommendations and it helped a little but it still runs cool and eventually you get cold water in the shower (it has a tankless DHW coil). I left the Hi and Lo DIFF at the default at 10. Should the DIFF be changed on the Lo? Original control (L8124) called for DIFF of 25.
Same issue as MaineMark. I have a tankless hot water heater installed for DHW and a long shower will get COLD water at some point. I changed the Hi to 210 and the Lo to 190 in accordance with Burnham the boiler manufacturer recommendations and that helped a bit. They recommended a DIFF of 25 on my old L8124 controller. I left the Hi and Lo DIFF settings at the default of 10. Should i adjust the Lo Diff on this new L7224 controller to maintain better DHW heat?
(Nov 16, 2014) frank hollis said:
Limits are set, but the burner starts only when the low limit is reached, and stops when the upper end of the low limit is reached. The burner will not run to the upper limit.
Apologies Vince B.
I fixed broken html and added links to Honeywell's instructions for these two controls in the article above. Please take another look.
We are seeing several complaints and notes about confusion in the operation of this control.
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.
(Nov 17, 2014) FRANK HOLLIS said:
Unit is now operating, and low and high limits are working, but I am not able to change the numbers using the push buttons. Can anyone help me? I contacted Honeywell to talk to a technician, but this can no longer be done probably due to legal issues. Sad!
Frank the control has a "lockout to prevent tampering" that can make setting the control tricky.
Try the button pushing procedure shown in Honeywell's
(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
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.
2 Feb 2015 Anonymous (Gregg) said:
I have a Burnham V14A-T boiler with tankless coil, aquastat is a (Honeywell L7224U1002). I'm not the brightest crayon in the box, but I am the owner/head technician/service engineer for a small CNC machine tool service center. I usually get it right when it comes to figuring out how things are supposed to work, But I must admit, I'm a bit unsure about this.
I recently replaced the old mechanical aquastat on my boiler (about a week ago) it went up in smoke due to poor connections/corrosion in the push loc wire terminals. Any way I replaced it with the Honeywell L7224U1002 unit.
It works I have heat and hot water but not as I understand your explanation of operation, I have second and third guessed myself into a state of frustration, So hopefully you can shed some light on this situation
It seems as though the low limit has complete control over boiler temp. when t-stat is calling for heat. With lo limit at 140 it circulates 130-ish water through the radiators,(140-10 diff) burner running constantly circulator on at 140 off at 130 which is not hot enough to do any good. never gets close to the 210 hi limit.
If I set the lo diff to 25 as suggested to prevent circulator from turning on until 165 I get 115-ish water running through radiators(140-25 diff)It seems the diff is on the - side of lo limit.
The only way I can get hot water for heat is to set lo limit up to 190-10 diff then I get scalded every time I wash my hands, not so bad for me but the wife and grand kids have some unkind things to say about it. If I reduce the lo limit to a safe level the house can't get up to temp. So what am I missing or is this the way its supposed to work?
The old one did pretty much the same thing, we just got used to it. But reading on this site left me with the impression that I did not need to have my boiler at 190 ALL the time, And what you have written makes complete sense to me but that's not what I'm seeing here. 140 domestic 190 heat would be great, Can you help?
Anon - dull crayon
Notes from a not so sharp pencil out of the box: I'm just thinking as typing here. First you can read in the FAQs above that other fellows have had trouble getting this control to behave. I don't know but I suspect that at least part of the problem is that the Honeywell engineers included an anti-tampering feature on the control - to the joy of the HVAC service company - that can make button pushing and control setting sometimes not give you what you thought you set.
Have you taken a look at the quick guide PDF we link in the article above - see
I would not set HI above 200F out of worry for pressing on the TP valve - that is running a bit hot and close to TP valve spill temperatures.
With LO at 140F the HI should be controlling on a call for heat and the LO and DIFF should be sleeping unless the boiler temp is down in the LO / DIFF range
SO I don't assume the control is not working and given that you're a smart Crayola I will for now assume you wired everything correctly and set the control correctly.
SO I ask - could there be another reason the boiler is simply not heating up to the higher temperatures: such as bad flame, or a thick coating of crud on the heat exchanger?
NO because you can foul up the control by crossing its settings - putting the LO up into the too-high range of 190 which then gives you a boiler so hot that the tankless coil is giving scalding water to the sinks. (serves you right)
Yeah it is acting as if the control is crossed somehow.
You could install a tempering valve at your tankless coil so as not to get scalded - that's a good idea in any event.
But I would try this
- turn off power to the control
- wait a minute
- turn power back on
- follow the instructions to set the HI LO DIFF to 200 140 25
I include this note from the PDF
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:
HLo= High Limit.
LLo = 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.
The following is for people whose boiler does not include 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.
6 Feb 2015 Gregg said:
Thanks for the reply Dan, I have read all of the suggested material ad nauseum, Both Before and after installing the 7224. Before because I was having trouble with the old mechanical unit. This site actually led me to purchase the 7224.
I can get past the anti tampering lockout, as evident by the fact that I can raise and lower boiler temp by raising or lowering LO LIMIT and/or LO LIMIT DIFF. I have verified that I can circulate 130 degree water, I can circulate 210 degree water, and a few temps in between. That is not the problem here. The problem is when calling for heat, the burner will run constantly until T-Stat is satisfied because the circulator turns on at the LO LIMIT (140)setting.
The circulator will run until Boiler Temp is down to 130 (at -10 diff) then the circulator stops, burner continues to run. round and round it goes, Boiler temp. never gets above LO-LIMIT setting much less to HI LIMIT. If T-Stat is not calling for heat then burner runs until (140)LO-LIMIT is reached and everything stops until boiler cools down to 130, then starts again. Hi LIMIT is ignored completely. The way I understood it to work is the LO-LIMIT is ignored if there is a call for heat. Yes?
5 Feb 2015 Gregg said:
Thanks for the reply Dan,I just saw your response in FAQ's I will try cycling power and resetting values again
Let us know what happens, Gregg, and before tearing out any more hair consider you're part of a club of folks who've had trouble setting this control. If still you see no success I'd give Honeywell a call at their tech support line. (The company emphasizes that the control we are discussing should be installed, adjusted, or repaired by a trained HVAC service technician).
Telephone for Honeywell Technical Support with Aquastats or other Heating Controls
1 (877) 841-2840
001 (480) 353-3020
5 Feb 2015 Gregg said:
I'm way past pulling Hair Dan, remember Kojak? Anyway, I cycled power and set values Twice-No Dice. same thing. I did however remove circulator lead while T-stat was calling for heat, the boiler temp rose to HI-LIMIT and shut off. but when I reconnected circulator temp went to Hi-LIMIT -10 burner started, circulator kept running, boiler temp went down to 128 at that point I disconnected circulator again, it would have kept going down until whatever water temp was returning from house radiators.
My question is should circulator have stopped when HI-LIMIT -10 was reached then stayed off and restarted only after HI-LIMIT was again reached? Current settings are HL=190, Hdf=10 LL=140 Ldf=25. Duu=OFF ASC=0 PC=OFF
This may be a bit general but on a call for heat the circulator will normally run until either the thermostat is satisfied and the call for heat stops (and turns off the circulator) OR until the boiler temperature drops down into the range controlled by the LO and LO-DIFF (Ldf in your notes).
6 Feb 2015 Gregg said:
Hi Dan, I messed around a little more last night, this unit would work (better)if the LO-LIMIT diff was on the upper side of LO-LIMIT but it definately is not. I proved that to myself again last night. with LL set at 140 (diff 10)circulator will draw boiler temp down to 130 (Diff at 10) and down to 115 (diff at 25) On temp rise, burner still shuts off at 140. SO my settings now are HL 200 Hdf 10 LL 180 Ldf 10 Am I reading something into this that isn't there? I'm sure I read that the LL diff of -10 is fixed and the adjustable up to 25 goes plus from LL MINUS 10 PLUS 25IN other words a LL of 140 and adjusted diff of 25 nets me 155 on temp rise before the ciculator is "ALLOWED" to turn on? Is this correct?
I don't know this control as well as I do the old R8182 series. On that control the 10 diff from the HI is hard-wired while the DIFF from the LO is adjustable as described in our article on that control. This L7224U has a programmable DIFF on the HI as well as the LO controls but we ought to re-read the documentation.
Have you considered giving your heating service company or Honeywell a call?
I did call the honeywell tech line number that you provided, they refused to talk to me because I'm not a licensed contractor,
I called 2 licensed contractors that I know personally and they don't know anything about this control.
[Click to enlarge any image]
Have you asked your local heating service company to help with this problem and have you then asked if they'd call Honeywell to get help with the control? If they learn how to install and troubleshoot the control surely that'll be good for their own business.
Indeed often HVAC suppliers and manufacturers, recognizing that their principal customers are service companies and installers, are reluctant to talk with a homeowner as it infuriates their main customers.
When I first walked into a local HVAC supplier in NY, running my own heating service company with a partner, we had to hand over a business card and convince the supplier that we were "in the business" too (which we were) just to get the time of day. Even then they were nervous, as there were other old-time customers sitting around (looking at the girlie calendars) and who were less than welcoming to a new competitor.
Over time, when the fellows saw we were serious (it helped to walk in through the back door and to arrive covered in soot and smelling of oil burners as well as to know the common names for controls and parts) our relationship blossomed and the supplier was very helpful.
In the 1980's I was working on oil fired heaters in partnership with a woman engineer who was very sharp. Carol S. had built her own car, solar panels, designed computers, and other stuff. And she was very straight and clean-talking (dinked was about the strongest language she'd use). I stepped carefully in demeanor and speech so as to avoid stepping in something. Carol was the boss.
So when she got particularly sooty while squeezing around a dirty oil fired boiler, and when two round black blobs of soot marked her coveralls, I didn't even think of saying a single word about it. Stay focused.
We walked into the service area of JEM supply in Poughkeepsie where about six old timers were sitting around jawing when they should have been out sucking the soot out of oil fired heaters. Silence fell instantly, rolling in not on cats feet but like a silent tsunami of fog. The old guys stared. Behind the counter you could have heard a pin drop.
Nobody said a thing. They all looked at me. They looked at Carol. They looked back at me.
Carol was the boss of our outfit and had more time in the field than I did, but obviously, and as soot blotches demonstrated, she was a woman. Those blots didn't compute easily for the fellows over at JEM supply. They looked at Carol again. They looked away. They looked at me, expecting for me to speak as we walked up to the counter. It was a woman, but obviously we were working on heating systems: Carol was even sootier than I was. We both smelled bad. At least I did.
Carol stepped up to the JEM Supply counter and spoke in her own deep and confident voice.
"Gimme a Sunstrand J-pump 43A, left-hand rotation, with the internal bypass factory set, and a 23-A pump strainer"
Yes maam, said the JEM clerk in a too-loud voice. We took our parts and left. The tsunami of silence surely receded but not before we were driving our truck out of the lot.
See DINK FACTOR - Remember Carol S for another version of this report about our heating company. But don't call us for repairs.
Back to bidness: Am I right that you're not seeing any error code on the L7224 aquastat display?
The LO range is between 110 and 120F (typically) but the company notes that IF this L7224U is being used to replace a Honeywell L8184A,C L7184A or L7284A,C then the LO Limit must be disabled - a step performed using the adjusting settings of the control (not by changing a jumper wire as is done on older aquastats). The instructions describe how this disabling is performed (Press the UP and DOWN buttons at the same time for 3 seconds, then pus the I button until LL_ is displayed, then press the DOWN arrow until you see OFF)
If you're still stuck we could see if Carol will come out of retirement and give us a hand.
(Feb 9, 2015) AUTHOR:Gregg (no email)
COMMENT:Hi Dan, I messed around a little more last night, this unit would work (better)if the LO-LIMIT diff was on the upper side of LO-LIMIT but it definately is not. I proved that to myself again last night. with LL set at 140 (diff 10)circulator will draw boiler temp down to 130 (Diff at 10) and down to 115 (diff at 25) On temp rise, burner still shuts off at 140. SO my settings now are HL 200 Hdf 10 LL 180 Ldf 10 Am I reading something into this that isn't there? I'm sure I read that the LL diff of -10 is fixed and the adjustable up to 25 goes plus from LL MINUS 10 PLUS 25IN other words a LL of 140 and adj diff of 25 nets me 155 on temp rise before the ciculator is "ALLOWED" to turn on? Is this correct?
Page 5 of the Aquastat instructions found at inspectapedia.com/heat/Honeywell_L7224U_Aquastats.pdf describe four operational states of the control and gives operating sequence and troubleshooting tips - on p. 5 and continuing.
As I read the company's drawing, for this aquastat the LO-DIFF is a temperature set *below* the LOW LIMIT. **IF** there is no call for heat, then if boiler temperature falls below the LOW LIMIT minus the LO-DIFF then the switch will "make" and the burner will turn on.
And **IF** there is still no call for heat, then the burner will remain on until the boiler temperature reaches the LOW-LIMIT setting.
IF there IS a call for heat the circulator will not run until boiler temperature has reached (or passed) the LOW-LIMIT setting. At that point the circulator will turn on. (The burner may happen to be on or off depending on what has preceded). P. 6 says that
on a call for heat the circulator starts when the water tempeature is above the Low Limit setting (if applicable). Boiler temperature is checked. Burner starts when water temperature is below the HIGH LIMIT setting.
The burner will be turned off if
- the HIGH LIMIT is reached or exceeded
- the thermostat is satisfied (this will also turn off the circulator)
Continue reading at AQUASTAT HI LO DIFF SETTINGS that explains the best settings to use & how & when to disable the LOW & DIFF controls or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see MANUALS for HEATING SYSTEM CONTROLS for a list of aquastat installation & repair guides
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1 March 2015 Well sensor Resistance said:
I am sure my well sensor was bad not only from the condition of it but from error code 1 being displayed. Is there a resistance check that can be done throught the connector to determine this. I am sure the controller is smarter then me but I just want to confirm.
Symptoms included short cycling after lo limit was reached and of course error 1. I pulled out the well sensor and saw it was kinked abnormally. I bought a new sensor and mounted it in a new 3/4" deep well for better immersion and mounted the control directly on the well. I have since ordered another well sensor so I can remount the control to it's original location on the boiler panel. At the time I did not realize there are optional sensor lenghts available for different mount locations.
Anyone have any resistance values for the sensors.
I'm looking but haven't seen this test value.
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