Heater Aquastat vs Boiler Short Cycling FAQs

Boiler cycles on and off continuously Q&A
Boiler cycles on and off too rapidly Q&A

Aquastat Problems Causing Boiler Short Cycling

Tip: See step by step diagnosis and repair advice at SHORT CYCLING HEATING BOILER 

If the boiler turns on and off too rapidly - is it due to a bad aquastat?

My boiler is firing on and off rapidly.

It seems that the contactor inside the aquastat that controls the burner is opening and closing when heat or hot water is called for. Can this be replaced individually or does the entire aquastat need replacing? (Feb 19, 2014) Mark

Reply:

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

Then have your service tech check that the aquastat's temperature sensor is working.

See AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND

Mark Thomas said:

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

Reply:

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

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

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

One would think that the problem then would lie in the

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

BUT if your system is wired as they usually are in the U.S., it is the heating boiler temperature that is sensed by the aquastat on the boiler (not on the indirect fired water heater tank) that turns the burner on and off.

If that equipment is working properly the boiler will turn on when boiler temperature (not your indirect water heater temp) drops to the LO or cut-in and the boiler would keep running until boiler temp reaches the HI or cut out temp.

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

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

vs

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

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

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

See complete diagnosis and repair details

at SHORT CYCLING HEATING BOILER 

Heating boiler short-cycles on and off until thermostat is satisfied

@InspectApedia Publisher,

I am in the US.

What we're trying to figure out is why the boiler comes on for 4 minutes, reaches it's HI limit and goes off for 8 minutes, repeatedly until the room is at set temp which is often many hours.

I have observed that the 8 minutes is all it takes for the boiler temp to cool off 10-11 degrees to get below the Hi temp differential so it can come back on. The circulator pumps seem to be working.

We've also just discovered there may be an issue with the thermostat at well. On 2023-01-15 by Anonymous -

Reply by InspectApedia Publisher

@Anonymous,

Yes we understand that that's the question; we've suggested several things to check, including both settings (which you are confident are correct) and over-firing - something that your heating tech can examine.

It may also be diagnostic to consider when this boiler on-4 off-8 minute cycle first occurred. Since new installation, since work was done on the boiler, or since something else changed?

If you're in the U.S. and your heat is controlled as typical,

The articles in this series explain how the aquastat turns the boiler on or off in response to temperature:

The thermostat, on a call for heat, turns on the circulator for that zone.

The aquastat, in response to the temperature in the boiler, will turn on the burner when needed.

Keep in mind that I'm quite limited in what I can see of your system from here .

Seems as if the next thing to do is ask your tech to check the firing rate - the nozzle size in gph.

by Anonymous - how do I find out the right firing rate for my boiler?

@InspectApedia Publisher,

I assume the firing rate isn't something they can tell me via email or phone and it's at the point where we have no option but to set up at service call? They charge $149/hour and I don't understand why it's not covered by my "gold contract". I believe it had been in the past.

I believe I've read all articles suggested. We know it has been taking hours to come up to set temp for years now. I t may have coincided with changing the thermostat or it may coincide with having the aquastat replaced. We're not sure.

Reply by InspectApedia Publisher

@Anonymous,

No it means that the technician has, if you are correct in the number you give is actually the nozzle that's actually installed, downfired the boiler which would be in response too and overfired boiler in the 1st place. Now it's time too get actual data on what's actually installed.

Ask your tech to check the firing rate - the nozzle size in gph Of the nozzle that is actually on the burner.

And then ask the technician to investigate what you have described as a too short and too frequent burner on cycle.

Please report back to us what you are told when you have done that.

You need a heating service call at your home, by an experienced service technician.

Do let us know what you're told - what she or he finds - as that will help other readers who've slogged through this problem.

by Anonymous - boiler firing range on burner label?

@InspectApedia Publisher
I went down and found that label. The way I understand the info online, since the label does not show a "firing range" but does show 2 nozzle sizes, does that mean the year a 60-80A was installed, it was not a correct one for this burner?

by InspectApedia Publisher - Oil burner nozzle spray patterns and delivery rate

@Anonymous,

Please use our on page search Box and search for the phrase

Oil burner nozzle

And you will see we have articles explaining about spray patterns and angles and delivery rate in gallons per hour. That will be more accurate and thorough than if I try to make that up de novo here off the cuff.

The firing rate given on the data tag in hyour photo is 0.85 along with the nozzle type and pattern, but that tells us what the oil burner manufacturer or boiler manufacturer recommended, NOT necessarily what nozzle a service technician has installed.

by Anonymous

@InspectApedia Publisher,

I was just looking back at the service records I got from my provider and it looks like they've always used a 75-80B nozzle (no brand listed) but one year they used a 60-80A Delavan.

I'm trying to decipher the info here about nozzles. Are these 2 nozzles interchangeable or if not, which is the proper size? The boiler is a Beckett RSA85LN-TB. It appears the oil filter has always be #3.

by InspectApedia Publisher - checking the firing rate of the oil burner

@Anonymous,

About the changing the delay back to 5 - making no difference, that's a helpful diagnostic step. So we're back to checking the firing rate of the oil burner.

by Anonymous - TX9100uc heater settings on our L7224& Aquastat

@InspectApedia Publisher,

I also apologize for how frustrating it must be for you to work with someone who knows nothing about these systems.

When you mentioned the dipswitches, I realized that I typo'd when mentioning which model I have, which is actually the TX9100uc https://www.luxproducts.com/wp-content/uploads/2017/07/tx9100uc_manual_en.pdf

This model has all the settings accessed through programming, and my settings include:
Early recovery: on
System mode: furn
Fan: gas (what directions say for a boiler)
Delay time: 2. default is 5 but I was trying to force it to run more often to get it up to temperature.
Temperature swing is set at 1 (range 1-9 with 1 being smallest) Trying to force t to keep temp once it got it.
Offset: 0 because a boiler doesn't have second stage heat. (default 2, range 1-9)

As far as the backlight apparently sending a signal to the boiler, the manual says: The display screen is lighted to assist viewing at nighttime, or in locations with low light levels. A press of any button on the front panel will light the display for approximately 10 seconds.

The L7224U aquastat settings:
HL_ is set at 190 (was 180)
Hdf is set at 10
LL_ is off
duu is off
ASC is set at 0 sec
tPL is off
PC- is on

Do all these settings look good to you?

Last night I changed the delay back to 5 as suggested, and this morning I see it makes no difference. The boiler is still doing the 4on/8off sequence.

by InspectApedia Publisher

@Anonymous,

About your thermostat

Delay time: 2. default is 5 but I was trying to force it to run more often to get it up to temperature.

Try setting that back to 5 and see what happens.

by Anon

@InspectApedia Publisher,

Ok, I've changed that setting already. Thank you again. Heat is already set lower for the night, so I will check back tomorrow.

I do wonder if the thermostat is malfunctioning somehow as well as whatever the other issue is, since the instructions say to hit any button to turn on the backlight. Earlier when the set temp and room temp were the same, it clicked and flashed the set temp 3 times then "heat" 3 times then the boiler came on.

However, after being near the thermostat checking settings, and the detected temp raising above set because I was near, I tried again out of curiosity to see if that would happen again and it didn't.

I'll experiment with that again tomorrow as well, but I doubt it's a coincidence.

Trouble with LUX 9100U thermostat control signals to burner

@InspectApedia Publisher,

This conversation actually started on Jan 4. When it was moved to here to aquastat, I started a new comment/question.

We're now wondering why the Lux 9100u thermostat sends and electrical signal to the burner when I'm only touching a button once to turn the backlight on.

It takes 2 pushes to change anything. On 2023-01-15 by Anonymous -

Reply by InspectApedia Publisher -Temperature swing vs burner cycles per hour

@Anonymous,

You probably have the LUX9100U THERMOSTAT MANUAL [PDF] Lux Products, originaL source: https://www.luxproducts.com/wp-content/uploads/2017/07/tx9100u_manual_en.pdf.

I don't see any discussion from Lux that approaches your question of when the TT is signaling the boiler. For sure, you'd think that just touching to wake up a display ought not "do" anything else.

BTW there are some dip switch setting recommendations in the manual that you might compare with those on your thermostat.

Do take a look at their discussion of the Swing Setting starting on p. 23:

TEMPERATURE SWING AND OFFSET SETTING: A thermostat works by turning your heating or cooling system on and off whenever the room temperature varies from the desired set-point temperature.

The amount of this variation is called the swing. Generally your system should cycle on about 3 to 6 times per hour. A smaller swing number makes the system cycle more frequently, so the room temperature is more precise and constant.

A larger swing number will make the system remain on for a longer duration each time and decreases the number of cycles per hour. There is only one Swing setting, and this determines the cut-in and cut-out points for both the first and second stages (if present), in both Heat mode and Cool mode.

Why is our heating boiler firing at a very regular intervals

Why is my hot water baseboard boiler firing every 24 minutes for about 2 minutes and 40 seconds each time even though the thermostats are off?

The boilers seems to be on a timer but the thermostats are off.

I have a Honeywell triple relay aquatstat on the boiler. Thanks. On 2017-01-05 by John

Reply by (mod) - check for a shorted thermostat wire or ...

John,

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

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

Look for a circulator that never shuts off and a/or a thermostat that is not being satisfied (or shorted thermostat wires that call for heat continuously). - Daniel F.

by John

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

I have a gas water heater that is not heated by the boiler.

I was thinking about disabling the LO limit but the regular interval firing is the confusing issue. Thanks.

Causes for Hydronic (hot water) heating boiler rapid on-off cycling of the burner

Causes for Hydronic (hot water) heating boiler rapid on-off cycling of the burner

OK. So if the circulators are on the return side of the loop, and the pipe coming into the circulator is hot not just right at the circulator but several feet away from the boiler then hot water is circulating through the system.

When a heating boiler is cycling on and off to frequently but we are certain that hot water is running properly through the piping and radiating devices, then there are other things to check including

CIRCULATOR PUMP RUNS INTERMITTENTLY

- be sure to step through the diagnostic suggestions in this article as some of them could explain what you are experiencing.

AQUASTAT HI LO DIFF SETTINGS - above on this page

AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND

Watch the boiler temperature and pressure gauge when the boiler runs.

If its aquastat and aquastat sensor are working normally, when the boiler turns OFF the water temperature will be close to the HI setting on the aquastat and when the boiler turns back on, depending on the aquastat model, the water temperature will be 10-15 degrees below the HI.

You didn't give the fuel for this boiler. If it's heating oil and if the boiler is over-fired (oil burner nozzle size in gph or oil pressure too high) that could also cause the boiler to "get ahead" of the building's heating piping: water in the boiler heats up very rapidly and then is exhausted quickly by the circulator.

Let me know what you find. On 2023-01-15 by InspectApedia Publisher -

by Anonymous

Thank you, I appreciate the help. I know how busy you must be and you still find time to help on this board

Is it odd that the lower limit on the aquastat was turned completely off? The boiler temp was at 70 when we looked at it this morning and when it first came on awhile later for the day setting, it had to come up 110 degrees before it turned off at the high limit of 180. It took maybe 15-20 minutes.

It was only at Christmastime this year during sub-zero temps when factoring in wind chill (very unusual here) and the heat didn’t come up even 2 degrees after literally all day, that I started timing it and saw it runs 4 minutes and off 8 before coming back on again for only 4. This morning was the first time I noticed it was on much longer when first on in the morning.

by InspectApedia Publisher

@Anonymous,

The low limit on the aquastat should be off as you don't have a tankless coil for your hot water.

About the temp not coming up quickly - that sure looks as if either a circulator isn't running OR a heating line is air bound.

PLEASE READ AIR-BOUND HEATING SYSTEMS - home

as that will be more complete than having me try to re-create all of that guidance here, off-the cuff. Let me know if anything you read seems unclear.

by Anon

Does the transformer look normal to you?

Any thoughts on why the gauge at about 172 doesn't agree with the aquastat at 180 or how to fix that?

It looks like the other settings I tried posting didn't actually post. Do these look right?

HL_ is set at 180
Hdf is set at 10
LL_ is off
duu is off
ASC is set at 0 sec
tPL is off
PC- is on

According to the manual, there should be more settings but this is all it shows me when cycling through.

As far as the Lux 9100uc thermostat, I have the temperature setting at 64 for night and 66 for day. I used to let it go lower at night but for years now I don't set it higher when I'm the only one home (and use a small electric heater) because oil costs so much, the last couple years, especially this year.

I believe these problems started when we replaced the old thermostat because it stopped holding it's programming, or it just occurred to me that it may coincide with the aquastat being replaced.

There are other settings I've tried changing to try to make the boiler keep heat at set temp or even reach it, but they don't seem to make a difference and it still takes hours to come up even one degree. It was only this year that I started timing it and see it runs 4 minutes and off 8 before coming back on again for only 4.

This morning was the first time I noticed it on much longer when first on in the morning which seem to me to coincide with your idea that the high and low settings may be off.

Delay time is set at 2 with default being 5.
Temperature swing is set at 1 (range 1-9 with 1 being smallest)
Offset is a 0 because a boiler doesn't have second stage heat.(default 2, range 1-9)

I also realize that a high pitched noise when the boiler first came on is gone. I'm not sure if that's good or bad, but I believe it was there least year as well, maybe longer.

by InspectApedia Editor

@Anon,

Those are reasonable settings

Boiler short cycling for 4 minutes on

Does anyone know why an oil-fired boiler baseboard heat system would only run for 4 minutes then go off for 8 minutes, then back on for only 4 minutes, repeatedly, even when set temperature on the digital thermostat has not been reached? On 2023-01-04 by Anon -

Reply by InspectApedia Publisher - oil-fired boiler baseboard heat system cycling

@Anon,

There are various causes of heater cycling:

1. The boiler is getting up to its shut-off t(HI LIMIT) emperature in just 4 minutes possibly because a circulator isn't pushing hot water around the heating zones very fast or not at all, or because the system is air-bound or partly air-bound.

2. The boiler heats up to its HIGH LIMIT very quickly because even though circulators are running, the boiler is over-fired: the oil burner nozzle size is too big (too many gallons per hour) for the heating application

See our detailed diagnosis and repair advice at SHORT CYCLING HEATING BOILER