Hot Topics: Burner Overheating

bright blue stove burner flame circle's Forum features conversations between more than 250,000 experts and novices in over 120 categories. Each week, we highlight a conversation that may help you with a project. This transcript has been lightly edited.

Original Post: Burner overheating and sometimes continues to run when turned off?

TFastle - Member

Hello, in my home I have baseboard heating with a Burnham P-206-W gas boiler. The night before last I woke up to some banging noises, jumped out of bed and went to the mechanical room. It was very warm and the boiler temperature was near max (210-220ish I would guess, didn't have my glasses) and making hissing noises.

I quickly unplugged it but was at a bit of a loss as to what to do. After a bit of thought I grabbed my gloves, pulled the front panel and shut off the gas valve. I then plugged the system back in so the zone valves would open and the circulation pump would kick on and cool the very hot unit. Obviously, since the mechanical room is an interior room in the house this was a bit of a concern and led to very little sleep.

The next morning I called the HVAC guy that has done a bit of work on it in the past ( helped me add a couple of zones for an addition). He came out and thought that is was likely there was air in the system and it air locked. He purged the system, fired it up and ran it through a couple of cycles. It seemed to work fine but I did notice that one time it heated up beyond the 170 it had been shutting off at. I paid him, he left and it seemed to be working fine. Long story short, I wanted to be absolutely SURE there was no issue so I spent the day monitoring and writing down the On/Off cycle times and temps of the boiler.

They were not as consistent as I had hoped, usually kicking off (turning off the boiler) about 165 but sometimes at 155 and even up to 180 once. Then it went to 180 again and continued to run. Here's where it got odd—I unplugged the power but the boiler kept running full steam. I am not an expert but I thought I had remembered that it would (and should) shut off when powered down. It didn't. I waited a minute or so and it did not shut off. Since the circ pump was now off the heat began going up fast so I pulled the cover and shut the gas valve, got to about 200.

At this point I have decided that it had nothing to do with an air lock and I had a faulty Aquastat (Honeywell L1848 J 1009). I had replaced it a couple of years ago because the contacts for the circ pump had gone bad. I called the HVAC guy and he came back, again purged, ran it through some cycles, agreed that there was some issues with how consistent the kick on and off temps were and said he would get me the Aquastat if I wanted to put it on.

I did that yesterday, got it up and running in the afternoon and then ran the unit for a couple of hours closely monitoring the on/off cycle temps. After adjusting the temp, it seemed to be working fine although once, early in my monitoring, it did go to 180 (had it set for 165ish) which concerned me. Later I decided I would turn it off for the night and monitor it again in the morning. Well, I unplugged the power and low and behold, again the burner continued to run (not the pilot light, the burner). At this point I am pretty sure something is wrong since all the other times I have unplugged it the burner shuts down immediately (with the exception of the time above that I described before replacing the Aquastat).

Well, another night of very little sleep. Like everyone, since my family is in the house, this has me a little worried. I am not sure what happens if my burner runs full tilt until there's a failure but, whatever that failure is, I'd guess it's ugly. So, a couple of questions. Am I correct in that my burner should shut off when the power is cut?

If so, then what could cause this? My guess is it had nothing to do with the AquaStat and that I have a problem with the gas valve - probably sticking intermittently. It looks like the "gas valve" is a block of components, mechanical and electronic. In a situation like this what part of the "gas valve" is likely the culprit (I would guess the mechanical valve or a solenoid) ? Do gas valves go bad sometimes (never, rarely, often)? Does this make sense or am I way off base? Should I be looking elsewhere?

Sorry for how long this is but I wanted to include the pertinent information. Any input would be GREATLY APPRECIATED! I would like to get this thing going before the weekend - supposed to be cold! )

poorplmbr - Member

What is it you are exactly "unplugging"? A boiler should NOT have a plug! It absolutely should be hard wired in with an emergency switch outside the boiler room and a service switch at the boiler itself. As for running hotter than the set point, this can happen when the boiler is heating a small zone.

Grady - Forum Topic Moderator

You unplugged the boiler & the burner kept firing? If so, it does sound like a stuck gas valve. Gas valves are not repairable & must be replaced. This isn't something someone without the test instruments should do.Gas pressures need to be checked & sometimes adjusted.

The boiler being plugged in concerns me. There should be TWO switches, wired in series. There should be a service switch at the boiler (within reach when working on the burner) and a second emergency switch with a red wall plate located outside the mechanical room.

TFastle - Thread Starter

It is not hardwired and never has been. So I am unplugging the power cord to the Aquastat to turn it off. Did not seem to be a problem when inspected for addition, nor have the three HVAC professionals that have seen it seemed to think it was an issue. While hard wired and switched would be nice it's not part of my problem.

Why would the boiler sometimes exceed the upper limit when heating a small zone? It's my understanding that if the upper limit is 170 then, when the temp of the boiler get's to 170 the Aquastat should shut it off whether one zone or multiple zones are calling and this has been my experience in the past.

TFastle - Thread Starter

Thanks for the reply Grady.

Yes, the burner was fired with power to the boiler disconnected, twice now. This last time (last night) I was surprised and waited a minute or two to see if it would go off and it did not. At that point turned the gas valve to off and it, of course, shut down. So a stuck valve is not unheard of?

As I thought about it last night it's really the only thing I could come up with, a valve that sticks intermittently. When you say "gas valves are not repairable" I assume that means the whole block that comprises the valve is only sold as one unit and must be replaced.

Also I noticed that the insulation on the wire that goes from the Aquastat to the gas valve (looks like standard two strand wire, almost like a lamp cord, probably 16 or 18 gauge) has melted slightly, I'm sure from when it got so hot. Should that be a special kind of "heat resistant" wire?

With regard to the switch, I really never thought about it and it's never been mentioned. The house is 30 years old (I've been in it for six), been well maintained but maybe that has something to do with it. Regardless it would be good to have it hard wired and switched and will look into getting that resolved as well.

Again, thanks for the input!

Grady - Forum Topic Moderator

It's not uncommon for a boiler to overshoot it's high limit by 10* or so but 20+ is too much. With you killing power to the aquastat & the burner continuing, it has to be the gas valve sticking open.

Grady - Forum Topic Moderator

Sorry, I missed the part about the wires going to the gas valve. Factory wiring to a gas valve is almost always wire rated for 105*C. If the wire jacket has melted, that usually would indicate a lack of draft issue causing excessive heat on the wires.

TFastle - Thread Starter

Appreciate it Grady.

I have the HVAC guy coming out on Monday to replace the gas valve.

You were correct on the wire, it was 18/2 limited fire protective signal cable. I locates some fire protection 18/2 and replaced it with that. I suspect, the night that it woke me up is when it got melted. The gauge was very near maxed if not maxed which is scary. Do you have any idea, under normal use with it set to cycle off at 160, how hot the cabinet (metal cover - top in particular) should get. The HVAC guy thought it was bit hot.

When it's running it does get to the point where you don't want to lay your hand on more than a couple of seconds. I haven't taken the temp yet but plan to. It does seem hotter than I remember it but suspect it's because it's been off most of the time the last few days and, when it is on, all zones are calling all the time and it's running more than if the house was up to regular temp.

I am running it now and watching it closely. I set the temp dial on the Aquastat to 160 (based on the looks I am guessing that dial is a fairly rough estimate) and it's been cycling off regularly at 154-155. I thought it was suppose to cycle back on 20 degrees lower (or is that not correct?) but it's been kicking on mostly around 143-146 but even kicked back on one time at 149. Is that odd or cause for concern or more related to the fact that I am on the lower end of the temp range?

Again sorry for the length but hope to get this right soon. Your input is very much appreciated.


Grady Forum Topic Moderator

As I recall, some variants of L8148 have fixed differentials others are adjustable but don't take that as gospel.

Regarding the top jacket being "hot", most have fiberglass insulation. I wouldn't normally expect the top to get too hot to touch. Your insulation may need replacing. Easy enough to do but make sure any combustible (paper) backing is removed.

TFastle - Thread Starter

I did pull the top cover and there is insulation there but it did look at bit tired. I will poke around and see what is good for that and get it replaced (unless you have a specific suggestion—sans the paper of course).

The unit seems to be running flawlessly this afternoon and evening although I'll continue to watch it closely and turn if off at night until I have the new gas valve installed on Monday.

Mucho Thanks for the input, help and suggestions!

Grady - Forum Topic Moderator

There is a material called kaowool but it's hard to find in sheets. If there's a company around you who installs commercial kitchen exhaust hoods, they should have it.