gas furnace turns off gas after 2 minutes


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Old 01-12-19, 03:34 PM
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gas furnace turns off gas after 2 minutes

Hi,

I have a tempstar furnace with a Honeywell S8610U ignition module and a Honeywell q3451 style burner pilot (it has a single spark/flame rod, so there is only one connection to the spark on the ignition module). When calling for heat with a nest thermostat the exhaust fan turns on, pilot lights and furnace burners light up. After a little while the fan kicks in. This seems to tell me that the limit switch is operating correctly as it is turning on the fan once furnace warms up. All good so far.

After about two minutes the burners shut off, and power is cut to the thermostat. The fans continue to run for another minute or two (I am assuming this is due to the limit switch not reaching its cool limit yet).

After approximately 30 seconds, once the power is restored to the thermostat, the pilot will light up again and repeat the cycle. Sometimes I have to adjust the temperature on the thermostat in order to force the heat to come on again.

The ignition module does not report any specific error codes, it seems to only report the flame current (which seems to fluctuate between 4μAand 7μA.

If anyone has suggestions what could be causing the temporary shutdown, or what should be looked at it would be greatly appreciated.


thank you
 
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Old 01-12-19, 03:50 PM
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I am assuming this is due to the limit switch not reaching its cool limit yet.
No.....your problem is your furnace is overheating and opening a high heat limit switch. When that happens the burner shuts off but the blower stays on to cool the heat exchanger. That limit switch is self resetting and the burner will restart.

You need to find out why it's overheating. Dirty filter, too many closed registers, blocked return grill, A/C evaporator coil clogged with dust are all possible problems.
 
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Old 01-12-19, 03:52 PM
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Just from what you described, as an observation it sounds like your furnace is reacting to your stat and probably do not have a problem with the furnace itself.

Have you tried bypassing the stat to see if the furnace runs normally.
 
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Old 01-13-19, 02:36 PM
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thank you so much for the quick responses.

I have tried bypassing the thermostat and the same behavior occurs. I am pretty sure it's not the thermostat since the power goes out on the red wire to the thermostat after the burners shut off. This leads me to believe that it is some kind of limit that has been reached.

thank you PJmax for the info, this definitely sounds like the problem. I did not know that the blower continues once the upper limit is reached by the limit switch, but this makes perfect sense. The timing of the shutdown is almost exact every time the furnace starts so it must be the limit switch triggering the shutdown otherwise it would be more random. The filter is brand new and all of the outlets in the home are open, the unit gets very little usage. I was wondering if it is worthwhile trying to replace the limiter switch, it's a cheap and easy attempt to try out before calling a tech. Current one is 60t11 L200-20F, which as I understand turns the fan on at 180 and shuts off the furnace at 200, I see online 200-40 models which I am guessing would just open up the fan earlier.

I'll have to do some more research to see if return grill or a/c evaporator coil cleaning is something that I am able to look into myself.

This may have been a problem for a while in my furnace, but my old thermostat was not able to detect and report power cut on the red wire. It had a backup battery so I am assuming it would just relight the burners and continue.

btw. I had a repair person come in and they fiddled with nest for 30 minutes thinking that was the problem (which is easily eliminated by bypassing the thermostat). In the end they quoted to replace the pilot burner and clean out the pilot orifice but that made no sense to me so it is really helpful to get your opinion.

thanks again
 
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Old 01-13-19, 03:38 PM
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The R supply to the thermostat is after all the limit switches so that if any open.... the stat goes dead and you know there is a problem. With battery stats this problem is not readily visible.

An L200 would be a high limit safety. The blower should be on well before that. Typically a blower stat is an L140. There will be several safety stats on the furnace. One or two high heat limits. One or two flame rollout switches.

There may have been an issue with the pilot but that wouldn't allow the burner to run for two minutes and shut down.
 
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Old 01-13-19, 04:37 PM
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thanks PJMax, yes the blower kicks in pretty soon after the burners start. I only see one L200 switch at the back which I am assuming is the fan limit switch (L200). Is it safe to assume that if the low limit is triggering the fan, the switch is operating correctly and detecting the high limit which would make it trip? Or is it worthwhile giving replacement a shot?


There seem to be two rollout switches but they don't appear to have tripped (I could not press down the reset switch on either). The bottom part of the switches (metal sensor) however does seem a little rusty. The flame seems to be getting pulled inside of the furnace so I don't really see how a flame could reach them, unless the metal bracket holding them is overheating and somehow causing them to trip (and the reset button is no longer functional).
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Old 01-13-19, 04:42 PM
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Flame rollout does happen. It can be caused by overfiring, a defective heat exchanger or a partially clogged flue. That furnace may use a timed blower on/off function thru the control board.
 
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Old 01-13-19, 06:33 PM
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Dex,
Just throwing this out there since you are playing with this anyway. What you can do is just remove the overheat switches, one at a time by just removing the screws and leaving them wired away from the heat to see if any of them kick off the furnace.

If it is one of the switches, with it being removed from the furnace it wouldn't feel the heat and the furnace should run its normal cycle and could narrow down the problem without just parts changing.

Just a thought, hope it helps a little.
 
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Old 01-13-19, 06:47 PM
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It sounds like a good idea but could be risky. If the furnace is overheating and you unmount the high heat sensor.... it won't shutdown. Best to use a voltmeter to check them in operation.
 
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Old 01-13-19, 06:48 PM
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thanks, might give this a try.
I also tried removing the air filter on the intake just to see if the new filter is restricting the airflow too much, but all it did was buy me around 30-40 extra seconds of runtime before it tripped.
The old thermostat had a battery in so it was probably masking the problem for a long time.
 
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Old 01-15-19, 10:04 AM
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just wanted to say thanks again for all the suggestions. I hoked up a voltmeter (with alligator clips) across the limit switch in the back of the furnace (set on V dc) and measured the voltage. It measures 0 until the burners shut off at which point it measures around 30+. So at least now I know for sure that it is the limit switch tripping and the furnace is overheating.

I am wondering if anything can be deduced based on the temperature coming out from the outlets in the rooms, the air is not very hot but the air flow is fairly good. Just thinking if there is an easy way to tell if the AC evaporator coils are clogged or not.

When I first moved into the home the filter was filthy so the switch may have been tripping for years and not be noticed (due to battery backup in the thermostat), perhaps over time this messes with the limits of the switch. I think I might still try replacing the limit switch in case it is getting tripped earlier than necessary but this may just be my wishful thinking.

thanks again!
 
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Old 01-15-19, 10:12 AM
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only visual inspection or a static pressure drop test across the coil can help you determine if the coil is plugged.
 
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Old 01-15-19, 10:41 AM
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Good airflow and not very hot air seems to point to a defective limit switch, perhaps tripping before it's set point. I'd change that switch and see how it operates.
 
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Old 01-15-19, 02:43 PM
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just a thought...had very similar problem once....but my furnace gave an error code...found a plastic bag partially obstructing the outside air intake..would also check the flue as pjmax suggested.
 
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Old 01-15-19, 04:18 PM
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I only mentioned the flue because the OP asked how a flame rollout switch could trip.
There is not a flame rollout problem here.

Doesn't pertain to this thread.....
Blocked air vents for a two line system can be a problem. Many aren't covered with a screen and are a delightful place for kids toys..... and little balls. Snow can also be a problem on low exiting pipes.
 
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Old 01-20-19, 02:00 PM
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today I tried replacing the limit switch and noticed that there is a column behind the limit switch that seems to be damaging the limit switch (it is slightly touching it and burning itself onto the switch). Does anyone happen to know what this column behind the switch is and if it should be touching the switch?
 
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