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Old 01-23-2014, 09:11 PM   #1
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Furnace Troubleshooting Question

The past 2 mornings I've woken up in the morning to a colder house. I have a Nest thermostat and the house was colder that the set temperature. In both cases, the thermostat was 'orange' and indicating heating.

Yesterday morning I noticed that the there was no combustion in the furnace, however the exhaust fan was on. I flipped the breaker off/on and lower/raised the heat. after that, the next call for heat turned the exhaust fan on and burners ignited.

This morning it was the same, however I couldn't immediately get the burners lit with what I had done yesterday. Since it was day 2 of the issue, I noticed that after the exhaust fan turns on, the burner igniter (no pilot light, not sure what it's called actually..) doesn't glow orange. After doing the song and dance, I started to get ready for the day, and after about 20 minutes, lowering and raising the temperature caused the burners to eventually light. we had a furnace repair guy here today and because the situation eventually was resolved, he couldn't troubleshoot that. everything he checked out looked ok.

i'm not quite sure how to troubleshoot this if it happens again, how would i go about doing that or what should i look out for. i have a multimeter and don't have any problems measure V. to my knowledge, the voltage for heat is low voltage and a constant signal. is that correct? the circuit doesn't transmit data does it?

any help would be appreciated.


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Old 01-23-2014, 11:00 PM   #2
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I had a similar problem with my furnace a year ago. The combustion fan would come on, and after a few minutes it'd just go off, and the igniter never fired (as you described, the heat coil never activated.)

Turned out for me it had to do with a drain plug. There's a drain plug on my furnace that the installer never removed. Turned out water had built up inside the furnace and found its way to the limit switches. These are the switches that indicate whether there is sufficient air pressure and so on for combustion. I took the plug off the drain line (a waterfall worth of water gushed out!), put a hose on it, and replaced the limit switches and have not ever had a problem since.

Not sure if this is actually required, but in my case it did solve the problem. Worth looking at, since you still are able to ignite sometimes. Even if it's not a water issue, it could have to do with the limit switches. They are mechanical after all and I've heard they can fail. I paid something like $30 per switch for two of them to replace them, and the job only involved unscrewing the old ones and attaching some little rubber hoses and wires to the new ones in the same configuration as the old ones, and then screwing the new ones into place. (Obviously, turn off your gas valve and disconnect all power to the furnace before opening it!) Haven't had one misfire since.

Other than that wait for more qualified experts. ;-)



Last edited by fdmillion; 01-23-2014 at 11:02 PM.
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Old 01-24-2014, 11:27 AM   #3
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Check to ensure the air intake and exhaust piping outside doesn't have ANY snow within 12" of it. Also at night when its really cold condensation can form on the piping out let and especially the bug screen if installed during a call for heat. When the tstat is satisfied the furnace turns off and then the condensation will freeze over the opening. The next time the furnace tries to fire it thinks it has a blocked exhaust and wont come on giving you a "Pressure switch stuck open" or some such error code. By time the service tech shows up the next morning it has had time for the sun to hit it and thaw it out enough for the unit to work- no faults to be found.
So check your exhaust piping.
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Old 01-24-2014, 11:37 AM   #4
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Look for a flashing LED which gives you an error code B4 resetting it and find the code in the manual or on the back of one of the doors and let us know. Check your vent pipes too. Post the brand and model # and some pics of it with both doors off.
"Cut it twice and it is still too short".
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Old 02-23-2014, 07:37 PM   #5
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after my last post the situation was intermittent, maybe 1x per week. after some troubleshooting, i learned that my multimeter is shot and had to get a new one. finally got the new one in and checked the pressure switch. looks like it closes fine as both wires measure 24v against ground.

the furnace vents up through the chimney. would freezing exhaust still be an issue?

also, it happened again tonight. we went out for a bit, came home the exhaust fan was on but no pilot or flame again. so brought out the multimeter again, pressure switch v looks ok. turn off the tstat, and back on they light. we've had some mild weather here in the NE the past couple of days, and the temp has been above freezing. so i imagine that it wouldn't be a condensation issue?

i don't see any flashing lights anywhere, and even if there were, there is no window in the circuit board bay. once i open that door, the door switch opens, and when its closed (by the door or my hand), the furnace may kick on.

i caught it the issue happen once, and it didn't look like the smart valve ever attempted to ignite the pilot. it could also have been that i missed it glowing orange, but i think i would have caught it.

anything else that would contribute to no pilot light besides the pressure valve? any suggestions on testing the smart valve? i don't see anywhere i can get a probe to measure anything.

model # is HTC5100BFE2

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Old 02-23-2014, 08:42 PM   #6
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I missed what furnace brand & model you had?

If you have a board with a diagnostic led that turns off with the opening of the furnace door...Temporarily tape the door switch closed to be able to read it next time it fails.
Check your smart valve for it's own diagnostic led as well.

There is a trouble shooting wiring harness for diagnosing the smart valve pilot while it's operating but it costs out close to what a replacement pilot assy does.
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Old 02-23-2014, 09:06 PM   #7
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It's a tempstar HTC5100BFE2


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