Troubleshooting furnace not kicking on.


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Old 11-08-11, 10:38 AM
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Troubleshooting furnace not kicking on.

I have an older, low-voltage (24VAC) furnace which, the last three years has religiously "burned up" older non-smart thermostats (mercury based I guess). I say burned up because my solution was to simply replace the thermostat and things would start working again.

Symptoms are that I could enable the blower just fine from the thermostats, but the furnace itself would never kick on. Visual inspection of the furnace showed the flame kicking on for a few seconds, but then stopping (no pilot light on this furnace) instead of continuing. As mentioned, replacing the thermostat would correct this.

Last year, I replaced the old-fashioned style thermostat (when it broke) with a Honeywell RTH2310 I got from Wal-Mart. It worked fine all last winter and summer and the last couple of weeks as I've started using the heat again until this morning. Same symptoms as before.

This time I really want to understand what is causing these things to go south on me... I busted out the voltmeter and checked my voltage across the R terminal to each of the others for cooling, heating and the blower respectively. All showed about 27V (seems a bit high, but don't know if this is a concern). I replaced the batteries on my thermostat and also reset the circuit for the furnace itself. No change.

I'm wondering if perhaps a fuse on the thermostat itself has blown... it still seems to function fine, but I switched my voltmeter to resistance mode and tried testing between the prongs on the thermostat that slide into the terminals. I assume current needs to flow between these prongs... My ohm-meter gave no readings here, but I don't have a "before" reading to compare with.

Will probably try directly wiring the R and W wires together tonight when I get home just to verify that the furnace will kick on, but any other thoughts? Happy to replace the tstat again if I have to, but would rather figure out why they seem to keep dying.
 
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Old 11-08-11, 06:12 PM
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Not sure what would cause this to burn out the stats.

The newer stats are just a simple on/off switch.

Jumper the two wires together and see what happens.
 
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Old 11-08-11, 08:37 PM
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Did so. No change. The furnace "clicks", the igniter glows, but nothing ignites and the igniter eventually goes off.

Opened up the furnace, removed power, turned gas off/on then reapplied power. No change (same behavior as above).

Perhaps the control module that decides when to let gas come out is burned out? I'll probably just give someone a call.

My furnace is a Snyder General ComfortMaker GUD060A014AIM. This problem actually sounds very similar, though I'm not sure I will attempt measuring voltages inside the furnace. :-)
 
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Old 11-08-11, 09:42 PM
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Ok, got adventurous and have been trying some volt readings within the furnace. From the place where the wires run in from the thermostat, two wires run up to what appears to be a transformer. If I put my voltmeter across these two wires at the transformer, I get 26-27V when I have my thermostat wires R and W wired together.

There are two additional wires running from the transformer to the gas valve. I stuck my voltmeter across these wires at the terminal on the transformer and observed the following:
  1. An initial click occurs. Time passes and the igniter lights up.
  2. Shortly thereafter, another click is heard. However, I don't observe any voltage across what appears to be the two gas valve terminals.
  3. The igniter goes out.

I don't know if this furnace has a pressure switch, but I'm guessing it doesn't or the igniter wouldn't kick on (I don't hear any sort of inducer fan)?

So not sure what is supposed to tell the gas valve to start emitting gas... and what controls sending power to those two wires.
 

Last edited by Variant; 11-08-11 at 10:30 PM.
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Old 11-09-11, 04:49 AM
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I am not able to find any info on your model.. Is there a small fan on the exhaust pipe?
 
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Old 11-09-11, 05:04 AM
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Couple links:
  1. Very similar to mine.
  2. Ditto (same guy)
  3. Not the exact same model number, but looks almost identical. The control board housing (?) at the bottom on mine is all enclosed is the main difference. Filter at the bottom as well on mine.

Will go check for fan on exhaust pipe and take some pics if possible.
 
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Old 11-09-11, 06:08 AM
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Ok, took a bunch of pictures as well as a video. Should all be viewable here.

Doesn't appear to be an exhaust fan from what I can see (you'll see what I believe to be the exhaust in the album above).
 
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Old 11-09-11, 06:43 AM
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Well, get a load of this... after buttoning the system back up subsequent to taking and posting the pictures this AM, the darn thing kicked on. Burners and all. It seems to be working "right" -- at least for now.

I've opened and closed it multiple times, so no idea what was so special this morning. I did go toggle the blower on and off manually immediately after reassembling it, but that was it.

Very odd.

PS: Of note, last night I whacked the gas valve a couple of times when the igniter was lit up just to see if it was stuck or something. Didn't make a difference at the time, and didn't whack it at all this morning. Maybe coincidental, but worth mentioning.
 
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Old 11-09-11, 04:57 PM
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Sounds like the gas valve is getting power, but it's not picking the valve open to allow the gas flow.

I'd plan on getting a new gas valve, or start saving up for a new furnace.

Also, RED FLAG on your gas line going into the furnace!! Flex should NOT be used! it has to be the black pipe.
 
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Old 11-18-11, 07:13 PM
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Hi Jay, all... just a follow-up. Things worked OK for a week or so, but then the furnace started exhibiting the same behavior. Seems like my post #4 above was the key, although I wasn't sure at the time I was measuring correctly.

HVAC guys came out and said the ignition module was bad. Apparently they didn't read any voltage across the wires connecting the igniter module with the gas valve at the expected time.... they swapped it out (it WAS a White Rogers 50E47-140 and NOW is a 50ER7-843) and all is well again.

I did ask about the gas line, and the HVAC guys said it was definitely not code now but probably was acceptable back whenever my beast was put in. Sounded like replacing it would be an ordeal.

Future project?
 
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Old 11-18-11, 08:28 PM
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Did you get the control replaced?

I would do the change out on the gas line soon, unless you are planning on a new furnace soon.
 
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Old 11-23-11, 08:26 PM
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Thanks, Jay. I am renting, so will bring it up with the landlord.

An update -- all was fine for about 5 days. Tonight got home from work and no heat. This time the new control board has a blinking yellow light indicating "reversed polarity" and even when calling for heat, the igniter doesn't come on.

I did some basic measurements with my meager skills per the control board's documentation. Didn't measure any voltage (well 0.1) between L1 and GND, and -- at least when calling for heat, between L1 and TH I got (oddly enough) 9.7v instead of the expected 95ish.

Maybe control board went out again, but wonder if something else is to blame.

Heading out for Thanksgiving so will have to deal with it when I return.
 
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Old 11-24-11, 06:24 AM
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If you are renting, quit messing with this and call the landlord you don't have heat! Your landlord needs to get this fixed.
 
 

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