Gas furnace wont start


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Old 10-07-16, 11:33 AM
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Gas furnace wont start

Hello,

I have a gas furnace (also AC system attached). The AC runs fine with no issues. I noticed at the end of last winter that the gas heat was not coming on. Again, AC works fine. When the thermostat is on "heat" mode, the desired temperature is above room temperature and the "flame indicator" on the thermostat lights indicating that it is trying to start the furnace, but the furnace does nothing. Does not make a sound. The fan will run on auto, so it does not seem like a power or thermostat issue. I cannot see any indicator leds on my 20 year old unit. I have removed both covers exposing the gas valve and burner side and the blower and circuit board side and cannot locate any led indicator light to see trouble codes.

Should a 20 year old system have this led?
Any other ideas to check?
I have a new $10 thermostat that I could replace with. Does it make sense that the AC and fan would work on auto if thermo was bad?

Thanks for any help.
 
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Old 10-07-16, 01:46 PM
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Doesn't sound like a stat problem. what's the make and model number on the furnace?
 
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Old 10-07-16, 02:29 PM
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A twenty year old gas furnace should have an inducer motor that starts up whenever the thermostat calls for heat.



Find the R and W terminals inside the furnace burner compartment. Use a wire to jumper those two wires together to see if that causes the inducer motor to start.

If it does, the thermostat isn't turning on the furnace.

If it doesn't investigate whether you have 120 VAC power to the furnace and then whether you have 24 VAC power to the R terminal. If you don't you may have a burned out fuse, transformer or other low voltage problem.
 
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Old 10-07-16, 03:04 PM
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The OP 's furnace has 120v as the AC and the blower are working normally.

If you remove the burner access cover.... you'll see an ID plate on the side wall.
Post the model number for us.
 
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Old 10-07-16, 03:47 PM
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It's a Tempstar, but I need to get the model number.

What I noticed today is that i do hear a hum from the small motor that must be the motor that drives the exhaust. It's actually really warm. almost hot. It even hums when the system is not on "Heat". I suppose it was warm like this all summer. Maybe this motor is bad? I smell a little electrical smell too. It's probably been like this all summer but I wonder if this is a hazard? It feels hot.

Thanks.
 
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Old 10-07-16, 05:01 PM
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That's the inducer motor. It should be room temperature when the furnace hasn't been running. If it doesn't run, the furnace won't light.

The motor sounds like it might be toast, but since it shouldn't be hot (even if it's faulty) when the furnace hasn't been running, there is likely a problem with the control board as well.
 
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Old 10-07-16, 06:10 PM
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One more update:

I checked the motor again and it actually was only humming and getting hot when the system was trying to heat. With the system on Cool, the inducer motor was cold. So, I tried moving the fins, and I am not certain, but the fins may have been slightly obstructed by something. Now the fins move freely. I switched it to "Heat" and the motor started up and the igniter fired. Not sure if the motor really was in a bind due to the fins, but it seems to be working now. I will keep an eye on it when cool weather comes. Thanks for all the input!
 
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Old 10-07-16, 09:00 PM
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It could have been the motor itself was bound or something like rust causing the blower blades to bind. Definitely keep an eye on it as it could need replacing soon.
 
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Old 10-08-16, 03:55 PM
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Often the inducer motor assembly that attaches to the furnace chassis can be removed pretty easily by removing a few bolts.


Removing the inducer motor will allow you to turn the fan by hand, and you will probably find that the motor has seized up, is hard to turn and will need to be replaced. The seized up motor is the usual reason why the motor just hums, and overheats, and then typically shuts off when a temperature limit switch shuts off the power to the motor to prevent a fire.

However, you MIGHT find something inside the furnace preventing the fan from turning. If so, you can remove the obstruction and probably be back in business.

Once you remove the inducer motor assembly (including the fan wheel) you can power up the inducer motor to see if it starts and comes up to speed properly.

Occasionally the fan will be broken.

DON'T try to replace just the motor or fan. Usually everything is too corroded together to get it apart.

Maost manufacturers provide an inducer motor assembly that includes a new motor, fan and sheet metal that allows you to easily bolt the new assembly to the furnace.

While this costs somewhat more money, it greatly simplifies replacing the failed parts. Make it easy on yourself and get the whole assembly.
 
 

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