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I have a NOMA Programmable thermostat attached to my furnace.

The set point is usually around 21 degrees, and the room temperature is reading around 17 degrees. The furnace is not coming on.

However, If I go down to the basement, and flip the power switch off and on, the furnace starts, and runs for about 10 minutes.

Is the problem in the thermostat or the furnace? Where should I start troubleshooting?

It's a gas Furnace with AC unit. Manufactured in 1998, no idea when it was installed.

It when it's running, it pumps out a fair amount of heat, so I doubt it's gas line pressure.

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What type of furnace (gas/oil/electric)? What brand/model? How old? –  gregmac Nov 22 '10 at 16:45
    
Does the furnace start if you manually adjust the thermostat? –  Tester101 Nov 22 '10 at 17:14
    
Accepted Eric Petroelje. He was correct in assuming it was a sensor. He was wrong about which sensor, but "Check All Sensors" is the correct answer to the problem –  Chris Cudmore Dec 21 '10 at 22:11

5 Answers 5

This happened to me, and it turned out to be a leaf that got sucked into the air intake. The leaf got sucked all the way across the house in the PVC, and was sitting right at the point of entry into the furnace (there was a screen there to prevent the debris from coming in.) This blocked new clean air from coming into the furnace, and then the safety mechanisms would shut it off after a few minutes.

Once I removed the leaf, it worked fine.

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up vote 3 down vote accepted

It turned out to be a blocked drainage hose, so the condensation was building up.

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this should be the accepted answer if it was the solution. –  mohlsen Feb 1 '11 at 13:09

I had the exact same symptoms with my natural gas furnace, and it turned out to be the flame sensor.

It's an L-shaped metal bar (but can also be straight, depending on model) that detects if there are flames actually coming out, and turns off the gas if not. It had a white high-temp wire connecting it to the control board. Taking it out, cleaning it with some steel wool fixed the problem for me.

flame sensor

If yours is like the above, you'll only see the back part of it and the bracket, as the L-shaped end is inside the heating unit.

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Will check when I get home. –  Chris Cudmore Nov 22 '10 at 21:10

Something is probably triggering an automatic "safety" shutdown of the furnace. When I've seen this, it's usually been one of two things:

  1. A bad ignitor - if the furnace doesn't detect that the ignitor has turned on, it will shut itself down. Most of the time this would prevent the furnace from firing up at all though.

  2. A backflow issue. If the chimney is clogged or if there is any kind of backdraft down the chimney that would cause exaust gas to come back down the chimney, that could trigger a sensor that would shut down the furnace.

Backflow seems more likely than an ignition issue in your case, but the error code readout on your furnace (usually a flashing LED) should be able to tell you exactly what the problem is.

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Chimney isn't clogged. It's at ground level, and the exhaust is blasting out when it's running. But it does smell a bit gassy. Maybe there's not enough airflow going in. –  Chris Cudmore Nov 22 '10 at 21:12
    
@chris - Could be an O2 sensor or some such thing as well if it's detecting that there isn't enough oxygen for complete combustion of the gas. –  Eric Petroelje Nov 22 '10 at 22:16

it's likely the furnace. but you didn't tell us anything about the furnace, so we can't provide any more detail.

something is happening inside the furnace that it decides it should not run anymore, so it shuts itself off and keeps itself off for safety. for example, if it's a gas furnace, you might have low gas pressure due to a faulty regulator or clogged jet and the flame is not sustainable. so if the furnace detects that the flame has gone out, it will shut down the furnace and refuse to come back on.

on my gas furnace, there are two lights on the front that tell you what the furnace is doing or if there is a fault. yours might have a similar indicator.

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