Furnace pilot light goes out intermittently

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Old 02-12-14, 08:25 PM
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Furnace pilot light goes out intermittently

I have a 20 year old Bryant gas furnace with a standing pilot light, which intermittently will go out. The furnace may run for days without a problem. The thermocouple has been replaced but problem still exists. The pilot flame looks good (blue) and covers a sizable area at the end of the thermocouple. I've noticed that when the weather warms up into the 40's or 50's, that's when the pilot will start it's intermittent problem. Whenever it goes out I have no problem relighting it and the furnace works fine until the next time the pilot light goes out.
 
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Old 02-12-14, 11:06 PM
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The pilot orifice might be plugged.

I would get that furnace inspected/serviced if you want to keep it running -> heat exchanger is probably cracked by now.
 
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Old 02-13-14, 08:33 AM
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I will check the orifice on the pilot gas line. I have another question. When the pilot light goes out and the thermostat calls for heat, should the inducer motor and then the blower motor come on? This is what happens. And since the burners never come on due to the extinguished pilot, cold air is blown through the ducts and out the registers.
 
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Old 02-13-14, 07:28 PM
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"heat exchanger is probably cracked by now." Why would the heat exchanger be cracked?
 
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Old 02-13-14, 10:29 PM
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^Every time the furnace goes through a cycle, the heat exchanger gets a little weaker from expanding/contracting. After 10-25 years (depending on many factors), of cycling probably 50+ times a day, the probability of having cracks in the metal are very high.

Cracks can cause co poisoning, high co in the exhaust, or flame rollout.

Get it checked out.

*Don't rely on a monoxide detector to tell you that there's a problem. A CO alarm is about as effective at preventing co poisoning as a smoke alarm is at preventing fires. By the time one goes off, you already have an emergency on your hands.

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"When the pilot light goes out and the thermostat calls for heat, should the inducer motor and then the blower motor come on?"

Yes, if blower operation is controlled by a timer on the circuit board.

Most induced draft furnaces to my knowledge aren't standing pilot.
 
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Old 02-14-14, 05:58 AM
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offplumb

I understand concerns about the heat exchanger. A service tech came out several weeks ago and inspected the furnace and didn't report any problems with the heat exchanger. He didn't report any problem with how the pilot flame looked or how it enveloped the thermocouple.

He was told about the pilot light issue. His response was that it is a not-so-uncommon problem with the older standing-pilot furnaces. He said with warming outside temperatures it is susceptible to flue drafts blowing out the pilot. I find this interesting and would like to know how this happens and if there is a remedy for this.

He also said the solenoid on the gas valve was a little weak. (?) After doing some research I'm assuming he meant the pilot valve solenoid since the pilot flame is what keeps going out. Can this be ohmed-out? If so, what are normal specs? I don't have a problem with replacing the valve, but would like to be certain that's the problem before spending several hundred dollars.

There is a pc board in this furnace. Is there normally anything in the circuitry that would cause the pilot solenoid to drop out thus causing the pilot to go out?
Or is it intended for the pilot to always remain lit no matter what else goes wrong? (Other than loss of gas pressure.)

As far as the service techs opinion, he seemed to lean more towards it being a draft issue. (He said replacing the gas valve MAY solve the problem.)

And yes, this Bryant furnace has an inducer motor and a standing pilot. Possibly one of the last models before they switched to electronic start.
 
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Old 02-14-14, 06:51 PM
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With gas valves, either the solenoid can open the valve, or it can't. The regulator in the valve is responsible for maintaining proper pressure. (pilot valve pressure is adjustable, but shouldn't be changed unless the orifice is clean)

The thermocouple produces just a little bit of current which holds the pilot valve open; when the flame goes out, pilot valve cuts off gas to the pilot and main burners. (when you push the button down to relight the pilot, you're physically opening the pilot valve and holding it there until the thermocouple heats up and starts producing current again)

The pc board is in no way connected to pilot operation - it just sends a 24v signal to open the valve and turns on the blower after a fixed time delay. There's no feedback on a standing pilot model.

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You can tell if the orifice is dirty by looking at the pilot flame: pilot flame colour orange or soft blue | furnace trouble shooting help

With a clean orifice the pilot should shoot into the heat exchanger, not go straight up.

He was told about the pilot light issue. His response was that it is a not-so-uncommon problem with the older standing-pilot furnaces. He said with warming outside temperatures it is susceptible to flue drafts blowing out the pilot. I find this interesting and would like to know how this happens and if there is a remedy for this.

Read more: http://www.doityourself.com/forum/ga...#ixzz2tLqoYbzw
That only applies if there's an open draft hood.

Probably 99% of standing pilot furnaces are natural draft.
 
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Old 02-15-14, 03:48 AM
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"I will check the orifice on the pilot gas line." You can clean it by pushing a long thin wire in it. If you post a picture of the pilot light assembly someone might have specific knowledge about it.
 
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