Gas furnace pilot won't stay lit

I have an older gas furnace. I replaced the thermocouple twice as I thought I may have picked up a bad one. The pilot will lite, the burners seem to be fine & the furnace will run until the temp on the t-stat is reached. Once the furnace kicks off, the pilot goes out. I am getting a good flame from the pilot & the burners. Am I looking at a bad gas valve, or what else could be causing the pilot to go out? It's obviously not the thermocouple.

 

There could be a small blockage in the pilot orifice or gas line.

 

I am the original poster. I had to reregister as the system would not log me in. Anyway, I lit the pilot again without turning up the t-stat. The pilot has stayed lit for over an hour. I removed the gas line from the valve to the pilot assembly & using air duster in the can blew out the line. We'll see what happens. Thanx.

 

my fathers furnace had a similar problem and it turned out to be a bad wire from the gas valve to the control board.

 

Well, I got up this a.m. & the pilot was out. I'm assuming that the furnace kicked on overnite. I'm going to remove the pilot assy. today & clean it. What's the best way to clean it? Soap & water?

 

I would suspect an electrical problem first before going through that.

 

What kind of electrical problem? I have checked all the connections. This furnace is at least 25 yrs. old.

 

In my father's furnace, we ended up replacing the thermocouple and the gas valve without any success of improvement. It was only after we found out that one of the wires that connects to the valve was somehow faulty, possibly broken, and depending on how it was bent, sometimes it made full connections other times it didn't. Once we replaced that wire , the furnace has run without problems.
How did we find it ? Pure endurance or luck , just keeping trying and checking all connections to see if current goes through. On that faulty wire , the current wasn't going through at least one time we checked.

 

That's a pretty tedious way of doing it but it could end up being cheaper in the long run. You can "ohm out" each wire that could potentially be the problem. At this point though, you may be looking at a problem with your GV.

 

This is a standing pilot system right? - the kind where you have to light it with a match?

If so, is this an old furnace with metal flue pipe, and exposed to the effects more of outside windy conditions and/or natural draft issues?

Have you observed to make sure the flame is engulfing the thermocouple good?

Have you stared at the pilot to see if the pilot flame wavers some, and wavers upward, and gets only to the outer edge of the thermocouple when it wavers?

Have you ever just sat and stared at the furnace during the call for heat to see what happens to the main burner flame, as the furnace shuts off? Like, do the burner flames instantly shut down? Or do they sort of herky jerky shut down? And/or maybe turn off, then come back on again, before shutting off for good? (Anything that way that could perhaps cause a concussion effect that blows out the pilot light.)

 

A good diagnostic method would have found the problem right away. Luck is only needed if you lack skill and experience.

Testing to see if there is voltage to turn on the gas valve at the gas valve terminals themselves would presumably have zeroed in on the problem without changing out the valve.


Before changing out an expensive part, you want to think of all the tests you can to catch some odd ball problem that might be causing the problem. Of course, the typical DIYer usually can't do that very well, and tends to leap to conclusions too readily.

It's easy to do.

 

Agreed, I now own a voltmeter. At that time we didn't have one. I used a light and wires to check for current

 

As a repairman, I always looked for indicators that might let me zero in on a suspected problem. That's an educated guess.

But before replacing parts, especially expensive parts that are time consuming to replace, I always looked for a variety of ways to test for a bad part. A gas valve that isn't passing gas may not have gas to it, may not have the wire to it powered, may have a broken wire or a broken connection. Checking for all the possible defects greatly reduces the likelihood that you will change out the part only to discover that the #&^$%$$ thing STILL doesn't work!


DIYers tend to be weak on imagining those tests, along with not a few repairmen who ought to know better.

I've learned this myself a few times!

 

No, I haven't done as u suggested. And it looks like I'm back to square one. For the past couple of weeks the furnace has been running fine, the pilot stayed lit. A couple days ago the pilot went out during the nite, & I've had to relight it every time I need heat. It blows out when the furnace shuts down. The flame from the pilot is good. Does the flame need to completely engulf the tc or just the tip? This furnace has got to be 30 yrs. old. Does anyone know what a new gas valve would run me if that is the problem? I'll have to get a contractor here to install it, the job is out of my league.

 

Well, I have an HVAC contractor coming next week to look at the furnace. I explained to him what was going on & he said that it could be the valve, a downdraft from the chimney shutting off the pilot or a cracked heat exchanger. Could a cracked heat exchanger cause this issue? I was under the impression that if the heat exchanger was cracked then the furnace was could emit co2. I have alarms in the house & they have yet to go off. Opinions?

 

Ok, I looked at the flame as u suggested. When the furnace shuts down the flame goes off instantly. When the burner flame went off, the pilot stayed lit, then I heard a click from the gas valve & the pilot went out.

 

When you light it and it goes, are you lighting just the pilot?, and actually see the pilot going, and staying on, before the main burners come on?

If so, it is rather mysterious to me how the pilot would be influenced by the burner flame. Or maybe it is being influenced by the thermostat when main valve (MV) voltage goes away after call for heat is over with. Does this furnace have a wall mounted thermostat? Is this one of these pilots that grows larger when there is a call for heat?

 

I'm not sure what you're getting at. I light the pilot, then turn the t-stat up to call for heat. The pilot does stay on thru the heat cycle, but shuts off when the heat cycle is over with, so I have to keep relighting it when I want heat. This is an older furnace with a standing pilot. And yes, I have a programmable wall mounted t-stat, which is about 3 yrs.old. Are u suggesting that the t-stat is malfunctioning? Bear with me, I'm no expert when it comes to furnaces. Thanx for the replies.

 

The most likely reason for the pilot to be extinguished when the burners shut off is that the pilot burner and pilot orifice are dirty and need to be cleaned.

A clean pilot has solid blue flame and looks and acts like a small blowtorch --- and is relatively rewsistant to being blown out.

As a pilot becomes dirty it looks and acts increasingly like a candle flame that has yellow flames and is easily blown around and out.

Removing and cleaning the pilot burner and pilot orifice will likely solve the problem