Furnace question

[fudgie]

I have a goodman GMNT-1004b LP gas furnace. 92% eff. When it kicks on, only 1 or 2 (outta 5) gas chambers light, then it goes out. It'll try again and maybe all will light or none. Sometimes it will be fine and light right away. Did this a few years ago and the tube ends were rusty and had a bad ignitor. I took out the orifaces and they were clear. Cleaned the ends of the tubes, they were rusty/burnt but all open. When it does light its a good, clear (well blue), unobstructed flame. I replaced the ignitor this morning. I was thinking either a bad thermocouple at the end of the run or bad flow/pressure switch. Anyone know? Plus give me a ball park fig as I want to do it myself since the heat/air place wont do it without cash upfront.
We use it as a secondary/third source of heat and thermostat never goes above 55 deg. House is usually cool but keeps my pockets full. 

[Walküre]

Fudge - you can check the thermocouple with a millivolt setting on your meter, and put heat on the end.

Here's a link - pdf, page 4, left side:

http://support.fluke.com/FInd-Sales/download/asset/2434064_6121_eng_b_w.pdf

That will eliminate the thermocouple. Usually, though, it works or it doesn't. It just provides voltage to keep the valve open, and it's an on/off thing. It sounds more like you are getting strange air flow, not allowing parts of the burner to light, all the time. And that seems to point away from the switch, as well.

You can go through the entire pdf, there's a lot of good troubleshooting in there.

When you try to light it, you might take a long lighter (like the BBQ type), and while holding the pilot, see if you can manually light the unlit portions of burner. If you can, then it's usually an air thing. If you can't, then it's a gas thing.

It's easier in person to troubleshoot, but this is where I would start.

Roger

[fudgie]

Fudge - you can check the thermocouple with a millivolt setting on your meter, and put heat on the end.

Here's a link - pdf, page 4, left side:

http://support.fluke.com/FInd-Sales/download/asset/2434064_6121_eng_b_w.pdf

That will eliminate the thermocouple. Usually, though, it works or it doesn't. It just provides voltage to keep the valve open, and it's an on/off thing. It sounds more like you are getting strange air flow, not allowing parts of the burner to light, all the time. And that seems to point away from the switch, as well.

You can go through the entire pdf, there's a lot of good troubleshooting in there.

When you try to light it, you might take a long lighter (like the BBQ type), and while holding the pilot, see if you can manually light the unlit portions of burner. If you can, then it's usually an air thing. If you can't, then it's a gas thing.

It's easier in person to troubleshoot, but this is where I would start.

Roger

There is no pilot light. It has a electric ignitor. It glows red and then starts the gas. Cant really light the burners. Once it realizes the other burners are not on it shuts off and trys again. Maybe a second total. Its a hit and miss. It worked fine for 20 times and didnt light a time or 2.

[Bobbo]

Try cleaning or replacing the flame sensors.

[wilmont kid]

I install these these things and the problem is most likely the flame sensor. AKA flame rod. Its a metal rod about 3" tall and 3/16" in diameter. It sticks up into the flame and tells the gas valve you have flame and it's OK to keep open. Got 2 furnaces working this week by cleaning or replacing this sensor. Just clean it up with steel wool. If you want a spare, PM me and I'll send you one. They're less than $10.   Jeff

[fudgie]

I install these these things and the problem is most likely the flame sensor. AKA flame rod. Its a metal rod about 3" tall and 3/16" in diameter. It sticks up into the flame and tells the gas valve you have flame and it's OK to keep open. Got 2 furnaces working this week by cleaning or replacing this sensor. Just clean it up with steel wool. If you want a spare, PM me and I'll send you one. They're less than $10.   Jeff

Hell I thought it was the thermocouple.  flame sensor. Had it out today and rubbed it on my jeans. Its at the end of the row of burners. Usually when it does light its the 1st 2 burners. Not near the flame sensor. I'll do that in a few days if not tomorrow. Its next to the wood burner and its toasty down there.