Gas furnace troubleshooting

Page 1 of 2  
I'm having a furnace problem. I turn the furnace on. The draft inducer runs, the igniter lights up and then the gas goes on. It stays on just about till the blower starts and then the gas goes off. The blower keeps going for a little while then shuts off. I had an extra thermostat and changed that out with no change. I cleaned the flame sensor. The furnace has been running fine for years until this happened today. Anybody know what's wrong? Thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Shutdown at that point in the sequence sounds like flame sensor. You can put in a new flame sensor easy enough. Might be bad, and they're cheap. You cleaning it with fine steel wool? If a new sensor doesn't work, bite the bullet and get a pro in unless you want to fart around for days in a cold house. If you do that look around on the net for troubleshooting your model. Could be the motherboard. When I had that happen cleaning the flame sensor fixed it for a period of time. Had to clean it a few times a year. A couple years ago the fan relay went bad on the motherboard, so I had that replaced. Came with a new flame sensor. Haven't cleaned it in the two winters since then. The original that came on the new furnace wanted frequent cleaning right off the bat.
--Vic
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 18 Jan 2011 18:32:34 -0600, Vic Smith

Rather than just replace it, can't he test it by bypassing it. If it's normally closed during operation, bypass the sensor with a jumper wire whereever it's convenient to attach it, with alligator clips on the ends maybe. If it's normally open (I haven't worked on a gas furnace, but I doubt it) just disconnect it, at the right time if necssary.

Even if it were free, it means interrupting his work to go to the store. IF his testing gives uncertain results, he can guy one then.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Brand, model, approximate age would be very helpful. What do you mean "it stays on", the igniter or the gas? It sounds like it could be that power is removed from the gas solenoid when the blower comes on, maybe a bad connection somewhere.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Flame sensor or some overtemp sensor. Get the papers out and go through the possibilities on the schematic.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It sound to me like your high temperature cut-out is turning off your gas. Check your wiring diagram and use a voltage tester to see if that is where you are losing power.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thank you for your replies. I cleaned the flame sensor again and checked continuity on the flame rollout switches. I have continuity through the main limit switch but still have not been able to check the auxitllary limit switch. I'll have to take allot of stuff off to get to it and don't want to try till I get a new flame sensor tommorow. The furnace was made by Goodman Manufacturing Company and is a model GMP100-4. It's about fifteen years old. It's the gas that goes off around the time the blower starts. I've timed the burn time from 10 to maybe 20 seconds. I can't find any way to reset the thing after it goes off. I turn up waiting more than an hour before it will fire again. It does have an LED code but it doesn't blink but stays steady which is the all OK code. I'll try a new flame sensor tommorow and if that doesn't work I'll either call somebody or try to get to the aux limit. The board has to come off to do that.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

This morning I went down and turned the furnace on to see if my latest cleaning and connector wiggling had helped anything. It did the same thing and the gas shut off at about 12 seconds. I forgot to turn it off again. About an hour later it came back on while I was upstairs. I waited for it to go off but it didn't. Now it seems to be running normally. ..Great an intermittent problem..this will be easy to fix.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

That's Murphy's second law!!!!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

To reset the furnace from lock-out, try this: turn the thermostat down to a temperature cooler than the current inside temp. so that the thermostat isn't calling for heat. Turn off the power to the furnace. Now, turn the power back on. Turn up the thermostat so it's calling for heat. Does this reset it??
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

See, Vic, he stopped for the day to get a new flame sensor, when he probably could have bypassed the one he has for a testing purposes.

LOL. Ain't it always the way?

Maybe it will never do it again.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

You might be right, but I don't know how to bypass it, and don't want my advice to make anything worse. I just hardly do anything electric and use the "pull and replace" method for electric if the part is cheap and I suspect it's the cause. I do know a few things to keep me safe, and that's it. Last time I even put my ohm meter on anything was testing some fuel injectors. The results were "inconclusive" so I just nought new ones. That worked good. Besides, see below.

Based on what he said, it will. I dealt with the exact same problem for years, but cleaning the flame sensor always got the furnace going again for awhile. A new flame sensor stopped the problem entirely. Problem is, the sensor came with a new motherboard, so I'll never be sure which cured it. Just don't know. I sometimes had that intermittent problem too. I would hear it do the failed cycle, but it would "cure itself" before I got to cleaning the sensor. One thing I didn't mention is Bobo is to make sure he's cleaning the flame sensor, and not a rollout sensor. Didn't want to insult him. My Rheem manual had a variety of possible control systems, one with 2 flame sensors. I must have cleaned the rollout sensors 6 or 7 times before I caught on to the fact that I had 1 flame sensor, and was missing it. The flame sensor was pretty well hidden. What fooled me at first was after cleaning the rollout sensors the first couple times, it worked! Then it stopped working, so I dug in some more and found where I was wrong. Another example of an intermittent problem confusing the issue. But I like your idea of bypassing for a test. Next time I run into such an issue I'll try jumping a component if I think I won't zap anything.
--Vic
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Vic Smith wrote:

Flame sensor o/p is by the millivolts. My problem was with HSI couple times burning out. Flame sensor was never replaced.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

I replaced my HSI once, and have a spare on hand. When I was looking up HSI - never heard the term - I ran across this, which is a pretty good basic explanation of common furnace light off sequence. http://arnoldservice.com/Troubleshooting_Heating_Problems.htm
One thing isn't clear about Bobo's problem, and that's how long the furnace fires before shutting down. My Rheem would only fire for a few seconds when the flame sensor needed cleaning. Bobo seemed to indicate his is firing much longer. If that's true I take back everything I said.
--Vic
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 19 Jan 2011 19:21:51 -0600, Vic Smith

varies from say, ten to twenty seconds. After it started working again this morning I turned the heat down before I went to work.. When I returned home the problem started again. As it seemed to work fine when there wasn't much time between burns I thought warming it up might help so I put an electric heater in front of it for awhile and it's running fine now. I mailordered a flame sensor when it was running good this morning. I hope that does the trick. The mailorder place has boards for $125. Hopefully I won't be sorry I didn't order one. I sure have learned allot about modern furnaces the last couple of days. I think the flame sensor produces voltage when hot so it would be a tough thing to simulate.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Most furnaces like this have at least a few LEDs on the control board that show error information. Have you looked?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 22 Jan 2011 09:12:03 -0800 (PST), snipped-for-privacy@optonline.net wrote:

Yes the LED burns steady which is supposed to be all OK.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I'm no expert in this area, but here are a few thoughts.
The rooftop gas hot air heating unit where I work was doing the same thing. It would run for a brief period and then shut off. It could be reset and restarted, but then it would do the same thing. It turned out that it had a cracked heat exchanger. It is also a Goodman and is about 12 years old. In our case, the local gas company was able to diagnose the problem, and the technician said that there was a period of time back then when Goodman made models that had heat exchangers that tended to crack after about 10 years. He said they later changed the design of the heat exchangers which makes them last longer.
Since a cracked heat exchanger can send carbon monoxide gas into your home, be sure you have a working CO detector in your home.
In our area (Southern New Jersey) the gas company (PSE&G) will come out and check out heaters and other appliances, and they do heater, HVAC, and other appliance repairs. The have crews working both day shift and evening shift. The way they work it here is that they come out for free, and if they can tell what the problem is (which they almost always can do), they tell you how much it would cost to have them fix it. Then we have a choice of either having them fix it or having someone else fix it. There is no charge for the visit if we don't have them do the repair. When we call and say we have no heat, they almost always come out the same day withing a few hours. If your gas company has any kind of similar policy, that would be an easy way to get your problem diagnosed and probably fixed. They put the repair bill on your gas bill if you decide to use them to do the work.
Bobo wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bobo wrote:

I never have problems with my 34-year-old furnace.

I don't have a draft inducer fan.

I don't have an ignitor.

I don't have a flame sensor. I have a standing pilot light.

Hmmm. Coldest night of the year?
My furnace has been running more years just fine than yours has.

Your furnace is over-technologized just so you can save an extra $10 worth of natural gas per year beyond what your modern heat-exchanger is giving you.
Enjoy.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

His furnace is probably 20 or 30% more efficient than your 30 year old furnace.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.