I have a Gibson 90+ Gas furnace that is about 6 months old that i installed myself . I noticed in the past week or two that the unit is what i call cycling. It fires up and runs for a while then sometimes shuts down before the thermostat is satisfied. The gas valve shuts and the blower motor goes thru its cooling cycle and without any hesitation the inducer motor starts and the unit refires and runs properly. It doesn’t always malfunction.
I have checked the : Vent for obstructions,the condensation line, the air filter, the intake air for obstruction, gas pressure on both sides of the regulator, the flame sensor for dirt, loose wires, vibration of the unit, safety switch on the door, temp switch on the inducer ( vent isn’t getting to hot) I have replaced the batteries in my thermostat ( digital progrm.) . I get no light codes on the panel inside my furnace
. The unit has ran properly for the last 3 months and just started this problem. It doesn’t seem to cycle all the time. I am not very familar with HVAC repair but i am very interested in trying to find the problem myself. I have tried to find a book on HVAC repair but can’t seem to find anything local. Any information would be greatly appreciated. Thanks Andy
Edited 2/17/2003 12:38:44 AM ET by Andy3882
Edited 2/17/2003 12:40:32 AM ET by Andy3882
Replies
Since the problem is intermittant it may be tough to trouble shoot, but the one thing you didn't list as checking is the high limit switch in the heat exchanger. If it is opening early and shutting the fire cycle off the switch could be bad, or the temp in the heat exchanger is getting to high for some reason.
mine did the same thing last year. each time I manually shut if off/on it would stay good for a while. then it just got too much trouble to do it and the frequency was too often. The only difference in my problem and yours was there was trouble code displayed -- the guy came and installed a new circut board (not sure which one) and it has been running fine -- also, he thought the evaporator boot may have been cloged the one that is rubbber adn it pulls off the excess mositure from the fuel and humidifier -- it was filty but not the sole problem.
Do you have good air flow?
My first thought too. Check the air filters, dampers, registers, etc. The burner is probably cycling because it is getting to the high limit.
Yesterday I couldn't even spell plumber, today I are one.
Other possible causes: overfired burners; or cracked heat exchanger.
Run the furnace only, time the gas meter and post the time it takes for one revolution of the smallest fdial, the measurement of that dial (typically 1/2 or 1) and the btu rating of the furnace.
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Edited 2/17/2003 9:12:07 AM ET by Bob Walker
I know you said you checked the condensation line, but if the line is blocked up near where it connects to the furnace, the line would be clear but the problem would still be there.
I had the same phenomenon, and my blockage was in a Y fitting (wayward bug - a long story). I checked the line several times, and only decied to be 100% sure when I though I hear water in the furnace. When I popped the pip off the fan housing, about a liter of water came out.
'Debugged' the fitting, problem solved.
You might consider double checking the line, and inlet and exhause pipes.
Had a Carrier 90+ that caused some grieff a while back turned out to be a bad exhaust air pressure switch. I finally tracked it down with a tester after it got worse and worse to where I could make it repeat to be able to find it. Then when I went to get the part I knew I was on the right track when I saw 2- 100 bags of them on the wall. Plus they gave them to me plus a spare.
Isolate the failure if you can make it cycle and fail. Start with the suspect sensor and work out from it until you find the break in the sensor circuit. Also study the schmatic that comes with it first. It sounds like all the first steps you did were good to get to the next level. I also had a power switch(light switch) that was bad cause the installer didn't have one and used a old plastic discarded one he had replaced that was on the floor of his van.
Andy,
I had an intermittent problem with my 90+ for 2 years. It turned out that the intake was sucking in air from the exhaust vent and snuffing itself out. The flame sensor would then cycle the unit off and the thermostat would still be calling for heat and the unit would fire back up. The worst part was finding the problem, it would only happen with just the wrong wind conditions. Those conditions usually were accompanied by a brutal storm in the winter. By the time the tech would get here the wind would change and the furnace would work fine. They thought I was nuts. Finally they sent a new tech and he had seen this happen before. An extra foot of pipe and an elbow or three and it was fixed.
Ours was doing this recently, and since the fix was different than anything else listed, I'll throw it in the soup. The thermostat was getting old and out of adjustment. Talk about a relief. At least (in our case) it was a quick and easy fix. Had one of those dial types with the spring in it.
" To the noble mind / Rich gifts wax poor when givers prove unkind" - Wm Shakespeare, Hamlet, III,i,100
Andy, I'm not an expert on this but I did install my own efficient furnace (Tempstar). Could it be that your blower speed is too high which is cooling things ahead of their time in the furnace?
Mine had a different problem (to whom it might interest). My fan would stay on an inordinate amount of time after the cooldown. I opened the fan switch up and re-calibrated it (that sealed part). I took plenty of time watching it go through its cycle and it's been working fine since.
I know you checked the vent for obstructions, but did you check it for slope? We had a similar problem that was corrected when the exhaust line was adjusted so that it drained properly. Condensation was apparently getting trapped in the line, blocking the exhaust. Our tech originally thought it was the gas pressure sensor.