posted on 11/15/2007
I am new to this site and would appreciate all tips that may help me resolve my problem. SORRY this note will be long, but I want to share as much detail as possible.
I bought my house 3 years ago. It has a Ducane 75,000 BTU Propane furnace. I don't have the model handy. The furnace was installed in 1999. Two years ago I had the Honeywell Smartvalve replaced because the furnace would not light. I have had no problems until now.
About 1 month ago I put a new filter in and turned it on to make sure it was going to work this winter. It lit normally and warmed the house temp up to the set point on the thermostat and shut down normally. Thought I was OK. Weather got cold here about 2 weeks ago. I turn on the furnace, inducer runs, burners light, warms the house up and t-stat turns furnace off normally. Temp dropped in the house a little while later, t-stat calls for heat, inducer kicks in, burners light, they stay lit for about 5 seconds and the burners shut down. This cycle continues a number of times, the burners do not stay lit long enough for main blower to come on and then it appears ignitor/smartvalve lock out. Inducer is running the whole time.
At this point if I turn the furnace off for about 2 to 3 hours. It will light, heat the house up, shut down and then start the same burner on/off cycle. Almost seems like residual heat in the furnace is keeping it from relighting after the first run?
Furnace tech looked at it. The small piece of metal that is used to reference flame to the probe on the ignitor assembly was gone. He replaced the entire ignitor assy and also the three burners (they were shot). I estimate he cycled the furnace 10 times after the new parts were in and it worked perfectly. Each cycle was no more than a minute or 2. He leaves, I let it run, heats the house up and shuts down normally.
Today I turn it on, heats the house up, and then the cycling started when it tried to light the second time. Call tech back. He goes all through it. Checks condensing hoses, no leaks, condensing lines are not plugged, lines to pressure switches are open, takes "vestibule" cover off to check for cracked exchanger etc. He checks/cleans furnace very well in my opinion, spent a couple hours on it (no charge) and does not find anything obvious. Checks gas pressure etc. Seals up the plenum and tries it. Works fine. He must have cycled it 10 times. But the longest he let it run with main blower running was about 2 minutes.
He leaves, I let furnace bring house up about 5 degrees and t-stat shuts it off. About 30 minutes later t-stat calls for heat, inducer starts, pilot lights, burners kick in but once again - they run for about 5 seconds, then shut off and the cycling continues. I have to call tech back tomorow, AGAIN!
This is what I know:
Gas pressure was checked and it's set correctly.
Condensing part of the furnace is clear. No plugged lines - holes into Inducer are open, no hose leaks etc.
Inducer Blower has been removed, no water in furnace. Inducer runs fine and creates good draft.
I assume SmartValve is OK or it wouldn't light and stay lit the first on/off run cycle.
Burners light, burn clean when they stay lit - no backdrafts or soot.
Can't be flame roll-off switch because I never have to manually reset it.
I don't think it's the thermal limit (HI Limit) switch in the plenum? I have not checked it with a VOM after it's run a while to see if it's opening. I can't believe the thermal is getting residual heat and stopping furnace from running the second heat cycle? If it was the thermal switch I do not believe the burners would light and run for the 5 seconds?
What about the pressure switches? Don't these either work or totally fail?
Could it be the main control circuit board? But again, why is the furnace working fine the first time it runs through a heating cycle?
I have the impression, even though this tech has been repairing furnaces for 17 years he is a little stumped. I don't want to get into just changing parts until it's fixed. I can replace do that myself!
Sorry this is so long, thanks in advance for any suggestions!!!