Bryant gas furnance short cycle problem?

[cprkw]

I have a late-1980s Bryant forced air gas furnace in the basement that has acted poorly from the day I bought my split level home in October

2000. Every now and again, when it first comes on, it appears to work normally: the blower starts, the electronic pilot clicks, the gas burner flames, and very soon I have warm air coming through the vents. However, after about 10 minutes, the electronic pilot clicks or multi-clicks (even though burner flame is still present), which is followed by flame putting itself out and then relighting. All the while, the blower is going fine and itself does not also cycle on and off. And this goes on and on and on, every 5-20 seconds, until I set the thermostat back at least 1 degree below the current temp in the house. That first winter (2000/01) in my house, I had service techs from 2 separate companies check it out at different times. The first one was unable to suggest a fix other than to try a new thermostat in the living room. So, I bought a new programmable and a new non-programmable thermostat, tried them both, and had the same result. Given this information, the 2nd service tech thought a new "Gas Furnace Control Center" would be in order (Part No. HH84AA021). When that didn't fix it he said that, while being inconvenient (takes much longer to heat the house this way) and disconcerting to hear the pilot lighting and relighting, it was not dangerous and that eventually I should buy a new furnace. In the meantime, I began keeping notes of when this occurred and environmental factors. Everytime this problem came up, I would note the indoor vs outside temps, humidity, cloud cover, snow on ground (I live in D.C. area), etc. I also tracked indoor conditions (when filter last changed, open/closed position of doors in house), and soon discovered a correlation between the furnace short-cycling and the upstairs bathroom door being closed OR if the shower is being used upstairs. This info did not appear to help my two technicians, but once I had this understood, I simply heated the house before using the shower and all was mostly well. No, my furnace problem wasn't solved, but 80% of the problem appeared to be related to that one door upstairs.... ...that is, until the past 24 hours. Now the heater short-cycles within 10 minutes of igniting everytime. I will soon begin looking for a service tech from a 3rd company, but before I do I wanted to query this on-line community for ideas about my problem. Your thoughts and suggestions are appreciated!

[profft]

It's your 3-wire pilot burner, either the heat switch is going bad or the pilot flame is not completely filling the pan. Have the pilot burner replaced or cleaned, whichever is appropriate and your problem should go away. As to why it happens when you take a shower, it's your vent fan sucking more air than can infiltrate so your house is running at a more negative pressure than it does when the vent fan is off.

[Jack Kramer]

I had something similar happening on my furnace. Though my pilot would light and re-light about 10 times before the burners stayed lit. I had a new gas valve installed and all has been well since.