So my wife said, "do something about the furnace coming on every night." I replied, "I'll turn it down" and she said, "no, because then it will be too cold in the morning." I said, "I'll program it" and she again thought this was a bad idea because then she might be cold at night. The furnace runs pretty much every night because in San Francisco it is almost always cool in the early hours, plus she sleeps with the windows open. After explaining about having the cake and eating it too, she told me to turn it off for the summer, which I did, for the past 4 months. Finally had an electric bill under $250. I wonder at this point if turning it off for that period of time contributed to the failure below?
Anyway, it starts to get cooler around now so I turned it on and of course, no heat. Most of these furnaces are the same or follow the same initialization cycle. Mine was getting to the point where it cycled the larger fan and the smaller fan to evacuate the gas combustion area, turned on the electric glow plug and then should have tripped the gas valve. At that point it didn't and presented a blinking error code 21. This means something like "the fire didn't ignite" but it is more cryptic as it really means that either the gas is off (you didn't pay the bill), the CPU is not working, the igniter wasn't working or the gas valve didn't open; basically the furnace expected a gas fire and never detected one.
After checking all the sensors, I found I had no voltage to the gas valve itself. The most interesting sensor is the "flame rollout sensor" which detects the length of the flame in the combustion chamber. After another argument with my wife about not touching the thermostat while I am up to my elbows in the furnace surrounded by 6 blow torches and a fan that could give a wood-chipper a run for its money, I found that yes, indeed, no voltage to the gas valve. In her defense, she argued that she didn't want cold air blowing on her at night even though I was at risk of life or limb. I explained unsuccessfully that the cold air was only during the time I was testing but of course, that wasn't a good enough excuse and I slept in my sons room.
I finally decided that the gas valve relay on the CPU motherboard wasn't being tripped and when I opened the board up, I found two cheapo brown caps, both 47uf and 63V had pissed all over the place. I suspect had I called the service man (or allowed the one my wife called while I was at work today) to come over, he would have replaced the motherboard at a cost of $350 for the board, $150 for the initial diagnosis and $250 for installation labor.
The final thought here is that I often find I don't apply my electronic skills liberally across all areas. Furnaces have always sort of spooked me because of the 220V, gas, heat, etc. Not that long ago I had an A/C fan short to the motor frame. It was really hot, I was sweating like crazy, took a shower and thought, what the heck, maybe the fan isn't starting because of a starter cap, let's give it a spin (fresh out of the shower, naked, covered in water. I woke up on the floor with a large cut on my arm where I yanked it out of the box fan when I was nearly electrocuted by leaning on the grounded frame while spinning the live inner blade of the fan. Someone told me older A/C unit fans are electrically isolated using rubber grommets on the motor. In this case, the fan motor shorted to its frame and then I touched it as well as the grounded A/C ventilation generating the NDE.
Anyway, all is well, my wife is happy with the heat and I have $350 + $150 + $250 = $750 I shouldn't have, so what to buy, what to buy?
Jerry