gismo
Touchdown! Greaser!
My furnace has been acting up lately. It uses spark ignition and a single wire "flame sensor" probe. The symptoms are that when the thermostat calls for heat, the induction blower runs, the pilot gas valve opens, the ignitor lights the pilot and then instead of the main gas valve opening, the main gas valve goes "clunk, clunk, clunk..." a few time about half a second apart with several second pauses between groups of "clunks". The first time this happened, I cleaned off the "flame sensor" a little (scraped it with a screwdriver) and it worked OK for a while but the problem came back. Then I pulled the flame sensor out and cleaned it up real good with a wire brush leaving it shiny. This didn't seem to help much but then I discovered that there was some resisitance between the ground on the ignition/flame sense controller and the furnace so I added a jumper wire between them. This worked great for a while but several days later it began to act up again in the same way.
The big question is whether this is a problem in the controller (RobertShaw 715U) or the flame sensor itself. The sensor appears to be a single wire rod about three inches long with two of those three inches inside a ceramic insulator. It's definitely not a thermocouple so I assume it senses the flame by conduction. I traced out part of the controller's schematic and it looks like there is a low current 120 v source source appled to the probe and something that looks like a synchronous detector connected as well.
I'm wondering is there is supposed to be some kind of coating or plating on the sensor that has burned off over the years (the furnace is getting close to 20 years old) that's causing my problem or if it should work with a freshly cleaned probe for more than a week. In that case I would assume the controller is the problem. It's definitely not the gas valve as I've measured the voltage on it's coil and found little or nothing when it's off but supposed to be on. Also I can jumper from the pilot valve terminal to the main valve terminal and make it work.
I suppose I could just get a probe and try it, but I'd rather not waste the money and worse yet the time to go to the distributor across town for the wrong part. The probe costs something like $20 and the controller is more like $150.
The big question is whether this is a problem in the controller (RobertShaw 715U) or the flame sensor itself. The sensor appears to be a single wire rod about three inches long with two of those three inches inside a ceramic insulator. It's definitely not a thermocouple so I assume it senses the flame by conduction. I traced out part of the controller's schematic and it looks like there is a low current 120 v source source appled to the probe and something that looks like a synchronous detector connected as well.
I'm wondering is there is supposed to be some kind of coating or plating on the sensor that has burned off over the years (the furnace is getting close to 20 years old) that's causing my problem or if it should work with a freshly cleaned probe for more than a week. In that case I would assume the controller is the problem. It's definitely not the gas valve as I've measured the voltage on it's coil and found little or nothing when it's off but supposed to be on. Also I can jumper from the pilot valve terminal to the main valve terminal and make it work.
I suppose I could just get a probe and try it, but I'd rather not waste the money and worse yet the time to go to the distributor across town for the wrong part. The probe costs something like $20 and the controller is more like $150.