Hi All,
OK here's my question for you all.
I have a Bryant 90 Plus furnace it's probably 20-25 years old maybe. OK OK I know you are all boo hiss hiss but here is the deal, I also have a computer server farm in the basement that puts out a heck of a lot of heat - which during the winter I divert into the house. So the furnace hardly ever gets used and I have not yet suffered the heat exchanger issues of clogging and rusting (I replaced the blower motor 2 years ago and looked up at the bottom of the heat exchanger and it looked perfect no rust, etc.). However, I run the fan 24x7.
Anyway, the thing is controlled by a Honeywell programmable thermostat - an older unit, it's round in the traditional Honeywell style and runs off the 24v ac power with a digital display. No batteries and no place for them.
Anyway my wife and I go out to the beach for the weekend and we get a call from my 20 year old son Sunday night on the way home claiming the furnace thermostat is broken because it's displaying nothing. Bear in mind that as of yet I have not turned on the furnace this season so I have not bothered changing the filter for the last 3 months. Apparently my son decided putting on a sweater in a 65 degree house was too difficult so he decided to turn on the furnace.
So I get home and yes the thermostat display is blank. I pull it off the wall and measure voltage at the terminals and I'm getting only 17 volts. I replace it and I go downstairs and sure enough the filter is clogged up, I replace that and close the blower door and the furnace is blinking code 33. I check the rollout switch, it's not tripped and has continuity and I get 24 volts each side of the switch. Wires connections are all tight. I try turning the furnace off and on a couple times, nothing.
Finally I pull the blower motor cover and check for voltage at the C and R terminals and get nothing. Ok of course, the blower motor cover is open, I manually close the cover switch, and I get my 24 volts. So I reinstall the cover and suddenly the blink codes disappear, the draft motor fires up, a moment later the gas valve opens and fires up and we get a beautiful blue flame in the burner box inspection port. Furnace begins operating normally, display reappears on the face of the thermostat.
What just happened here? Is this thermostat incompatible? It's been working for years on this furnace BUT maybe once or twice a year I've seen the display blank out, and then come back on 20 minutes later. But the thermostat doesn't lose programming when that happens so maybe it's just seeing a power reduction?? Why would the furnace supply 17 volts?
Was it the clogged filter that tripped some limit switch? If so, what? And why didn't whatever limit switch that got tripped untrip itself once I replaced the filter and turned the furnace off and on. Why was I getting 17 volts at the C and R terminals at the thermostat when the thermostat was off the wall sitting in my hand?
I'm wondering if maybe I have bad contacts at the blower door safety switch. That switch - part HR54ZA006 - seems to control all power to the blower are the contacts in it worn? When I replace the filter in this furnace I have to open the blower cover and I often do it when the fan is running (never when the furnace has active gas in the heat cycle though) maybe that's caused arcing that created a high resistance contact in the switch?
Perhaps the switch had a high resistance contact that was reducing power to the furnace and causing reduced power to the thermostat and causing the control electronics to spazz out?
OK here's my question for you all.
I have a Bryant 90 Plus furnace it's probably 20-25 years old maybe. OK OK I know you are all boo hiss hiss but here is the deal, I also have a computer server farm in the basement that puts out a heck of a lot of heat - which during the winter I divert into the house. So the furnace hardly ever gets used and I have not yet suffered the heat exchanger issues of clogging and rusting (I replaced the blower motor 2 years ago and looked up at the bottom of the heat exchanger and it looked perfect no rust, etc.). However, I run the fan 24x7.
Anyway, the thing is controlled by a Honeywell programmable thermostat - an older unit, it's round in the traditional Honeywell style and runs off the 24v ac power with a digital display. No batteries and no place for them.
Anyway my wife and I go out to the beach for the weekend and we get a call from my 20 year old son Sunday night on the way home claiming the furnace thermostat is broken because it's displaying nothing. Bear in mind that as of yet I have not turned on the furnace this season so I have not bothered changing the filter for the last 3 months. Apparently my son decided putting on a sweater in a 65 degree house was too difficult so he decided to turn on the furnace.
So I get home and yes the thermostat display is blank. I pull it off the wall and measure voltage at the terminals and I'm getting only 17 volts. I replace it and I go downstairs and sure enough the filter is clogged up, I replace that and close the blower door and the furnace is blinking code 33. I check the rollout switch, it's not tripped and has continuity and I get 24 volts each side of the switch. Wires connections are all tight. I try turning the furnace off and on a couple times, nothing.
Finally I pull the blower motor cover and check for voltage at the C and R terminals and get nothing. Ok of course, the blower motor cover is open, I manually close the cover switch, and I get my 24 volts. So I reinstall the cover and suddenly the blink codes disappear, the draft motor fires up, a moment later the gas valve opens and fires up and we get a beautiful blue flame in the burner box inspection port. Furnace begins operating normally, display reappears on the face of the thermostat.
What just happened here? Is this thermostat incompatible? It's been working for years on this furnace BUT maybe once or twice a year I've seen the display blank out, and then come back on 20 minutes later. But the thermostat doesn't lose programming when that happens so maybe it's just seeing a power reduction?? Why would the furnace supply 17 volts?
Was it the clogged filter that tripped some limit switch? If so, what? And why didn't whatever limit switch that got tripped untrip itself once I replaced the filter and turned the furnace off and on. Why was I getting 17 volts at the C and R terminals at the thermostat when the thermostat was off the wall sitting in my hand?
I'm wondering if maybe I have bad contacts at the blower door safety switch. That switch - part HR54ZA006 - seems to control all power to the blower are the contacts in it worn? When I replace the filter in this furnace I have to open the blower cover and I often do it when the fan is running (never when the furnace has active gas in the heat cycle though) maybe that's caused arcing that created a high resistance contact in the switch?
Perhaps the switch had a high resistance contact that was reducing power to the furnace and causing reduced power to the thermostat and causing the control electronics to spazz out?