I just yesterday installed a new (old stock-the furnace is 4 years old but was never hooked up) Trane XR-80 100k BTU 80% efficient nat gas furnace in my shop.
I am using a Honeywell electronic thermostat utilizing only the R and W terminals on the thermostat and furnace (this should cause the blower fan to be in auto made, controlled by the furnace computer board if I understand right). I can hear it's internal relay clicking appropriately, so I think it's good.
The furnace appeared to work normally while I was checking it, raising the shop to 70 deg, and the burner, and then blower shutting off when the stet-temperature was reached, but it appears to have a problem:
1) I ran the furnace a bit this morning, then turned it off and opened the doors for awhile to let moisture out. I closed the doors, and I turned the furnace on.
The furnace lit normally and ran for the time I was in there, and I left to have dinner (the shop was 36deg, and I set 68deg), expecting it to be warm when I returned..
At some later point, the flame apparently shut off (for no reason?) and the room was only 41 deg when I returned (thermostat was set well higher than the room temperature, so the burner should have been on), but the blower motor kept running,
I turned the mode switch on the thermometer from HEAT to OFF expecting the blower motor to stop (since the heat exchanger was stone cold and had been for a while I suspect), but it did not even after waiting 5 minutes.
I removed the front panel and the safety switch shut the blower down. I closed the switch and the furnace appeared reset. I then moved the thermostat mode switch from OFF to HEAT on the thermostat and the furnace lit and ran normally again.
The red DS-1 LED trouble code light blinks continuously 1 blink/sec with the furnace powered (but not running), and when heat is commanded, blinks about 2 blinks/sec.
I looked up the Trane codes and it said this is how it should blink if working properly, but obviously the blower motor should not (?) just keep running forever with the flame out for a considerable period.
When powered (115v to the unit, but not activated), the furnace emits a very quiet transformer hum (barely noticible).
As an aside, last night when leaving the shop I turned the gas supply off (until I observe it for a while, I am reluctant to have it running overnight unattended), but forgot to change the Thermostat mode switch to OFF, and when the shop cooled below the thermostat set point, I guess the furnace tried to light (but couldn't because there was no gas pressure). When I walked in this morning, the blower fan was STILL running.
Shouldn't the furnace sense that the heat exchanger is cold and turn the blower motor off? In the "old days" there was a "Fan Control Relay" that in "Auto" commanded the fan to run only when the heat exchanger was above a certain temperature.
There also a "Limit Switch" that shut the gas off if the heat exchanger temperature exceeded a set limit.
I know modern furnaces have computer boards, but I would think they function about the same way?
So I guess my questions would be:
1) For what would my burner have shut off tonight (there is good gas pressure and plenty of airflow through the furnace, so I am sure the internal exchanger temp never rose above any "limit" sensor), and-
2) Why has my blower failed to shut off TWICE now (continuing to run for hours) after the flame went out while the unit was running (room temp still below set temperature) ?
Any suggestions welcome-
Bob
I am using a Honeywell electronic thermostat utilizing only the R and W terminals on the thermostat and furnace (this should cause the blower fan to be in auto made, controlled by the furnace computer board if I understand right). I can hear it's internal relay clicking appropriately, so I think it's good.
The furnace appeared to work normally while I was checking it, raising the shop to 70 deg, and the burner, and then blower shutting off when the stet-temperature was reached, but it appears to have a problem:
1) I ran the furnace a bit this morning, then turned it off and opened the doors for awhile to let moisture out. I closed the doors, and I turned the furnace on.
The furnace lit normally and ran for the time I was in there, and I left to have dinner (the shop was 36deg, and I set 68deg), expecting it to be warm when I returned..
At some later point, the flame apparently shut off (for no reason?) and the room was only 41 deg when I returned (thermostat was set well higher than the room temperature, so the burner should have been on), but the blower motor kept running,
I turned the mode switch on the thermometer from HEAT to OFF expecting the blower motor to stop (since the heat exchanger was stone cold and had been for a while I suspect), but it did not even after waiting 5 minutes.
I removed the front panel and the safety switch shut the blower down. I closed the switch and the furnace appeared reset. I then moved the thermostat mode switch from OFF to HEAT on the thermostat and the furnace lit and ran normally again.
The red DS-1 LED trouble code light blinks continuously 1 blink/sec with the furnace powered (but not running), and when heat is commanded, blinks about 2 blinks/sec.
I looked up the Trane codes and it said this is how it should blink if working properly, but obviously the blower motor should not (?) just keep running forever with the flame out for a considerable period.
When powered (115v to the unit, but not activated), the furnace emits a very quiet transformer hum (barely noticible).
As an aside, last night when leaving the shop I turned the gas supply off (until I observe it for a while, I am reluctant to have it running overnight unattended), but forgot to change the Thermostat mode switch to OFF, and when the shop cooled below the thermostat set point, I guess the furnace tried to light (but couldn't because there was no gas pressure). When I walked in this morning, the blower fan was STILL running.
Shouldn't the furnace sense that the heat exchanger is cold and turn the blower motor off? In the "old days" there was a "Fan Control Relay" that in "Auto" commanded the fan to run only when the heat exchanger was above a certain temperature.
There also a "Limit Switch" that shut the gas off if the heat exchanger temperature exceeded a set limit.
I know modern furnaces have computer boards, but I would think they function about the same way?
So I guess my questions would be:
1) For what would my burner have shut off tonight (there is good gas pressure and plenty of airflow through the furnace, so I am sure the internal exchanger temp never rose above any "limit" sensor), and-
2) Why has my blower failed to shut off TWICE now (continuing to run for hours) after the flame went out while the unit was running (room temp still below set temperature) ?
Any suggestions welcome-
Bob
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