I have a 12 year old Bryant 90 Plus gas furnace. Earlier this year I had to have the draft inducer fan blade replaced (it had cracked and was making a rattling sound; only the plastic fan blade was replaced, not the inducer motor). The furnace was working fine until a recent cold snap when I began noticing that it was taking longer to get the house up to temperature after recovering from my low overnight and daytime set-back thermostat settings. I also noted that the inducer seemed to be running much longer than usual before the furnace would ignite.
With information obtained by searching various internet forums, I performed the following tests: removed & cleaned flame sensor w/steel wool; inspected ignitor for cracks; checked PVC vent pipes for restrictions; checked condensate drain hose for blockage; checked pressure switch and draft inducer tubes for blockage; sucked on pressure switch tube and heard it ‘click’; ran control board self test (all relays operating properly). I also eliminated the flame roll-out sensor as a possibility since it appears to be a ‘manual reset’ type and has never been tripped. I then checked the voltage at the “ON” terminal of the pressure switch:
(1) With the furnace operating normally, I get a steady 26+ volts shortly after the inducer starts, then the ignitor glows, the burners ignite, and within 2 minutes the blower starts.
(2) During those times when the inducer runs but the furnace will not ignite, the voltage fluctuates rapidly between 0 and 26+ volts, accompanied by an audible clicking of the pressure switch (this is true whether I probe the switch itself or the appropriate test port on the control board). If I jump the pressure switch terminals for 10-15 seconds during this period, the ignitor will glow and the burners will ignite.
(3) Sometimes the burners will shut off after the furnace has been running for a while, even though the t-stat is not satisfied. The inducer continues to run, however, and when I check the voltage at the pressure switch it shows a steady 0 volts. If I switch off the power for 15 minutes, the system will usually start normally and may or may not continue to run until the t-stat is satisfied. BTW, the inducer motor gets very hot— almost too hot to touch.
This past weekend when temperatures moderated a bit, the furnace ran fine. Now we’re having another cold snap and the system is once again misbehaving. It’s as if the problem occurs only when the system is really being taxed, as when it runs for extended periods during recovery from low set-back temperatures. I suspect one of the following components but don’t know how I can confirm or rule them out:
(1) Inducer motor— is it possible that the motor is overheating and not spinning at sufficient rpm to create or maintain a sufficient draft?
(2) Pressure switch (is it normal for the voltage to fluctuate as described above)?
(3) Control board?
(4) The only other thing I can think of is that the high limit switch is being tripped— maybe the airflow thru the furnace is marginal and only during periods of extended running does the switch open; other times when the furnace cools down between cycles, the switch doesn’t trip.
Thanks for reading this very long post! Any help or suggestions are greatly appreciated.
With information obtained by searching various internet forums, I performed the following tests: removed & cleaned flame sensor w/steel wool; inspected ignitor for cracks; checked PVC vent pipes for restrictions; checked condensate drain hose for blockage; checked pressure switch and draft inducer tubes for blockage; sucked on pressure switch tube and heard it ‘click’; ran control board self test (all relays operating properly). I also eliminated the flame roll-out sensor as a possibility since it appears to be a ‘manual reset’ type and has never been tripped. I then checked the voltage at the “ON” terminal of the pressure switch:
(1) With the furnace operating normally, I get a steady 26+ volts shortly after the inducer starts, then the ignitor glows, the burners ignite, and within 2 minutes the blower starts.
(2) During those times when the inducer runs but the furnace will not ignite, the voltage fluctuates rapidly between 0 and 26+ volts, accompanied by an audible clicking of the pressure switch (this is true whether I probe the switch itself or the appropriate test port on the control board). If I jump the pressure switch terminals for 10-15 seconds during this period, the ignitor will glow and the burners will ignite.
(3) Sometimes the burners will shut off after the furnace has been running for a while, even though the t-stat is not satisfied. The inducer continues to run, however, and when I check the voltage at the pressure switch it shows a steady 0 volts. If I switch off the power for 15 minutes, the system will usually start normally and may or may not continue to run until the t-stat is satisfied. BTW, the inducer motor gets very hot— almost too hot to touch.
This past weekend when temperatures moderated a bit, the furnace ran fine. Now we’re having another cold snap and the system is once again misbehaving. It’s as if the problem occurs only when the system is really being taxed, as when it runs for extended periods during recovery from low set-back temperatures. I suspect one of the following components but don’t know how I can confirm or rule them out:
(1) Inducer motor— is it possible that the motor is overheating and not spinning at sufficient rpm to create or maintain a sufficient draft?
(2) Pressure switch (is it normal for the voltage to fluctuate as described above)?
(3) Control board?
(4) The only other thing I can think of is that the high limit switch is being tripped— maybe the airflow thru the furnace is marginal and only during periods of extended running does the switch open; other times when the furnace cools down between cycles, the switch doesn’t trip.
Thanks for reading this very long post! Any help or suggestions are greatly appreciated.