Troubleshooting Challenge: A Gas Furnace That's Heating Intermittently

November 8, 2010

The customer in this particular troubleshooting situation is somewhat savvy about his 12-year-old gas furnace. When he calls for service, he tells you that when things got very cold in the house, he disconnected the power supply to the unit, and then, after turning the 120-volt service switch back on, the operation seemed normal and the house got warm “for a day or so.” But the failure repeated, and he again followed the reset procedure, which got the unit operating again. And, even though he has heat at the moment, he’s called, and you have arrived to evaluate the situation.

What you find is the furnace running and the indoor temperature at an acceptable level. Your evaluation of the equipment includes checking the flame sensor (OK), checking the drain trap, and also checking the fuel pressure at both the inlet and the outlet of the gas valve (also OK).

Figure 1. (Click on the image for an enlarged view.)

As your next step, you go through the reset procedure by disconnecting the power supply for the appropriate time frame, and then turning the power back on. What you expect at this point is that the blower motor will run for 30 seconds on a system restart since that’s considered normal operation for this particular unit after a power interruption.

But, the blower doesn’t start until you hand-spin the squirrel cage. At this point, you’re leaning toward the diagnosis that the motor is failing (see the partial diagram in Figure 1), so you double-check two things: A test of the motor leads from the printed circuit board shows 120 volts, and, after turning off the power supply and removing the leads from the run capacitor, your digital meter shows a reading of 2.5 microfarads.

Your troubleshooting question: Which component needs replacing in order to eliminate the intermittent operation failure?

Compare your answer with ours by clicking on the PDF link below.

For information on Jim Johnson’s HVACR technician training DVD’s, covering a range of service and troubleshooting procedures, click here.

Publication date: 11/08/2010