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

The customer in this particular situation is somewhat savvy about their 12-year-old gas furnace, so when they call for help, they tell you that when things got very cold in the house, they disconnected the power supply to the unit, and then, after turning the 120V service switch back on, the furnace operation seemed normal and the house got warm “for a day or so.” But, the failure repeated, and they again followed the reset procedure which got the unit operating again. And, even though they had heat again, they called for service.

When you arrive, you find 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 checking the fuel pressure at both inlet and outlet of the gas valve (also OK). As your next step, you go through the reset procedure by disconnecting the power supply for the appropriate time frame and then turning the service switch back on, expecting the blower motor to 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 at the motor leads from the printed circuit board that shows 120 volts, and a check of the run capacitor with your digital meter that shows a reading of 2.5 microfarads.

Your troubleshooting question: Which component was replaced, eliminating the intermittent equipment operation failures?

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Publication date: 03/09/2009