Troubleshooting Challenge: A ‘No Heat’ Complaint
FIGURE 1: Furnace wiring diagram.
In this troubleshooting situation, the equipment that needs servicing is a condensing gas furnace in a relatively small home of approximately 1,200 square feet. The capacity of the unit is 40,000 Btu and it is equipped with an LED readout system.
When you arrive at the customer’s home to respond to the complaint of “not heating,” you find the LED sequence flashing … one short flash, then three long flashes.
Observing the LED sequence, you’re prompted to take three steps in accordance with the wiring diagram shown in Figure 1 and consulting the troubleshooting procedure guide.
1. Make a visual inspection of the FRS (flame rollout switch), and it indicates it is closed.
2. Use an ohmmeter on FRS, which shows that it is closed.
3. Use an ohmmeter on LS (limit switch), which shows infinity.
These checks lead you to a further investigation, and the component at the top of your suspect list is the indoor blower motor. You find it cool to the touch, and after isolating the wiring connections to the motor (COM (Common), HEAT, COOL, and SPARE1), your ohmmeter shows a reading of infinity when checking from the white wire to any other color.
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Publication date: 11/7/2016
For information on Jim Johnson’s HVACR technician training DVDs, go to http://techtrainassoc.com/training-videos/hvacr-training-videos/.
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