In this month’s troubleshooting situation, you’re responding to a customer’s complaint that his furnace isn’t heating properly. The equipment is an 80 percent AFUE, natural gas, induced draft unit that employs a hot surface ignition system. A schematic diagram of the electrical system is shown in Figure 1.

Upon your arrival, you set the thermostat to initiate a cycle and note that the induced draft motor operates, but then shuts down before ignition is accomplished. The customer confirms that what just happened occurs “sometimes,” and also says that there are times when the burner does come on, but then turns off before the fan motor starts. He also explains that there are times that the burners come on and the indoor blower motor starts, but then everything shuts down before the house gets warm. The customer then tells you that the furnace worked OK during the last heating season, and that during the summer he decided to add an additional duplex receptacle near the furnace room.

After confirming that the customer tapped into the furnace electrical receptacle to add the second receptacle, you disconnect the furnace power cord and test the receptacle at the points shown in Figure 2.

Your results are:

A - 115 VAC

B - 115 VAC

C - 0 VAC

Your two-part troubleshooting question: What is the cause of the intermittent equipment operation, and what do you do?

Compare your answer with ours here.

Publication date: 9/5/2016

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