Service & Maintenance / Extra Edition / HVAC Light Commercial Market / Rooftop Units

Troubleshooting Challenge: A Rooftop Unit That Won't Shut Down

March 7, 2011
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Figure 1. (Click on the image for an enlarged view.)

In this month’s troubleshooting situation, our customer has called to say that the older package unit that heats and cools his office is managing to warm up the area, but then seems to cool down too much before providing heat again. When you arrive, the first thing you note is that the indoor blower motor is operating, and that the fan switch on the thermostat assembly is set in the position shown in the diagram in Figure 1.

You also note that:

• The discharge air temperature is what you would expect it to be for this gas-fired heating system.

• Turning the thermostat to a lower set point and waiting a reasonable time proves that the indoor blower motor doesn’t shut down.

When you access the unit’s electrical system, you note that the burners are operating normally since the thermostat is again calling for heat, and you begin your troubleshooting procedure by placing a voltmeter at H1 and H2 on the IBDR coil and confirm 24 vac. When you instruct the customer to turn the thermostat down to a lower set point, you note that the burners shut down, and after a delay, your voltage reading at IBDR is 0 vac, while the indoor blower motor continues to operate. Your next voltage test is at #2 and #4, M1 and M2, and your reading is 0 vac.

Your troubleshooting question: What is your next step in solving this problem?

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

For information on Jim Johnson’s training DVDs for troubleshooting HVACR electrical circuits, click here.

Publication date: 03/07/2011


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