Service & Maintenance / Extra Edition

Troubleshooting Challenge: A Thru-the-Wall Package Unit That Won't Heat

In this month’s troubleshooting situation, we have a customer who has called to say that their condominium has no heat. The particular equipment in this case is a thru-the-wall package unit that provides cooling in the summer and employs a natural gas system to heat the building in the winter. The customer also informed the dispatcher that they had previous service on this equipment in the summer, and that the technician who responded to that request for service replaced the contactor. The unit operated normally through the rest of the cooling season, but failed to provide heat at the first start-up in the winter.

When you arrive and test cycle the unit by setting the thermostat to call for heat, you discover that the burners ignite, but after a delay, the system shuts down on the high limit switch due to the fact that the indoor blower motor did not operate.

When you consult the wiring diagram (click here for a PDF of the diagram), you note that the motor receives its 230-volt power supply via the wiring shown from T1 and T2 on the contactor. You also note that the replacement contactor that was installed in the previous service call is a two-pole component, and that the technician who performed the repair maintained the wiring as it is shown on the diagram.

Your troubleshooting question: What is the underlying cause behind this situation that is preventing the blower motor from operating in the heating mode?

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

Publication date: 03/05/2012

PDF - March Troubleshooting Answer

For information on Jim Johnson’s HVACR technician training DVDs, go to

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