In this troubleshooting situation, the customer has called to report a “no heat” problem. The equipment is a 5-ton, 230 VAC single-stage gas/electric package unit that employs an integrated furnace control and a constant torque blower motor.
When you arrive, you find that the building temperature is far below the set-point of the thermostat, and the indoor fan motor is operating, but the temperature of the air being delivered from the supply registers indicates that the furnace is not operating.
On the rooftop, your first step is to consult the schematic for the equipment (see Figure One). Based on the manufacturer’s information regarding the fault code presented by the LED, you check to find the protective control device that is responsible for an open circuit in the control segment of the equipment for the heating mode.
Your investigation reveals that the MRLC (Manual Reset Limit Control) is open. After disconnecting the power supply, you manually re-set the limit control, and after waiting to allow for any necessary control system re-set to occur, you find when you restore the power supply that the sequence of heating operation functions normally, igniting the burners and operating the indoor fan motor.
Your troubleshooting question: What is the next step you need to take in servicing this equipment?
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