In this troubleshooting situation, the equipment is a gas pack, and the resident has called for service because the building “isn’t warm at all.” After confirming the conditions reported by the customer and checking to be sure that the filter is clean, you perform an initial check of this rooftop unit, and note that the fault code light on the printed circuit board shows a sequence of three flashes.

When you confirm the code as “High Limit Switch protection device open” you take the next step of checking the wiring diagram, a photo of which is shown in Figure One.

After disconnecting the power supply and confirming that the MRLC (Manual Reset Limit Control) located in the burner compartment is not tripped open, you take the next step and test the LC (Limit Control) located in the blower compartment. When you disconnect the wiring and check this control with an ohmmeter, it shows continuity.

As your next step you replace the service panels and restore the power supply. You observe that the draft inducer motor and indoor blower motor starts. Then, after an appropriate delay, the ignition system functions, and the burners ignite. Allowing the cycle to continue and monitoring the operation of the equipment, you note that after several minutes of run time, the burners turn off and the fault code light again shows three flashes. When you disconnect the power supply, remove the blower motor access panel, and check the LC, your meter shows OL. After disconnecting the power supply and waiting for 10 minutes, your follow-up check of the LC shows continuity.

Based on the results of your testing, you use a dual port manometer to perform a static pressure test by checking directly at the return air inlet of the equipment and directly at the supply air outlet. Your manometer shows a TESP (Total External Static Pressure) of 1.2 inches of water column.

Your troubleshooting question: What is the next step you need to take in servicing this equipment?

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