This month’s troubleshooting situation involves an up-flow gas furnace that serves as both the heating source in the winter and as the air handler in a split system for cooling in summer. The condensing unit is on a pad next to the building on a concrete slab. Since there is no crawl space or basement, the connecting tubing and wiring for the condensing unit are routed through an underground chase from the equipment room to the outside.

Several days before your arrival, a failed transformer was replaced in this equipment. When you find the unit sitting dead and confirm that there is 120 VAC present at the furnace wiring connections (see the diagram in Figure One), your next step with the Interlock Switch in a closed position is to check the primary connections of the transformer, which shows 120 volts applied. Your next check at the secondary connections of the transformer shows 0 volts.