I arrived to do a diagnostic on a lennox gas furnace Upon arrival system off, furnace doors off. Found furnace dirty, replaced with customer’s own. Heat pump and gas furnace, set to emergency heat. Cycled, furnace combusting well. House is now heating, possible sequencing problem between thermostat, relay and outdoor heat pump. Furnace is high efficient, recommend natural gas to be heat source. Found thermostat dropping call for heat, dropping to 16VAC. Thermostat bad, replaced and cycled monitored system operating normally
