System only has between 5 and 7.5 kw heat in operation. Filter is clean, thermostat batteries are new, and both heaters are coming on and going off as needed. Most likely cause of the issue is system is pulling in cool air from an outside source. Most likely location would be the vent in the living room. No air appears to be coming from it so it may have come detached. Informed customer they may want to cover that vent if it gets really cold outside. System appears to be operating normally at this time.