DCM2 service 2 of 2 Heat: Changed (1) 16x20x1 filter,, Performed Heating Maintenance Per checklist, tenant mentioned on arrival that the system would not go into cool mode and would only blow hot air, after checking operation found that the thermostat was not sending 24v to the reversing valve, recommended to replace thermostat homeowner said that she has a spare nest thermostat that they will try first.