Heat pump / heat strip
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- Heat pump or heat strip: what is wrong? A rooftop heat pump runs the AC backwards with a reversing valve. It loses capacity as the outside air cools and most lock out between about 25 and 40°F. A heat strip is a 1500 W element in the AC's supply air that adds a little heat. Expect: Above 40°F outside, supply air 15 to 25°F warmer than the return in heat pump mode
- In heat pump mode, is the compressor running? Listen at the roof, and feel the condenser end. Compressor running in heat mode makes the outside coil cold. Expect: Compressor on, outside coil cold, inside air warm
- Voltage at the reversing valve solenoid in heat mode? The solenoid sits on the valve near the compressor. Its coil is 12V on some units and 120V on others: read the wiring diagram inside the shroud. Meter it with heat called and cool called. Expect: Coil energized in heat, de-energized in cool (or the reverse; check the diagram)
- 120V at the heat strip element with heat strip selected? Drop the ceiling shroud. Heat strips need the fan running; a limit switch and a thermal fuse sit in series with the element. Expect: 120V across the element terminals with the fan running
Causes and repairs
Reversing valve stuck
The coil is doing its job and the valve won't move. Sometimes switching heat to cool a few times with the compressor running frees it. A stuck valve is sealed-system work; on rooftop units that usually means replacing the unit.
Tools: none
Try cycling modes a few times. Still stuck: quote the unit replacement.
Control box or thermostat setup
The valve never gets told to shift. The thermostat isn't set for heat pump, or the control box output is dead.
Tools: multimeter
Confirm heat pump mode on the thermostat. Meter the control box output to the valve. Replace the box if the output is dead with the call present.
Reversing valve coil open
Voltage arrives and the coil is open. The coil slides off the valve stem.
Tools: hand tools, multimeter
Replace the coil (same voltage). No refrigerant work needed.
Low ambient lockout is normal
Below about 40°F a heat pump has little heat to move. Most controls lock it out between 25 and 40°F and expect the furnace to take over.
Tools: thermometer
Use the furnace or the heat strip. Nothing is broken. Show the owner where the outdoor sensor sits.
Heat strip element open
Power off. Ohm the element: about 9 to 10 Ω for 1500 W. O.L is an open element.
Tools: multimeter
Replace the element or the heat strip assembly.
Limit, thermal fuse or relay
Power off and ohm the limit and the thermal fuse: both should read near 0 Ω cold. An open thermal fuse means it overheated once: look for low airflow. Both good: the control box relay output.
Tools: multimeter
Replace the open part and fix the airflow cause. Replace the relay or control box if the output is dead.
Defrost not keeping up
Heat pumps frost the outside coil and defrost on a timer or sensor. Solid ice means the defrost sensor or control failed, or it is too cold out for the unit.
Tools: multimeter
Check the outdoor or defrost sensor (CCC2 shows E4 or E5). Ohm it against its chart. Below 40°F use the furnace.
Thermostat not set up for heat pump
Many thermostats only show heat pump mode when they detect a heat pump unit or when an installer setting says so. CCC2 decides from the unit's sensors and DIP settings.
Tools: small screwdriver
Set the installer option or DIP switch for heat pump per the thermostat manual. Power cycle.
Keep going on air conditioning
Learn it
Prove it
- AC hums and trips
- AC runs but barely cools
- AC ices up and barely blows
- AC kicks off on hot afternoons
- AC does nothing, thermostat is blank