Heat pumps and heat strips

Heat pumps heat by running the AC backward. They work less well as it gets cold outside.

The short version

• Heat pumps have a temperature cutoff. Below it, the system switches to the furnace or heat strip.
• A heat strip is just a resistive heater. Low heat output from it is normal; it's meant to take the chill off.
• If heat mode blows cold, check whether it's a heat pump in cold weather before you diagnose.
• Check the reversing valve operation and the thermostat settings for heat pump mode.

Numbers

• Useful range: above about 40°F; lockout 25 to 35°F
• Heat strip: 1500 W, about 10 Ω

Try it on a service call

Put the lesson to work on a hands-on troubleshooting case.

• AC kicks off on hot afternoons

Keep going on air conditioning

Learn it

• Rooftop AC
• Capacitors
• Airflow and coils
• Thermostats and AC control boards

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

See it in 3D

• Rooftop AC 3D wiring model

On the job

• Air conditioner
• Thermostat
• Heat pump / heat strip