This device measures outdoor temperature and heat-mode compressor run-time to determine if an electric heat pump system needs to be switched into auxiliary heat or emergency heat mode.

It allows use of less expensive and more readily available single-stage thermostats on a multistage heatpump system. ALSO prevents that annoying run-all-night-blowing-cold-air behavior that heat pumps are so famous for! Saves wear on compressor.

Code follows the following rules:

Below a certain outdoor temperature, actuate aux heat.
If outdoor temp keeps falling, below a certain lower temperature turn off compressor heat and use electric heat only.
Regardless of outdoor temp, if compressor heat has been on constantly more then 10 minutes, actuate auxiliary heat until thermostat cycles off.
Regardless of all other logic decisions, if compressor has been deactivated by time or temperature, there is a 3 minute restart delay.

Has two relays and controlls 2 heat pump systems.