Carrier heat pump (basic model) runs at 0 or below
A basic model heat pump with an air handler with electric strip heat generally will continue to run with a call for heat regardless of the outdoor air temperature unless there was an outdoor thermostat (an accessory) installed. There is still heat in the outdoor even at temperatures of zero and below. Generally, there is a balance point of approx 30-35degF that allows the heat pump to heat your house to 68degF without the need for the electric heaters. Below that, the heat pump can still extract heat from outside and "pump" it into your house; however, it will need help from the electric heaters. In temperatures below the balance point, the heat pump will run non-stop. As the temperature in the house falls approx 1 - 2 degF below the setpoint, the electric heaters will energize. So it is not uncommon for the system to run non-stop with the temperature falling a few degrees below setpoint. The colder the outside temperature, the more often the strip heaters will cycle on/off. If your system is well-maintained and operating at proper performance, it is still cheaper to run your heat pump than it is to run on straight electric heat. If you live in a climate where below zero temperatures are normal during the winter months, you might want to research heat pump options that offer higher COP's and increased performance in colder temperature than the basic contractor's model. Start with initiating communication with a local contractor. You will need to consider initial cost and the pay-back period based on your local energy costs and your specific demands you want your system to meet.
Hope this helps!
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