Heat pumps: Operating in warm weather

Can a heat pump system be run on the heating cycle when it is warm outside and the ambient outdoor temperature is over 70 degrees Fahrenheit? I’ve been taught that this mode of operation can cause damage to the system.

The Money Pit Answer

A heat pump can be a very efficient means of HVAC comfort in temperate climates. Running its heating function during warm weather is a rare circumstance but can be done safely, according to Randy Scott, vice president of product systems management for Trane.
"Running a heat pump at a 70-degree outdoor temperature is actually within the normal band of operation, so there would be no adverse affects on the reliability or longevity of the system," says Scott. "In fact, with the majority of the heat pumps that Trane manufactures, we actually go up to 72 degrees outdoors, and indoor temperatures can range anywhere from 60 to 80 degrees."
Heat pumps are typically rated at two different points: 47 degrees outdoors with 70 degrees indoors, and 17 degrees outdoors with 70 degrees indoors. So, for example, if you had a three-ton heat pump, it would produce three tons of capacity at a 47-degree outdoor temperature. And as you go up as high as 72 degrees, it'll actually produce more capacity than a nominal three-ton unit would, with more than 40,000 BTUs of output at that higher outdoor condition. Scott adds that at a higher outdoor temperature, the system's output can also become higher than its nominal capacity.
 Your heat pump can be operated in its heating mode when the outdoor temperature is at or slightly above 70 degrees. Be sure to consult your specific unit's operation instructions and ratings for more details.