Comfort·6 min read
Stop setting your thermostat back at night
The habit that saves money with a furnace does the opposite with a heat pump.
The habit that saves money with a furnace does the opposite with a heat pump.
Setting the thermostat back a few degrees overnight and recovering it in the morning is a habit most people learned with a furnace, where it reliably saves money. With a heat pump, particularly one with electric strip heat as backup, it frequently does the opposite, and it's worth understanding why before you carry the habit over.
A furnace has a large amount of excess heating capacity relative to the steady-state load of the house, so it can recover a setback quickly by simply running at full output for a while. A heat pump's output capacity is closer to the house's actual heat loss, especially in cold weather when its capacity has already fallen with the outdoor temperature. Asking it to recover several degrees quickly can exceed what the heat pump alone can deliver.
When that happens, most systems are configured to bring in auxiliary or emergency heat, electric resistance strips, to make up the difference and satisfy the thermostat faster. That backup heat is far more expensive to run per BTU than the heat pump's compressor, often three to four times the cost. The morning recovery from a nighttime setback can burn through more in backup-heat electricity than the setback saved overnight, netting out worse than just holding a steady temperature.
This isn't universal. A heat pump with generous excess capacity relative to the house's load, in a mild climate, or with a well-configured smart thermostat that manages recovery gradually and avoids triggering aux heat, can still benefit from a modest setback. The failure mode specifically involves a big setback, a fast-recovery demand, and a system whose controls default to backup heat rather than a slow, heat-pump-only ramp.
