Avoid Deep Thermostat Setbacks with Variable Capacity Heat Pumps
Dec 02, 2020
Energy savings is a key selling feature for all variable capacity heat pumps, including ductless heat pumps. Ductless and variable capacity heat pumps boast greater efficiency because they are far more efficient when running at low speed than at full power. Also, unlike earlier generations of heat pumps that only operate at one speed, variable capacity heat pumps have a wide range of operating speeds.
Variable capacity heat pump testing has shown that setting temperatures back five to ten degrees initially saves energy by reducing the immediate heating need. However, recovery from a such a deep setback causes the heat pump’s efficiency to plummet. The system will run in the most inefficient full power mode to quickly recover a room or house back to the original setpoint. Depending on the type of heat pump equipment and its configuration, this full power mode could result in the compressor and the backup heat source operating simultaneously.
Instead, allowing a variable capacity heat pump system to maintain a set temperature is the most energy and cost-efficient strategy. This allows the equipment to operate for long periods of time in its most efficient stage. Maintaining a set temperature can minimize, if not eliminate, the need for the equipment to run in its full power mode.
What, if any, setback is ideal? The ideal setback is hard to define, but it is possible to provide homeowners with guidance that will maximize their comfort without increasing their energy bills. For short periods (less than 6 hours), a ductless heat pump is most efficient without a setback. For medium-duration periods (6-12 hours), the best setback is two to three degrees. This prevents the system from operating at full power for an extended recovery period. If the homeowner is away for longer than 12 hours, then a standard heat pump setback of five to eight degrees is the best choice for maximum energy efficiency. Several brands offer a sophisticated soft recovery at the end of the setback period, which avoids high-speed operation but still achieves the desired temperature.
- < 6 hours: No setback
- 6-12 hours: 2-3 degree setback
- 12+ hours: 5-8 degree setback, depending on desired level of savings
If you are looking for resources to help homeowners understand how to operate their equipment, check with your manufacturer for operating instructions or reference resources from the NW Ductless Heat Pump Project: