Can programmable thermostats save money when used with residential heat pumps?
This has been a matter of debate, and I’m not sure if I can put that debate to rest, but I’ll tell you what I know.
Heat pumps require a thermostat with one stage of cooling and two stages of heating – two sets of contacts for heating – one set closes at the setpoint temperature (first stage) and the other is set for about two degrees lower (second stage). During normal operation the heat pump runs whenever the first stage of the thermostat calls for heat. If the weather is very cold and the heat pump cannot provide sufficient heat, or if there is a problem with the heat pump, the temperature in the house drops until the second stage is energized. The second stage of the thermostat energizes the back-up or emergency heat source – this can be an electric resistance heater, a gas furnace or any other automatic source of heat. If the outside temperature rises, temperatures in the house rise – shutting down the heat pump. As temperatures in the house continue to rise they reach the cooling setpoint, which sets the reversing valve in the cooling position and cycles the compressor to maintain temperatures at the cooling setpoint. The difference in temperature between the heating setpoint (let’s say 70°F) and the cooling setpoint (say 78°F) where the heat pump is not running in either heating or cooling mode is called the "dead band." A dead band of at least 6–8 degrees is recommended to minimize unnecessary heat pump operation and save energy.
If the house has a standard programmable thermostat (with a 1-2 degree difference between the first and second stage setpoints) and the temperature in the house is two or more degrees below the first stage setpoint, then when the setback period ends, the back-up heating will be energized and will run until the second stage setpoint is reached. At this point the heat pump continues to run until the first stage setpoint is reached. An efficient heat pump running by itself normally provides two to three watts of heat for every watt consumed, or a coefficient of performance (COP) of 2.0 to 3.0. Electric resistance heat provides one watt of heat per watt used, or a COP of 1.0. If both run at the same time, the COP of the heat pump drops to about 1.5 or less, which is a serious loss in efficiency of the heat pump.
Ramping Recovery Thermostats – Use Them If Backup Heat is Electric Resistance
To prevent inefficiency, thermostat manufacturers developed special models that do not allow the emergency heat to come on under normal warm-up conditions. They accomplish this by learning the thermal characteristics of the house and then starting the heat pump early enough to reach the first stage setpoint temperature at the required time without using emergency heat. In other words, they trade a longer warm-up period for improved efficiency.
There has been a lot of confusion about heat pump thermostats – strictly speaking, nearly any thermostat with two heating stages and a single cooling stage can control heat pumps and be called a "heat pump thermostat." In order to avoid the inefficiencies noted above, it is necessary to use a thermostat that can de-energize emergency heat during recovery – I call these units "ramping recovery thermostats." Manufacturers have different names for this feature – White Rodgers calls the feature "energy recovery management (ERM™)," Robert Shaw calls it "energy efficient recovery mode," Honeywell uses "Adaptive Intelligence Recovery™." When shopping for a heat pump thermostat, look for these designations and make sure they lock-out emergency heat during recovery.
So, my answer is – yes – thermostats designed for heat pumps that prevent the use of emergency electric heat during recovery will save money and energy when compared to a non-programmable thermostat or a conventional heat pump thermostat without the ramping recovery function. Please note that the foregoing discussion only applies to heat pumps with electric resistance backup and does not apply to systems that use a gas heater or other combustion heater to supply emergency heat – these units do not require a ramping recovery thermostat – just make sure that if the heat source is installed in ductwork downstream from the heat pump, it is rated for installation downstream from a cooling coil.