Energy Savings

A reminder for heat pump owners

Plus, understand the three stages of heat pump operation

A malfunctioning heat pump can cause excessive energy use, and you may not know it until you get your electric bill.

Man holding up a thermal imaging device.
Denison uses a thermal imaging device to monitor heat gain, but a digital cooking thermometer will allow members to check the temperature of air going into their heat pump system and the temperature of air returned by their system.

If you have a heat pump set to heat mode, make sure the outside compressor is operating when the indoor air handler is running. If the outside unit is not operating, the auxiliary resistance heat will automatically come on to heat your house; you won’t know the compressor is not operating because the house will be warm.

Everyone should be familiar with the Auxiliary Heat (Aux Ht) indicator on their thermostat. The auxiliary heat — which uses resistance heat strips located in the air handler to warm air — will automatically come on to compensate when the heat pump is not creating enough heat on its own. However, it’s a much less efficient way to heat your home, requiring two to three times more energy than a properly operating heat pump by itself.

The auxiliary heat is designed to operate when the indoor temperature is three degrees cooler than the thermostat setting. It generally operates when you raise the thermostat setting more than two degrees, which may be normal when you get up to a cold house in the morning or come home to a cold house in the evening. That is a good time to notice where your auxiliary indicator is and make sure that it goes off when the indoor temperature is within two degrees of your thermostat setting.

If you have a heat pump, it is best to increase your heat setting by only two degrees at a time to prevent your auxiliary heat from operating.

UPDATE from PEC Senior Energy Service Advisor Chris Denison

There are basically three different stages of heat pump operation. Your best tool to evaluate performance at each stage is a digital probe cooking thermometer. To know your heat pump, you must know the difference between return air temperature — what goes in your filter grill — and supply air temperature — what comes out of the vents. This is referred to as the “split” temperature.

  1. Heat pump only: The outside compressor is used to create heat.This is the most efficient operation of your heat pump system because it is only using the motor, but is usually only effective when the outside temperature is above 40 degrees. The split temperature between the supply air and the air coming out of your vents should be 25 degrees or more.
  2. Heat pump with auxiliary heat: If you increase the thermostat setting by more than two degrees, the auxiliary heating elements will come on to help the compressor create additional heat. Or if the system is not achieving the desired setting, the auxiliary heat will come on to help. The split temperature should be about 30 degrees depending on the amount of connected heating elements. Auxiliary heat will also operate during the system’s defrost cycle. If the system is having to defrost frequently, the thermostat should be switched to the emergency heat setting.
  3. Emergency heat: This setting is the most effective at creating heat when the outside temperature is below freezing. It also requires more electricity, so it’s more expensive to operate, but because it turns off the outside compressor and operates using only the resistance heating elements, it will achieve comfort and reduce wear and tear on the heat pump system. The split temperature in this setting should be well above 30 degrees, depending on the number of heating elements that are connected.

There are many variables that could impact each one of these functions — factory default parameter settings, adjustments made by your heating, ventilating and air conditioning contractor and how well your equipment performs. Try to find a knowledgeable HVAC contractor who will explain how your system works and help you get the most out of it.