An air source heat pump (ASHP) is a highly efficient heating and cooling system. However, to ensure consistent indoor comfort, it is often equipped with electric backup heat—also referred to as "auxiliary heat" or "emergency heat." The necessity for this backup heating stems from considerations related to thermodynamics, operational mechanics, and economics.
1. Reduced Heating Capacity in Cold Weather (The Balance Point)
Heat pumps operate by extracting heat from the outdoor air and transferring it indoors. As the outdoor temperature drops, it becomes increasingly difficult for the system to extract heat, causing the heat pump’s heating capacity to decline. Simultaneously, because heat is lost from the home to the cold outdoor environment, the building's heating demand increases. The temperature at which the heat pump’s heating capacity exactly matches the home’s heating load is known as the "balance point." When the outdoor temperature falls below this point, the heat pump can no longer meet the heating demand on its own; at this stage, electric backup heat is required to maintain the indoor temperature.
2. The Defrost Cycle
In cold, humid conditions, moisture from the air can condense and accumulate on the heat pump’s outdoor coils, forming a layer of frost that impairs its heating capacity. To remove this frost, the system temporarily reverses its operation (switching to cooling mode) to melt the ice. During this defrost cycle, the electric backup heating system activates automatically to prevent cold air from being blown into the home through the vents.
3. System Sizing and Economic Considerations
When sizing an HVAC system, one must account for both summer cooling and winter heating loads. In many climates, the equipment capacity required for peak summer cooling is lower than that required for peak winter heating. To avoid oversizing the heat pump—which could lead to poor dehumidification in the summer, higher upfront equipment costs, and frequent cycling (starting and stopping) of the unit—installers typically size the system to better align with the cooling load or the average heating load. During the coldest days or weeks of winter, the system engages electric auxiliary heating to cost-effectively bridge any heating shortfall.
4. Emergency Redundancy
Auxiliary heating also serves as a vital safety safeguard. In the event that the heat pump’s primary compressor fails—or if the system encounters extreme cold that exceeds its design limits—the electric auxiliary heating system ensures that household pipes do not freeze and burst, while keeping the home’s occupants safe.
Efficiency Trade-offs
It is important to note that electric resistance auxiliary heating is significantly less efficient than the heat pump itself. While modern air source heat pumps can maintain a Coefficient of Performance (COP) of 1.75 or higher even in cold weather, electric resistance heating has a COP of only 1.0—meaning it produces exactly one unit of heat for every unit of electricity consumed. Because auxiliary heating is more expensive to operate, modern thermostats are designed to activate it only when absolutely necessary, such as when a rapid increase in indoor temperature is required or during periods of extreme cold.
Technological Advancements
As technology continues to evolve, a new generation of "cold-climate" air source heat pumps (ccASHPs) has emerged, capable of maintaining their rated heating capacity at much lower temperatures—down to -13°F (-25°C), for example. Although these advanced models reduce the frequency with which auxiliary heating is needed, electric auxiliary heating remains a standard and practical feature in most installations, ensuring reliable heating and comfort throughout the year.
An air source heat pump (ASHP) is a highly efficient heating and cooling system. However, to ensure consistent indoor comfort, it is often equipped with electric backup heat—also referred to as "auxiliary heat" or "emergency heat." The necessity for this backup heating stems from considerations related to thermodynamics, operational mechanics, and economics.
1. Reduced Heating Capacity in Cold Weather (The Balance Point)
Heat pumps operate by extracting heat from the outdoor air and transferring it indoors. As the outdoor temperature drops, it becomes increasingly difficult for the system to extract heat, causing the heat pump’s heating capacity to decline. Simultaneously, because heat is lost from the home to the cold outdoor environment, the building's heating demand increases. The temperature at which the heat pump’s heating capacity exactly matches the home’s heating load is known as the "balance point." When the outdoor temperature falls below this point, the heat pump can no longer meet the heating demand on its own; at this stage, electric backup heat is required to maintain the indoor temperature.
2. The Defrost Cycle
In cold, humid conditions, moisture from the air can condense and accumulate on the heat pump’s outdoor coils, forming a layer of frost that impairs its heating capacity. To remove this frost, the system temporarily reverses its operation (switching to cooling mode) to melt the ice. During this defrost cycle, the electric backup heating system activates automatically to prevent cold air from being blown into the home through the vents.
3. System Sizing and Economic Considerations
When sizing an HVAC system, one must account for both summer cooling and winter heating loads. In many climates, the equipment capacity required for peak summer cooling is lower than that required for peak winter heating. To avoid oversizing the heat pump—which could lead to poor dehumidification in the summer, higher upfront equipment costs, and frequent cycling (starting and stopping) of the unit—installers typically size the system to better align with the cooling load or the average heating load. During the coldest days or weeks of winter, the system engages electric auxiliary heating to cost-effectively bridge any heating shortfall.
4. Emergency Redundancy
Auxiliary heating also serves as a vital safety safeguard. In the event that the heat pump’s primary compressor fails—or if the system encounters extreme cold that exceeds its design limits—the electric auxiliary heating system ensures that household pipes do not freeze and burst, while keeping the home’s occupants safe.
Efficiency Trade-offs
It is important to note that electric resistance auxiliary heating is significantly less efficient than the heat pump itself. While modern air source heat pumps can maintain a Coefficient of Performance (COP) of 1.75 or higher even in cold weather, electric resistance heating has a COP of only 1.0—meaning it produces exactly one unit of heat for every unit of electricity consumed. Because auxiliary heating is more expensive to operate, modern thermostats are designed to activate it only when absolutely necessary, such as when a rapid increase in indoor temperature is required or during periods of extreme cold.
Technological Advancements
As technology continues to evolve, a new generation of "cold-climate" air source heat pumps (ccASHPs) has emerged, capable of maintaining their rated heating capacity at much lower temperatures—down to -13°F (-25°C), for example. Although these advanced models reduce the frequency with which auxiliary heating is needed, electric auxiliary heating remains a standard and practical feature in most installations, ensuring reliable heating and comfort throughout the year.
