In an air source heat pump, supplemental strip heaters are typically energized during which condition besides extremely low outdoor temperatures?

In an air source heat pump, supplemental strip heaters are typically energized during which condition besides extremely low outdoor temperatures?

• A. During the cooling cycle
• B. During the off cycle
• C. During defrost cycles
• D. When the supply fan is off

Answer: (C)

The key idea is that supplemental electric strip heaters provide extra heat when the heat pump can’t meet the indoor load. They’re used not only for very cold outdoor temperatures but also during defrost cycles. When the outdoor coil is frozen and a defrost cycle starts, the system reverses to melt the ice, which temporarily lowers the amount of heat delivered indoors. To keep indoor temperatures steady during this brief interval, the electric strip heaters kick in and add heat. The other situations—cooling cycles, the system being off, or the supply fan not running—don’t rely on electric resistance heat to maintain comfort, so they aren’t the typical conditions for energizing the strip heaters.

The key idea is that supplemental electric strip heaters provide extra heat when the heat pump can’t meet the indoor load. They’re used not only for very cold outdoor temperatures but also during defrost cycles. When the outdoor coil is frozen and a defrost cycle starts, the system reverses to melt the ice, which temporarily lowers the amount of heat delivered indoors. To keep indoor temperatures steady during this brief interval, the electric strip heaters kick in and add heat. The other situations—cooling cycles, the system being off, or the supply fan not running—don’t rely on electric resistance heat to maintain comfort, so they aren’t the typical conditions for energizing the strip heaters.