Heat Pumps: The Indoor Unit

An infrared image of the indoor unit of a heat pump, showing high return air temperatures above 100 degrees F.

A blog post by Steven Winter Associates

Key Takeaways

  • Oversizing the system can cause the unit to cycle at warmer temperatures, which significantly reduces the efficiency of the system.
  • Ductless units are typically placed high on wall near the ceiling, but this leads to warmer return air temperatures which reduces both efficiency and capacity of the unit.

Executive Summary

To optimize heating performance of variable speed air source heat pumps (ASHPs) in cold climates, it’s important to select and size the system properly and consider the location of the indoor unit. Some studies have stated that oversizing heat pumps is acceptable, as compressors are more efficient at lower speeds. However, most ASHPs can’t modulate down to speeds below 20-30%, so to limit overheating, the unit will turn off entirely and begin to cycle. The fan will also run at consistently low speeds. Cycling and low fan speeds have been known to dramatically reduce the efficiency of the unit. Another issue that reduces performance is the location of the indoor unit. Building decisionmakers should consider low-wall systems. It is very common to see ductless ASHPs located just inches from the ceiling, but this leads to higher return air temperatures, which reduces the capacity and efficiency of the unit.

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