Choosing the right indoor units is a critical step in heat pump design. In the first part of this two-part series, I described ductless options: high-wall units, low-wall units, and ceiling cassettes. Here, I’ll look at ducted indoor units.

Using ducts to distribute heated and cooled air has several advantages. One of the biggest is that a single air handler can serve multiple rooms. As long as the duct system is properly designed and installed, it will provide more even temperatures than a design in which some rooms have ductless heads and others don’t.

The pros

Ducted designs also allow for a better match between the capacity of the indoor units and the needs of the rooms being served. Take, for example, a cluster of three bedrooms, each with a design heating load of 2500 Btu/hr. The smallest ductless heads available have nominal capacities of 6000 Btu/hr. Putting a ductless head in each room would mean 18,000 Btu/hr total. This severe oversizing can lead to temperature swings and efficiency losses. A better solution would be to install a single, 9000 Btu/hr ducted air handler that serves all three bedrooms.

Reducing the number of indoor units also reduces equipment cost and complexity. Less refrigerant piping and fewer fittings mean smaller quantities of refrigerant and fewer potential points of failure. For this reason, choosing ducted systems should reduce climate impacts associated with leaks of high-GWP refrigerants.

Ducted systems also allow separation of the equipment from the occupied space, reducing noise and visual impact. They can incorporate better filtration than the coarse mesh on ductless units. And ducted systems can also be used to distribute ventilation air. In addition, centrally ducted systems (described below) allow easy integration of supplemental electric heat using off-the-shelf controls and resistance heaters.

The cons

Ducted systems…