Multi-Split Heat Pumps: Sizing and Zoning Problems – GreenBuildingAdvisor

Q&A Spotlight

A Queens renovation needs seven HVAC zones but has a heating load of only 25,000 Btu/hr. Can a multi-split heat pump work?

Designing an HVAC system is one thing. Designing one with seven zones, a tight heating load, limited space, and a client who has already decided what equipment they want is another.

That’s roughly the situation facing GBA reader “danuspuchatek” (we’ll call him “Dan”), a self-described young architect who recently wrote in for some advice. He’s involved in a gut rehab of a 3000-sq.-ft., two-story house built in the 1940s in Queens, N.Y., climate zone 4A. The project includes a self-adhered WRB, additional insulation, and a conditioned, spray-foamed attic. Dan is hoping for a blower-door result below 3 ACH50 and an enclosure that meets local energy-code requirements.

His client wants to use a multi-split heat pump system—an outdoor unit that supplies several indoor units—rather than individual mini-splits. “I’ve read here [on Green Building Advisor] that it’s best to do a single-zone ducted unit per floor in these situations, but the owner wants to do wall-mounted mini-splits with a head in each room.” (Dan may have found this article from a few years ago.)

Dan used Manual J to estimate the house’s loads, which calculated as approximately 25K Btu heating and 22K cooling. Going with the client’s wishes, Dan has spec’d a 36K Btu Mitsubishi Hyper Heat unit, rated at 36K Btu cooling at 95°F and 42K Btu heating down to –13°F, that can support up to 11 indoor units. He figures he can spread the unit’s capacity throughout the house with seven air handlers:

• 4000-Btu unit in the cellar rec room
• two 6000-Btu units in the open floor plan first floor
• three 4000-Btu units on the second floor, one per bedroom
• 6000-Btu unit in the attic rec room

Dan writes: “I know the unit is oversized based on the calculation,…