Multi Split Design Advice – GreenBuildingAdvisor

Hi All,

Young architect here trying my best to employ some better design principles.

Working on a gut renovation of a 1940s detached single family home in northern Queens, NY. It is 3,000sf spread over 2 stories, plus cellar, and occupiable attic. The house is getting new self-adhered WRB (air barrier), spray foam to make a conditioned attic (I tried to advocated for exterior poly iso), R-12 poly-sio continuous insulation, and new R-15 or R-21 cavity insulation depending on cavity size. Shooting for less than 3 ACH50. Really, just current NYC energy code minimums, but leagues above where the house is currently.

The biggest question is the HVAC system. I’ve read here 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. I did a Manual J which put the house at ~25k BTU heating load and ~22k BTU cooling load. My strategy to work with the Manual J and the owner’s wishes was to spec a 36k BTU Mitsubishi Hyper Heat that supports up to 11 indoor units. This unit can modulate down to 13k BTU cooling and 12.6k BTU heating. This capacity will be spread across seven air handlers sized as best I can to meet the load of each space:

– 4,000 BTU unit in the cellar rec room

– two 6,000 BTU units in the open floor plan 1st floor

– three 4,000 BTU units on the 2nd floor, one per each bedroom

– 6,000 BTU unit in the attic rec room

I know the unit is oversized based on the calculation, but my argument here is that the single larger unit can have it’s minimum capacities split between more indoor units. The owner however would like redundancy built into he system and have two outdoor units, so that there would still be cooling in the house if one of the condensers failed.

I understand his concerns, but I think this approach will lead to issues like short cycling, poor humidity control, and increased cost, since the smaller condenser units essentially have the same minimum heating and cooling capacities as the 36k BTU condenser.

I essentially just wanted to see if anyone had any other advice or thoughts to share. Thanks!