Heat pump sizing for Puget Sound ADUs
Why oversizing kills efficiency, what the 2021 Washington Energy Code actually requires, and how we size mini-splits for a marine climate.
The code baseline
The 2021 Washington State Energy Code (residential) effectively requires heat pumps as the primary heating system for new dwelling units — including ADUs — through its credit-point system. Electric resistance and gas furnace primaries no longer accumulate enough credits to pass.
For an 800 sqft DADU built to code in the Puget Sound (Climate Zone 4C — marine), a single-zone ductless mini-split sized between 9,000 and 12,000 BTU/hr handles design heating load with margin to spare.
Sources:WA State Building Code Council
Why oversizing is the most common error
We routinely see 18,000 and 24,000 BTU mini-splits installed in 800-sqft ADUs by contractors who default to the principal-residence rule of thumb. In a tight, well-insulated ADU in a 4C climate, that is 2–3× the actual design load.
Oversized inverter-driven heat pumps short-cycle even in their modulating range, which collapses seasonal COP and causes the indoor coil to leave the dehumidification mode prematurely. NEEA's field studies have documented the seasonal-efficiency penalty repeatedly — the lab COP of 4.0 routinely drops to a delivered 2.5–2.8 when the unit is over-sized for the envelope.
Sources:NEEA
Our sizing method
We run a Manual J on every ADU at design-development. For a typical 800-sqft DADU with 2x6 walls, R-49 roof, R-30 floor, U-0.27 windows, and 2.5 ACH50 air-tightness, the heating design load at 27°F outdoor / 70°F indoor is 8,200–9,400 BTU/hr. A 9,000 or 12,000 BTU single-zone Mitsubishi, Fujitsu, or LG mini-split is correctly sized.
Two-zone systems only make sense above ~1,000 sqft or when the floor plan creates a true acoustic barrier (e.g., bedroom over garage). Otherwise the second indoor head is parasitic energy.
Sources:Building Science Corporation
Cold-climate variants and back-up strategy
Standard-spec mini-splits hold rated capacity down to about 23°F. The Puget Sound's 99% heating design temperature is 27°F at Sea-Tac and 23°F at Boeing Field. We spec cold-climate variants (HSPF2 ≥ 9.5) on every new ADU so the back-up resistance strip almost never engages.
On the rare design-day mornings when it does, the strip is sized for 4–6 kW, drawing 17–25 amps at 240V — well within the 60-amp sub-panel that we standardize for ADUs.
