Can a heat pump water heater go in a closet in my ADU?

Yes, as long as the closet gives the unit enough air. Section 110.3(c)7 of the 2025 Energy Code requires a heat pump water heater to meet one of four ventilation paths: a manufacturer's approved method, enough room volume, louvered openings to a larger space, or ducting. A small closet with a solid door usually fails unless you add louvers or ducts.

The four ways to meet the ventilation rule

1. Manufacturer's method. Install using a method from the manufacturer that performs at least as well as the options below.
2. Room volume (no ducts). The space must have at least 100 cubic feet per kBtu per hour of compressor capacity, or the manufacturer's minimum for this method, whichever is greater.
3. Louvers to a communicating space (no ducts). The closet must open to a connected space that makes up the required volume, through a single layer of fixed louvers or grilles. Net free area must be at least 125 square inches plus 25 square inches per kBtu per hour of compressor capacity, or the manufacturer's minimum, whichever is larger. Use a fully louvered door or two equal openings, one with its top within 12 inches of the top of the enclosure and one with its bottom within 12 inches of the floor of the enclosure.
4. Ducts. The space joined by ducts must make up the required volume, all duct connections and penetrations must be sealed, and exhaust ducts and ducts that cross pressure boundaries need at least R-6 insulation. If only the inlet or the outlet is ducted, the closet still needs low louvers or a door undercut of a set minimum size, and inlet and outlet terminations in the same space must point away from each other.

Compressor capacity comes from the AHRI 540 rating described in the code, and if two heat pump water heaters share a space, their requirements add together.

Other placement rules

• Backup heat: required when the unit draws unconditioned air, unless its compressor cut-off temperature is below the Heating Winter Median of Extremes for the nearest location listed in Joint Appendix JA2 (110.3(c)7A).
• Tank location: the prescriptive Option A and the climate zone 16 version of Option B require the storage tank in the garage or conditioned space (150.1(c)8).
• Condensate: the unit makes water as it cools the air, so it needs a drain to an approved receptor.
• Leaks and relief: a drain pan where a leak could damage the building, a relief valve discharge line, and seismic strapping.
• Service access and noise: leave room to reach the filter and controls, and keep in mind the compressor and fan sound if the closet is next to a bedroom.

What it means for your plumbing plan

Your plans should show the closet dimensions or louver net free area, the door type, any duct routes, and where the condensate and pan drains end, using the compressor capacity from the unit's specification. Coordinate ducts with the mechanical sheets and the circuit with the electrical sheets. See choosing an ADU water heater, the ADU Title 24 guide, and our plumbing plans.

Sources: energy.ca.gov, gocodebook.com.