The 2025 code made the decision for you: the standard design assumes a heat pump water heater in every one of California's sixteen climate zones. Fight that baseline with gas and your performance run starts in a hole. Accept it, and the compliance question is settled — what remains are the practical questions, and those are the ones that actually cause trouble on job sites.

How they work, in one paragraph

A HPWH is a refrigerator running backwards on top of a tank: it pulls heat out of the surrounding air and pushes it into the water. That is why it hits efficiencies two to three times a resistance heater (uniform energy factors in the 2s and 3s versus below 1). It is also why every practical quirk exists — the machine needs air to draw from, it makes that air colder, it dehumidifies (hence condensate), and the compressor makes fridge-level noise.

Sizing: buy recovery, not just gallons

Heat-pump recovery is slower than gas. The compressor reheats a tank over hours, not minutes, so the standard practice is to size up: where a 40-gallon gas unit served, a 50-gallon HPWH is the common floor; households of four-plus or tubs worth filling push toward 65 or 80 gallons. Hybrid modes with resistance backup cover surge demand, but every hour in resistance mode is efficiency you paid for and are not getting. A bigger tank that stays in heat-pump mode beats a smaller one leaning on its element.

Sizing also interacts with controls. Many units support load-shifting schedules — heat the tank midday on solar, coast through the evening. If the project claims battery self-utilization or has time-of-use rates, wire the HPWH on its own circuit with that in mind.

Where to put the tank

This is the question that ruins otherwise good installs. The unit needs air volume to feed on — the working figure is roughly 700 cubic feet of surrounding space (about a 7×12.5-foot room with an 8-foot ceiling), or ducting to somewhere that qualifies. The candidate locations, in rough order of preference:

  • Garage: the default in most of California. Big air volume, noise nobody hears, mild temperatures, floor drain often nearby. In CZ1 and the mountain zones, winter garage temperatures cut efficiency — still workable, just model it honestly.
  • Ducted closet: the ADU and compact-plan answer. A standard water-heater closet is far too small unducted, so you duct intake, exhaust, or both to adjacent space or outside. Budget the duct runs in the design, not as a field improvisation.
  • Basement or large utility room: fine on volume; mind the noise if a bedroom shares a wall — 45 to 50 dB is a window fan, tolerable in a garage, grating through drywall at 2 a.m.
  • Conditioned living space, unducted: legal, but the unit cools air your heat pump then reheats. In cooling-dominated zones that is a mild win; in heating season it is a quiet tax.

The two lines people forget

Condensate: a HPWH dehumidifies, so it drips — plan a gravity drain or a condensate pump, same as a furnace coil. The number of otherwise-finished installs waiting on a condensate path is not small. Electrical: most tanks want a 240 V/30 A circuit; 120 V plug-in models exist for retrofits and panel-constrained ADUs, trading recovery speed for electrical simplicity.

The 2025 baseline made HPWHs unavoidable. The difference between a good install and a callback machine is decided in design: the right gallons, a location with air and drainage, and a client warned about the hum before they hear it themselves.