ADU Heat Pump Water Heater Requirements
A newly constructed detached ADU on the prescriptive path needs a single 240-volt heat pump water heater, with two small-unit exceptions. A unit of 500 square feet or less may use an electric water heater with point-of-use piping, and a unit with one bedroom or less may use a 120-volt heat pump water heater. Additions and alterations follow separate lists.
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Two ways to shrink the footprint
Exception 1 to Section 150.1(c)8 allows an electric water heater with point-of-use distribution in a new dwelling unit of 500 square feet or less. Reference Appendix RA4.4.5 limits each hot water run by pipe size: 15 feet of 3/8 inch, 10 feet of 1/2 inch, 5 feet of 3/4 inch. A mixed run gets half the allowed length of each size, and the most direct path is required. A stand-alone tub is exempt. Prescriptively this is one instantaneous electric heater; multiple electric heaters need the performance path. Exception 2 allows a 120-volt heat pump unit where the dwelling has one bedroom or less.
A closet is a ventilation question
Section 110.3(c)7B, new in 2025, sets airflow for consumer integrated units. In an open room the space needs 100 cubic feet per kBtu/h of compressor capacity. The manual's example is a consumer-sized 4 kBtu/h unit, so 400 cubic feet. A smaller room can vent to a neighbouring space through louvers of at least 125 square inches plus 25 per kBtu/h, or 225 square inches in that example, in the upper and lower halves of the enclosure. See HPWH installation.
The other project types
- Detached: zones 1 and 16 add compact hot water distribution to the first option, and zone 16 adds field-verified drain water heat recovery.
- Addition with its own heater, Section 150.2(a)1D: a heat pump unit not outdoors, on a rigid R-10 base, with a communication port; or one meeting NEEA Tier 3; or electric point-of-use up to 500 square feet.
- Alteration, Section 150.2(b)1H: gas, a heat pump unit, or electric if the old heater was electric resistance.
Where to go next
The new-home options are on Title 24 water heater requirements; a design that keeps gas is covered in can a new ADU use a gas water heater. See the detached ADU report or start on the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
Only through the performance approach. The Energy Commission says multiple electric heaters are not allowed prescriptively, and the model compares the design with the prescriptive standard design.
Up to 15 feet of 3/8 inch pipe or 10 feet of 1/2 inch. For a mixed run, half the allowed length of each size is the maximum.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide