Heat pump water heaters under Title 24.

The prescriptive water heater is a heat pump

On a newly constructed single-family dwelling under the 2025 code, every prescriptive route to a compliant water heater runs through a heat pump. §150.1(c)8 gives three options and nothing else: a single 240-volt heat pump water heater with the storage tank in the garage or conditioned space, a single 240-volt HPWH meeting NEEA Advanced Water Heater Specification Tier 3 or higher, or a solar water-heating system with electric backup at a minimum annual solar savings fraction of 0.7. Anything else — a gas storage tank, a gas tankless, straight electric resistance — is off the prescriptive path and has to be traded for on the performance path under §150.1(b)1.

That is the headline, and it is the part most write-ups stop at. What follows is the rest: the two exceptions small dwellings get, the extra measures Climate Zones 1 and 16 pick up, what a gas water heater actually does to the compliance margin, the credits that buy it back, what “heat pump water heater ready” obliges you to build when you do install gas, and where additions and alterations part company. For how water heating fits the wider 2025 change set, see what changed in the 2025 code.

The three prescriptive options, and the fine print

Under all three options the water heater still has to meet the mandatory requirements for water heaters, and if a recirculation distribution system is installed, only a demand recirculation system with manual control is allowed. Beyond that:

• Option A — a single 240V HPWH, tank in the garage or conditioned space. In Climate Zones 1 and 16 it also needs a compact hot water distribution design meeting the basic compact design, and in Climate Zone 16 an ECC-verified drain water heat recovery system on top of that.
• Option B — a single 240V HPWH meeting NEEA Tier 3 or higher. The compliance manual calls this the simpler of the two heat pump options: in Climate Zones 2 through 15 nothing else is required. Climate Zone 16 still needs the tank in the garage or conditioned space plus a field-verified drain water heat recovery system.
• Option C — solar water heating with electric backup, installed per Reference Residential Appendix RA4 and rated at a minimum annual solar savings fraction of 0.7.

Two exceptions matter on small buildings, and they come up constantly on ADUs. An electric water heater with point-of-use distribution per RA4.4.5 may be installed in a new dwelling unit of 500 square feet or less of conditioned floor area. And for a new dwelling unit with one bedroom or fewer, a 120V HPWH may be installed in place of the 240V unit — which is the difference between a service upgrade and a plug.

Gas is not banned. It is priced.

Nothing in Part 6 prohibits a gas water heater in a new house. What the 2025 cycle did was remove it from the prescriptive menu, which moves the whole project onto the performance path — the energy model — where the gas water heater has to be paid for out of some other part of the design.

How much it costs is a modelling question, not a number the code publishes, and the honest answer is that it depends on the plan, the glazing, the climate zone and what else is already tight. But the mechanism is written down in the 2025 ACM Reference Manual, and it is worth understanding before you promise a client a gas tank:

• The Efficiency LSC standard design uses a heat pump water heater in every climate zone — a single HPWH at 2.0 UEF, located in the garage where the building has an attached garage, otherwise in conditioned space with inlet and outlet ducted outside, plus a compact distribution system in Climate Zones 1 and 16 and drain water heat recovery in Climate Zone 16. A proposed gas water heater is compared against that, whatever fuel it burns.
• The PV LSC standard design is fuel-matched. If the proposed system is gas, the standard design there is a gas tankless unit at 200,000 Btu/h and 0.81 UEF; if it is electric, it is the same HPWH. The source-energy standard design is a gas tankless unit in every case.
• You cannot solve it with panels alone. The Energy Code does not allow a tradeoff between the Efficiency LSC and the effect of the PV system on Total LSC unless battery storage is provided; with a battery of at least 5 kWh certified to JA12, a modest self-utilization credit lets part of the PV and battery LSC trade against Efficiency LSC.

In practice that means a gas water heater has to be answered with efficiency: envelope, glazing, ducts, distribution, equipment. On plenty of designs it is buildable. On some it is not, and that is much cheaper to learn from a model than from a correction letter — which is the same argument as the plan-check correction page makes from the other end.

Where the credits actually are

Most of the recoverable margin in water heating is in the distribution system, not the tank. The performance method compares alternative distribution designs to the standard one using distribution system multipliers: the standard trunk-and-branch system is 1.0, anything below 1.0 is a credit, anything above is a penalty.

Multipliers from Table 5-4 of the 2025 Single-Family Residential Compliance Manual. The expanded compact-design multiplier varies with floor plan and water heater location. A multiplier below 1 is a credit, because the system is modelled at less distribution loss than the standard case.

Look at the bottom row for a moment. A recirculation loop without demand control is modelled at nearly ten times the distribution loss of the standard system, which is why it will sink an otherwise fine design, and why the code allows only manual-control demand recirculation on both the prescriptive path and in alterations. On the equipment side, NEEA Tier 3 and Tier 4 rated HPWHs are modelled in the compliance software and earn credit against the standard-design heat pump, and a JA13-certified HPWH with basic load-up demand management earns a further credit that varies by climate zone — a mixing valve is required with it. Drain water heat recovery and the expanded compact distribution credit are both ECC-verified, meaning a rater and a CF3R.

If you install gas, you still build for the heat pump

§150.0(n)1 is mandatory, and it is the provision most often missed on a mixed-fuel design. Where a gas or propane water heater serves an individual dwelling unit, the building has to designate a space at least 2.5 feet by 2.5 feet by 7 feet tall suitable for a future heat pump water heater, with all electrical work per the California Electrical Code. What that space has to contain depends on where it is:

• Within 3 feet of the water heater (option A): a dedicated 125-volt, 20-amp receptacle within 3 feet of the water heater and accessible with no obstructions, connected to a 120/240-volt three-conductor branch circuit rated at 30 amps minimum; both ends of the unused conductor labeled “spare” and electrically isolated; a reserved single-pole breaker space in the panel next to that circuit’s breaker, labeled “Future 240V Use”; and a condensate drain no more than 2 inches higher than the base of the installed water heater, draining naturally without a pump.
• More than 3 feet away (option B): a dedicated 240-volt branch circuit rated at 30 amps minimum within 3 feet of the designated space, blank cover identified as “240V ready”; a reserved space in the main service panel for a future double-pole breaker, permanently marked “For Future 240V use”; either a dedicated cold water supply or the cold supply routed through the designated location before it reaches the gas water heater; the hot supply out of the gas water heater routed first through that location before serving any fixture; hot and cold piping there left exposed and readily accessible; and the same condensate drain.

Scope, from the compliance manual: these requirements apply to newly constructed buildings and to additions where a water heater is installed in the added floor area. They do not apply to alterations, and they do not apply when the water heater being installed is electric. The reasoning is plain enough — running the wire during construction costs a fraction of retrofitting it. This is the water-heating half of the 2025 electric-ready package; the space-heating, cooktop and clothes-dryer provisions in §150.0(t), (u) and (v) are a separate set.

Additions and alterations answer differently

This is where most of the confusion on a remodel comes from, because the two cases point opposite ways.

An addition that adds a water heater is governed by §150.2(a)1D, and the list is short: a single heat pump water heater with the tank not outdoors, set on an incompressible rigid insulated surface of at least R-10 and installed with a communication interface meeting §110.12(a) or carrying an ANSI/CTA-2045-B port; or a single HPWH meeting NEEA Tier 3 or higher; or, for additions of 500 square feet or less, an electric water heater with point-of-use distribution. The compliance manual works the case explicitly: an instantaneous gas water heater of 200,000 Btu/h or less added with an addition is not one of the circumstances that allows addition-alone compliance. Performance compliance remains available.

Replacing an existing water heater is an alteration under §150.2(b)1H, and the answer there is more permissive than people expect. The altered or replacement system has to meet one of several options, and the first one listed is a natural gas or propane water-heating system. The others: a single HPWH with the tank not outdoors, on an R-10 rigid insulated surface, with a §110.12(a) or CTA-2045-B communication interface; a single HPWH at NEEA Tier 3 or higher; a consumer electric water heater where the existing unit is electric resistance; or a system the Executive Director determines uses no more energy. Pipe insulation per §150.0(j)1 applies to newly installed and existing accessible piping, and recirculation on an alteration is limited to manual-control demand systems.

So a like-for-like gas water heater swap is still legal statewide under Part 6. What it needs is the documentation the city asks for: Whittier’s FAQ, for instance, tells a homeowner replacing a water heater to submit a completed CF-1R-ALT form obtainable at the counter — which is exactly the discretion §10-103(a)1C gives an agency on alterations that need no field verification.

Mandatory items that catch people

• Ventilation. The 2025 code added mandatory requirements for air-source heat pump water heaters: ventilation is required when installing one (§110.3(c)7B), and a HPWH installed so that its inlet air is outdoor air must have backup heat.
• Closets. The compliance manual warns that an unducted confined water-heater closet overcools its own air and significantly diminishes HPWH performance, and that ducting only the inlet or only the exhaust across the pressure boundary can increase heating and cooling load and interfere with balanced ventilation. Where the HPWH goes is a design decision, not a plumbing detail.
• Solar certification. Solar water-heating systems and collectors must be certified and rated by SRCC, by IAPMO R&T, or by an approved listing agency (§150.0(n)2).
• The certificate has to say so. The water heater, its efficiency and its location are among the items a plans examiner compares against the drawings, which is one of the reasons a CF1R comes back.

Where a city adds to this

Local energy amendments are the exception, not the rule, and most of the ones that make the news are about space heating rather than water heating — a heat-pump HVAC requirement is a different subject from §150.1(c)8 and the two should not be read together. These are water-heating overlays that have been verified against the city’s own codified text:

• Carlsbad carries seven sections of codified Part 6 amendments at CMC Title 18, Chapter 18.30, adopted by Ord. CS-437 (2022) and carried into the 2025 cycle by Ord. CS-498 (12/2/2025). Two are water heating: §18.30.050 adds §150(n)5, requiring a newly constructed residential building to derive at least 60 percent of its service water heating energy from on-site solar or recovered energy; §18.30.030 amends the nonresidential prescriptive service-water-heating requirements to a heat pump water heater with the tank in conditioned space, electric resistance, or solar at a minimum 0.40 solar savings fraction, with gas systems at or above 1 MMBtu/h held to 90 percent thermal efficiency. Carlsbad appears on neither the CEC’s 2022 nor its 2025 approved local energy standards list, so confirm scope and status with the building official before designing to it.
• Encinitas is codified and CEC-approved for the 2025 cycle — Ordinance 2025-11, approved 1/21/2026, covering single-family and multifamily additions and alterations at a permit valuation of $50,000 or more. It is a menu, not a mandate: a project picks any one measure marked available in Table 150.2-H, and both Heat Pump Water Heater and a Water Heating Package are on it, available to every building vintage. A DOE Home Energy Score of seven or higher, from a certified assessor, exempts the building.
• Santa Cruz runs a point system. SCMC 18.15.040 adds a local §150.0(w) requiring a major addition or major alteration — 350 square feet in a year — to install measures from Table 150.0-I totalling at least 9 points, plus that table’s mandatory measures, plus water-heating electric readiness per §150.0(n)1. A heat pump water heater alone is worth 12 points and clears the requirement by itself; the water heating package is worth 1. Homes built after December 31, 1991 are exempt outright, and verification is documented as an addendum to the Certificate of Compliance.
• Saratoga codifies its own §150.0(w), electric readiness for alterations, at §16-51.020: where wall framing is removed or replaced within 3 feet of a gas or propane water heater, the project owes HPWH space of 2.5 feet by 2.5 feet by 7 feet with a condensate drain, plus the circuit. It is the state’s new-construction readiness rule reapplied to remodels. Saratoga appears on neither the CEC’s 2022 nor its 2025 approved local energy standards list, so confirm status with the building official before designing to it.
• San Diego put its energy-exceeding requirements in CALGreen rather than the Energy Code: SDMC §1410.0501 adds CALGreen §4.201.2 requiring more energy-efficient central water heating in high-rise residential buildings, or solar water heating on the prescriptive path, filed with the CEC under Public Resources Code §25402.1(h)(2). The city’s own April 2026 notice describes it as central hot-water heating requirements for residential buildings using no more than 95 percent of their energy budget, with implementation pending CEC approval.
• Santa Clara is the useful counter-example. The city states on its reach-code page that the reach codes apply only to newly constructed buildings, that a detached garage converted to an ADU is an alteration and triggers nothing, and that replacing a gas water heater, furnace or stove does not force an electric replacement.

Where a local ordinance and the state code disagree, ask the building official rather than assuming — and check the code cycle the ordinance is written against, because several of these were adopted under the 2022 code and re-adopted under 2025 at different dates.

Prices

Flat fees, in writing before I start. No hourly, no bid package, no change-order games — the number on the quote is the number on the invoice.

One honest note

Water heating questions

If yours isn’t here, see the full FAQ or call and ask.