Does my ADU water heater need an expansion tank, drain pan, and relief line?

Most ADU tank water heaters need all three. The 2025 California Plumbing Code requires a temperature and pressure relief (T&P) valve with a compliant discharge pipe on storage water heaters, an expansion tank wherever a check valve, backflow preventer, or pressure regulator closes the system, and a drain pan wherever a leak could damage the building. Tankless heaters are treated differently.

Expansion tank

• When it's required: if your system has a check valve, backflow preventer, or other normally closed device that stops water from expanding back into the main, you need an expansion tank or other approved device, whatever type of water heater you have (CPC 608.3).
• With a pressure regulator: an expansion tank goes in the cold water piping downstream of each regulator (CPC 608.2).
• Installation: the tank must be listed, sized for the system, fastened to the structure, and installed on the building side of the check valve or backflow device. Pre-pressurized tanks must meet IAPMO/ANSI Z1088.
• Exception: an expansion tank isn't required for an instantaneous, non-storage water heater.

T&P relief valve and discharge pipe

• Storage water heaters need a listed combination temperature and pressure relief valve. Listed instantaneous heaters with an inside diameter of 3 inches or less are exempt (CPC 608.3).
• No shutoff valve may sit between the relief valve and the heater or in the discharge line (CPC 504.6).
• The discharge pipe must be full size, with no valves or obstructions, drain by gravity, not be trapped or exposed to freezing, and have an unthreaded end (CPC 608.5).
• It must end through an air gap into the drainage system or outside the building. Outside, it points down and ends between 6 inches and 2 feet above the ground, where you can see it.
• The relief valve may not discharge into the drain pan.

Drain pan

Where the water heater sits in an attic or on a floor-ceiling or floor-subfloor assembly where a leak would cause damage, it needs a watertight, corrosion-resistant pan at least 1-1/2 inches deep with a drain of at least 3/4 inch that ends at an approved, readily observable location (CPC 507.5). On a garage slab, where a leak can't damage finished construction, your inspector may not require one.

Don't forget

• Seismic strapping on every tank water heater.
• A condensate drain for a heat pump water heater.
• Venting and combustion air for a gas unit, where gas is allowed.

What it means for your plumbing plan

Your plans should show where the expansion tank goes, how the T&P discharge is routed and where it ends, and the pan and pan drain where required, along with the strapping note. Routing the relief and pan drains to an outside wall is much easier on paper than after the drywall is up. See water heater seismic strapping, choosing an ADU water heater, and our plumbing plans.

Sources: gocodebook.com, gocodebook.com, gocodebook.com, gocodebook.com.