What the tank actually does
Water grows in volume when it is heated. In a closed system it has nowhere to go, so pressure climbs every time the heater runs. One water-heater manufacturer’s manual notes that this can cause premature tank leaks that the warranty does not cover, and intermittent discharge from the temperature-and-pressure (T&P) relief valve, which is not meant to handle thermal expansion constantly (manufacturer manual hosted by DOE).
A typical tank has a rubber bladder with pressurized air on one side. Expanding water pushes into the tank and compresses the air instead of straining the heater and fixtures.
Hot-water heating boilers use expansion tanks too, but those belong to the heating system. The National Park Service design checklist lists hydronic expansion tanks and domestic hot-water expansion tanks as separate items (NPS mechanical QA checklist). Before replacing a tank, find out which pipes it sits on: domestic water is a plumber’s job, a boiler tank is a heating contractor’s.
The Philadelphia rule
Philadelphia’s plumbing code, section P-607.3, says that where a storage water heater gets cold water through a check valve, pressure-reducing valve or backflow preventer, a thermal expansion control device must connect to the heater’s cold supply downstream of all of those devices. The tank must be sized to the tank maker’s instructions so system pressure stays within the 80 psi static limit in section P-604.8, which also requires a PRV wherever building pressure exceeds 80 psi (Philadelphia Plumbing Code, Chapter 6).
Replacing a water heater in Philadelphia needs an L&I Plumbing Permit; the fee table lists a water heater at $37, or $31 for one- or two-family homes, and a licensed master plumber must oversee the work (Get a Plumbing Permit). That is usually when the expansion-tank question comes up: the installer has to meet P-607.3 on the new heater even if the old one ran for years without a tank. Our Philadelphia plumbing permit guide covers which other jobs need L&I. If the job is only swapping a failed expansion tank, ask the plumber whether they treat it as a permitted replacement or an exempt minor repair, and get the answer in writing.
How to tell whether your house is a closed system
Look, without touching anything, at the main water line just past the meter. A bell-shaped valve with an adjusting screw is often a PRV; a backflow device or check valve may also sit there. Building America’s heat pump water heater checklist describes a closed supply the same way, as one with a backflow preventer on the cold line, and calls for thermal-expansion control (HPWH installation checklist).
A common sign is a T&P valve or its discharge pipe that drips after the heater finishes a cycle, especially overnight. But dripping alone does not prove a missing tank. The same manual says supply pressure should be around 50–60 psi and never above 80 psi (manufacturer manual), so the fix might be the PRV rather than, or as well as, a tank. A good plumber checks both:
- Static pressure at a hose bib with the heater idle. Above 80 psi points to the PRV.
- Pressure at the end of a heating cycle. A big rise with normal static pressure points to thermal expansion.
- The existing tank, if there is one. Building America’s guide says expansion tanks typically last 5–10 years and that a bladder that lets out water instead of air has failed (Install a Heat Pump Water Heater).
- The T&P valve and its discharge pipe, which must stay open and unobstructed.
Never cap, plug or remove a T&P valve or its discharge pipe to stop a drip. Stay clear of hot discharge. Repeated discharge calls for a plumber promptly. If your pressure is uneven at just one tap, that is a different problem; see low pressure at one faucet.
What to ask before you approve the work
- What device makes this a closed system, and where will the tank go relative to it?
- What were the static and heating-cycle pressures, and does the PRV need work?
- What size tank, based on which manufacturer chart? The DOE-hosted manual’s chart sizes by heater capacity and inlet pressure (manufacturer manual).
- Will you set the tank’s air charge to match the house’s measured cold-water pressure, as Building America’s guide calls for (Install a Heat Pump Water Heater)?
- How will the tank be supported, and is the permit included?
If you are replacing the heater anyway, our tank or tankless guide and heat pump water heater basement-fit guide cover the larger choice. Code sections and fees were checked November 5, 2026; confirm with L&I and your plumber before work starts.