Codes generally require thermal-expansion control when a check valve, backflow preventer or PRV closes the water-heater supply; local adoption controls.
See when water-heater codes require thermal-expansion control, what closes a plumbing system, and how an expansion tank must be sized and installed.
An expansion tank is commonly required when a water heater is connected to a closed plumbing system, but it is not a universal requirement for every heater. The legal answer comes from the plumbing code and amendments adopted where the property is located.
A system becomes closed when a check valve, backflow preventer or another one-way device stops heated water from expanding back toward the supply. Some pressure-reducing valves (PRVs) have the same effect. In that arrangement, model codes require a means of controlling the pressure increase. A potable-water expansion tank is the usual method, although some codes allow another approved thermal-expansion-control device.
Model codes are not automatically law everywhere: a jurisdiction can adopt one in whole or in part, according to the Congressional Research Service. For a water-heater installation or replacement, ask the local plumbing or building department which code edition applies and whether a permit and inspection are required.
What the model codes say
The requirement follows the piping arrangement, not simply the presence of a water heater.
| Model-code provision | Trigger | Requirement |
|---|---|---|
| 2024 IPC 607.3 | A storage water heater receives cold water through a check valve, PRV or backflow preventer | Connect a thermal-expansion-control device to the cold-water supply downstream of those devices |
| 2024 IRC P2903.4 | Thermal expansion raises pressure downstream of a PRV, or a backflow/check device causes pressure to increase with storage water-heating equipment | Install a device that controls the increased pressure |
| 2018 UPC 608.3 | A check valve, backflow preventer or other normally closed device prevents building pressure from dissipating into the main | Install an approved, listed and adequately sized expansion tank or another approved device with a similar function |
The 2024 IPC text expressly addresses storage water heaters and says an expansion tank must be sized according to its manufacturer’s instructions. The 2024 IRC ties the requirement to a pressure increase caused by thermal expansion. The IAPMO explanation of 2018 UPC 608.3 applies its closed-system rule independently of the type of water heater.
This difference matters with a tankless heater. The cited IPC and IRC provisions focus on storage water heating, while the cited UPC language can require expansion control whenever the system is closed. A buffer tank, recirculation arrangement or other stored heated volume can also affect the answer. Do not treat “tankless” as an automatic exemption without checking the locally adopted text and the manufacturer’s instructions.
What makes a water system closed?
Look for a one-way restriction between the heater and the water source, including:
- a backflow preventer at the service entrance or meter;
- a check valve in the cold-water supply;
- a meter assembly with an integral check valve;
- a PRV that does not allow downstream pressure to dissipate upstream; or
- another normally closed device that blocks reverse flow.
The closing device might not be visible beside the heater. It may be in a meter box, underground or utility-owned. A utility meter project can also change a formerly open system: Clay County Utility Authority, for example, warns that installing a dual check valve may create a closed internal system.
Private-well systems require the same hydraulic check rather than an assumption. A well pressure tank can accept expanded water only if it remains connected to the heater without an intervening check valve or other isolation. The adopted code or heater instructions may still call for a dedicated listed device. For pressure-tank symptoms and tests, see water pressure tank problems.
The T&P valve is not a substitute
A water heater’s temperature-and-pressure (T&P) relief valve is a safety device, not the normal control for expansion during each heating cycle. IAPMO’s guidance treats the expansion-control device as additional to the relief valve, not a replacement for it (IAPMO code spotlight). Never cap, plug or install a shutoff valve on the T&P discharge.
Water dripping from that discharge during heating can be a thermal-expansion symptom, but it does not prove the diagnosis. Excessive supply pressure, incorrect expansion-tank precharge, or a worn or scaled relief valve can also cause periodic discharge, as an A. O. Smith expansion-tank manual explains.
A pressure gauge with a maximum-reading pointer can help. Record the static pressure, run enough hot water to start a heating cycle, stop water use, and monitor the gauge while the heater operates. A substantial rise during heating indicates that expanded water is not being accommodated. If an expansion tank is already fitted, possible causes include an incorrect air charge, failed diaphragm or inadequate tank size; Whirlpool describes this heating-cycle test.
Size and commission the tank correctly
Do not select a tank by pipe size or appearance. Sizing depends on:
- the stored water volume;
- incoming water pressure;
- starting and maximum water temperatures;
- the maximum permitted system pressure; and
- the manufacturer’s sizing and acceptance-volume data.
Use a tank listed for potable-water thermal expansion, not a generic hydronic heating tank. The A. O. Smith manual’s sizing chart changes the recommended tank size with heater capacity and incoming pressure, under stated temperature and pressure assumptions. It also requires the air precharge to match incoming water pressure while the tank’s water side is depressurized. A factory precharge is therefore not necessarily correct for the building.
Install the tank on the cold-water side in the hydraulically correct location—downstream of the device that closes the system and where it cannot be isolated from the heater during normal operation. Follow the selected tank’s instructions for orientation, support, clearance, pressure and temperature limits, and leakage protection. A failed diaphragm can leave the tank waterlogged, adding considerable weight to the connection.
The practical code check is simple: identify the locally adopted code, determine whether the heater is in a closed system, and verify which control method the authority accepts. If a storage water heater is downstream of a check valve, backflow preventer or non-relieving PRV, expect to need properly sized thermal-expansion control.