Usually, yes. A backflow assembly at the service closes the water system, and IPC 607.3 then requires a thermal expansion control device on each storage water heater's cold supply, downstream of all check valves and backflow preventers. Phoenix did not amend that section. The tank is sized to the manufacturer's instructions so pressure stays at or below 80 psi.
Why a backflow preventer creates the problem
Water expands when it's heated, and it doesn't compress. Watts explains the chain: "During no-flow periods in a system, pressure reducing valves, backflow preventers, and other one-way valves are closed, thus eliminating a path for expanded water to flow back to the system supply. That is when system pressure increases, causing a number of undesirable and potentially dangerous problems."
Before the assembly went in, that expanded water could push back into the main. After, it has nowhere to go. Pressure climbs every time the heaters recover, usually overnight when nobody is drawing water.
Watts says its potable expansion tanks are used to prevent "plumbing system and/or water heater damage as well as unnecessary relief valve discharge caused by excessive pressure from thermal expansion." So a relief valve that weeps after the heaters run, or a pressure reading that is higher after a quiet night than at midday, is worth a closer look.
What the code requires
The 2024 International Plumbing Code is direct. Section 607.3 says: "Where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer, a thermal expansion control device shall be connected to the water heater cold water supply pipe at a point that is downstream of all check valves, pressure reducing valves and backflow preventers."
It continues with the sizing standard: thermal expansion tanks "shall be sized in accordance with the tank manufacturer's instructions and shall be sized such that the pressure in the water distribution system shall not exceed that required by Section 604.8." Section 604.8 is the 80 psi static limit for building water pressure.
We checked Phoenix's amendments to the 2024 IPC and UPC under Ordinance G-7397. Neither set amends the thermal expansion rule, so the model text above applies in Phoenix. Other Valley cities that adopted the IPC without changing that section apply it the same way. Note the trigger: it's a storage water heater. The sentence doesn't address tankless units, so ask the manufacturer and your inspector about those.
The relief valve doesn't count. Watts puts it plainly: "A temperature and pressure (T&P) relief valve is not considered a thermal expansion device." Letting the T&P drip every night builds mineral deposits on the valve, which is the last thing you want on a safety device.
Where the tank goes in a commercial room
The placement rule comes straight from 607.3: on the cold supply to the heater, downstream of every check valve, PRV and backflow preventer. Amtrol's instructions for its larger commercial Therm-X-Trol tanks describe the same spot: "The THERM-X-TROL is installed in the supply line between the backflow preventer and the water heater."
In a mechanical room with several heaters, the practical questions are:
- Is there a check valve between the tank and each heater? If a branch has its own check valve or PRV after the tank, that heater is cut off from the tank's protection.
- Is the tank sized for the real system? Code sends you to the tank maker's sizing instructions, with the result capped by the 80 psi limit. A tank picked for one heater won't cover a bank of them.
- Was the precharge set? Amtrol's instructions say to "Adjust pre-charge to equal incoming pressure" before installing, and to make any pre-charge adjustments "at ambient temperature only."
- Can it leak safely? Amtrol says a drip pan to a drain is required "if leaking or flooding could cause property damage."
Keeping it working
Expansion tanks fail quietly. The air cushion leaks down over time, or a bladder ruptures, and the tank fills with water. Then it's just a heavy pipe fitting, and the pressure spikes come back.
Amtrol's guidance for these tanks is that "A professional plumber should check the complete heating system, including the THERM-X-TROL, yearly and more frequently as the system ages." An annual backflow test visit is a natural time to look at the expansion tank too, since the assembly and the tank are part of the same story. A pressure gauge on a hose bib, read first thing in the morning and again after the heaters recover, tells you a lot.
HQ replaces commercial water heaters and tests, repairs and replaces domestic backflow assemblies, so we look at both sides of this problem on the same visit. Upfront pricing, and the estimate is free.
For homes and small systems, our page on whether you need an expansion tank covers the residential version. If you're weighing a PRV with a built-in bypass instead, read whether a PRV needs a thermal expansion bypass. If the RPZ itself is discharging, why an RPZ backflow preventer spits water walks through the causes.