Missing Thermal Expansion Tank: What It Means on Your Home Inspection Report

Your inspector noted that the water heater has no thermal expansion tank: a small tank with an air cushion inside, connected to the water piping near the heater. Whether you need one depends on whether the house has a "closed" water system.

Water expands when it's heated. In an "open" system, Rheem's manual explains, the extra water flows back into the city main. A check valve in the cold water line creates a "closed water system"; it may be a separate backflow preventer or part of a pressure reducing valve, the water meter, or a water softener. A.O. Smith adds that backflow preventers in water meters may not be readily visible.

Why It Matters

Rheem's manual says thermal expansion in a closed system "can create a rapid and dangerous pressure increase in the water heater and system piping." That pressure can quickly reach the relief valve's safety setting, making it open during each heating cycle, and the repeated expansion and contraction can cause premature failure of the relief valve, and possibly the heater itself. In an undated technical bulletin (it cites a 2005 chart), Rheem went further, warning that repeated pressures can weaken the tank's welds and fittings: "water + heat + pressure + closed system = potential explosion!"

Bradford White's manual is blunt: "Do not operate this water heater in a closed system without provisions for controlling thermal expansion." The relief valve is the heater's "primary safety feature," a 2009 water-utility brochure says, not the fix; Rheem says replacing it won't correct the problem. A.O. Smith adds that an expansion tank helps avoid damage to appliances and toilet valves, and that a toilet that runs briefly at night for no apparent reason may be a sign of thermal expansion (see our page on a toilet that keeps running).

When it's needed: all three manufacturers tie it to a closed system. A.O. Smith says to install a thermal expansion tank if the heater is in a closed water system; Bradford White says provisions "shall be made" to control thermal expansion; Rheem suggests an expansion tank and notes other methods exist. A.O. Smith goes further: it says most systems today are closed, almost all homes now need an expansion tank, and it recommends one in any home without one.

Local rules vary. A 2019 North Carolina State Fire Marshal interpretation of the state's 2018 plumbing code says an expansion device is required when a storage water heater is supplied through a check valve, pressure reducing valve, or backflow preventer, but not for a tankless heater outside a closed-loop recirculation system. The City of Durango, Colorado, tells customers with a backflow preventer and a tank heater that an expansion tank is required. And systems change: the City of Enterprise, Oregon, says in an undated notice that it has been installing meter setters with check valves on new and replaced connections.

Urgent (beyond repair-soon): a relief valve that drips or discharges as the heater runs; water pressure above 80 psi, the maximum A.O. Smith says most codes allow; a tank with a bulged top or bottom, which that undated Rheem bulletin says means a Rheem tank saw over its 300 psi test pressure; or a relief valve or discharge pipe that's been capped or plugged, which A.O. Smith warns could cause an explosion, a safety issue (see our page on the relief valve discharge pipe).

What to Do Before Closing

• Ask the inspector what they saw. A check valve, backflow preventer, or pressure reducing valve; drips at the relief valve's discharge pipe; and a pressure reading, if taken. Under the InterNACHI standards of practice, one of the standards home inspectors work under, inspectors aren't required to measure water pressure or test relief or check valves.
• Ask the water utility whether there's a check valve in your service line or meter, as Rheem's manual suggests.
• Have a licensed plumber evaluate it. The plumber can confirm whether the system is closed, check the pressure, and size and install a tank to local code.
• Talk to your agent. Your real estate agent can advise whether to ask the seller to repair it or offer a credit within your inspection contingency.

For other items on your report, see more findings in our inspection report decoder.

What to Do After You Move In

The fix is a properly sized, properly charged expansion tank. A.O. Smith says it must be sized for the heater's tank capacity and pressurized to match the home's incoming water pressure. Rheem's 2019 installation bulletin says to put it on the cold water supply between the heater and the backflow device (or as the tank maker or local code directs), to support its weight, and that all expansion tanks may eventually leak. A.O. Smith also recommends setting the pressure reducing valve to 50 to 60 psi.

Until then, keep the discharge pipe open, and if the relief valve discharges large amounts of very hot water, A.O. Smith says to turn off the heater and call a qualified person. If the valve still drips after a tank is installed, it says the pressure may be too high or the tank may be defective.

Can You Fix It Yourself?

Installing the tank means cutting into the water line at a gas or electric water heater and meeting local code: a licensed plumber's job. What you can do: A.O. Smith suggests checking water pressure with a gauge on an outdoor faucet over 24 hours, and you can watch the discharge pipe for drips. Never cap or plug the relief valve or its pipe.

Frequently Asked Questions

Is a thermal expansion tank required by code?

It depends on where you live. Some jurisdictions, like North Carolina, require an expansion device when a tank water heater is on a closed system; a licensed plumber or your local building department can tell you what applies to you.

Do I need one if my system is open?

In an open system, Rheem says the expanding water flows back into the city main, where the pressure dissipates. But backflow devices can be hidden in the meter, so check with your water utility. A.O. Smith recommends a tank in any home without one.

Why is my water heater's relief valve dripping?

A.O. Smith says a small amount of dripping usually means the home's water pressure is too high or the house needs an expansion tank. Have a plumber find the cause; never cap or plug the valve.

The Bottom Line

In a closed water system, heated water has nowhere to go, and Rheem warns of a rapid and dangerous pressure increase. Ask the inspector and your water utility whether your system is closed, have a licensed plumber evaluate it before your contingency ends, and get a properly sized, properly charged expansion tank. Never cap a dripping relief valve.

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