Thermal expansion tanks: when a water heater needs one

Water gets bigger when it gets hot. In a storage water heater, every heating cycle pushes a little extra volume out of the tank, and that water has to go somewhere. In many older houses it simply backed out the cold line toward the street main, and nobody noticed. In a house where a check valve, a backflow preventer, or a pressure-reducing valve stands in the way, it cannot back out, so the pressure in every pipe, fitting, and fixture climbs each time the burner or element runs.

A thermal expansion tank is the usual answer. It is a small steel tank, teed into the cold supply near the heater, with a sealed air cushion behind a diaphragm or bladder. When the heated water swells, it pushes into the tank and squeezes the air instead of pushing up the pressure in the whole house.

This page covers when the model code calls for one, how to tell whether your system is closed, what a correct installation looks like, and how to check an existing tank without ruining its charge.

If your relief valve is already dripping, start with why the temperature and pressure valve drips, which sorts expansion from a fouled valve seat and a runaway thermostat. This page goes deeper on the expansion tank itself.

What a closed system is

An open system lets expanded water return to the public supply. A closed system has a one-way device somewhere upstream of the heater that stops that return. The International Residential Code treats the second case as a hazard that needs a fix. Section P2903.4 requires a means of controlling the increased pressure caused by thermal expansion, and its two subsections say when.

P2903.4.1 covers pressure-reducing valves. For water services up to 2 inches, a pressure-control device is required where thermal expansion pushes the pressure downstream of the reducing valve above the valve’s setting. P2903.4.2 covers backflow preventers and check valves. Where one of those, or any other device, is installed on a system with storage water heating so that thermal expansion raises the pressure, a pressure-control device is required.

The commentary to the 2015 IRC, quoted by Arapahoe County, Colorado, in its permit guidance, puts it plainly. A backflow preventer, check valve, or reducing valve in the water service or anywhere upstream of the heater creates a closed system, and thermal expansion can build destructive and hazardous pressures in the piping. The typical solution, the commentary says, is an expansion tank properly sized for the application, or another approved device designed for the purpose.

You often cannot see a check valve that a utility installed inside its meter assembly. If you are unsure, ask the water utility whether the meter includes one, and ask the plumber who quotes your next heater to state in writing whether the system is closed. A quote that skips the question is one of the gaps described in how to read a replacement quote.

Signs the pressure is climbing

Expansion problems tend to show up on a schedule. The pressure rises while the heater is recovering and falls when someone opens a faucet. That pattern is the clue.

• The relief valve spits or drips during or just after a heating cycle, and stays dry when nobody has used hot water for a while and the heater is idle.
• A toilet fill valve hisses or runs briefly with nobody touching it, or faucets give a short burst when first opened.
• A pressure gauge on a hose bib or the heater drain reads normal in the morning and noticeably higher after the heater has run, then drops back as soon as a tap opens.

None of those prove expansion on their own. A worn reducing valve can let street pressure creep up all day, which looks similar. The difference is that expansion tracks the heating cycle and falls instantly when you open a tap, while a failed reducing valve keeps the house high even when the heater is cold.

The IRC also caps static pressure in a house at 80 psi in Section P2903.3.1 and calls for an approved reducing valve where the main exceeds that. An expansion tank does not fix a house that is over 80 psi before the heater ever runs.

The relief valve is the last line of defense, not a pressure regulator for the plumbing.

The ICC’s own CodeNotes on water heater safety makes the related point: if the heater works, the system pressure is within code, and the relief valve is sound, there should be no discharge from the relief valve at all. Any discharge points at a system problem, a heater problem, or a bad valve. A drip on every recovery is the system telling you something.

How an expansion tank is installed

The manufacturer instructions for these tanks agree on the main points. Amtrol’s installation sheet for its residential potable tanks says the product must be installed by a licensed professional, and local plumbing codes govern the work. The summary below is so you can recognize a correct job, not a do-it-yourself procedure.

Location. Amtrol says to install the tank on the cold supply to the heater, between the heater and the backflow preventer or other one-way device. Rheem’s service bulletin says the same, with a note that some tank makers do not specify the cold or hot line, so the tank maker and the local authority decide. A tank on the street side of the check valve does nothing for the house.

Precharge first. This is the step most often skipped. Rheem’s bulletin says to measure the system’s static water pressure with a gauge, for example at the heater drain, after opening a faucet and letting the pressure settle. The tank’s air charge is then set to match that pressure with a tire gauge and a hand pump, before the tank goes on the system.

Amtrol says to adjust the air charge with the tank empty of water to match the cold supply pressure. Rheem and Watts both caution against going over 80 psi. The Watts instructions add that if system pressure is above 80 psi, a reducing valve has to be added, or the tank placed where static pressure is lower.

Support. A tank full of water is heavier than it looks on the shelf. Rheem says its weight has to be supported with suitable bracing where appropriate, and to check with the local authority for support methods. Amtrol says its floor-standing models may not be hung from piping, and that if a smaller inline tank is mounted inverted or horizontally, its body must be fully and rigidly supported. Its recommended position for inline tanks is with the water connection up.

Startup. After the water is back on, Rheem says to open a hot fixture until the air is out, restore gas or power, let the heater reach temperature, then recheck the system pressure. Amtrol says to purge air through a hot fixture before the first firing and not to set the heater above the tank’s maximum temperature. On an electric heater, the tank must be full before power is restored; the reason is in how to flush a tank.

Sizing without guessing

The IRC does not set a tank size. It requires a pressure-control device and leaves the selection to the product’s listing and instructions. Amtrol publishes a sizing chart that chooses a model from two inputs: the water heater’s capacity in gallons and the static supply pressure. The chart’s footnote states its assumptions: 40°F incoming water, a 150 psi temperature-and-pressure relief valve, and a precharge equal to static supply pressure.

Those assumptions matter. A larger heater, higher house pressure, or colder incoming water each means more expansion or less room for it, which can push the selection up a size. A precharge left at a factory setting that does not match the house throws off the arithmetic behind the chart.

Both Amtrol and Rheem call for the precharge to equal the house pressure. A tank that is the right size on paper but charged for a different house is the wrong tank.

Tankless heaters store very little water, and the Amtrol chart has a separate small line for tankless and point-of-use units of up to 2 gallons. Whether a given tankless install needs expansion control is a question for that heater’s manual and the local code official. The difference between storage and on-demand heaters is in tank or tankless.

Checking an existing tank

Expansion tanks wear out. The diaphragm can fail, the air charge can leak away, and the steel shell can corrode. Amtrol’s sheet warns that this product, like most pressurized products, may over time corrode, weaken, and burst, and says a licensed professional should check the whole system, including the expansion tank, every year and more often as it ages. Rheem and Watts both note that all expansion tanks may eventually leak, so the location should be one where a leak will not do damage.

Two common homeowner checks are worth understanding, with their limits.

• The knock test. Tapping the tank to listen for a hollow top and a dull bottom is folk practice, not something the manufacturer sheets cited here describe. Treat it as a hint at most.
• The valve check. Pressing the air valve and getting water instead of air means the diaphragm has failed and the tank needs replacing. Amtrol warns that checking the precharge lets a little air escape, so a careless check can itself leave the charge too low. It says to check the charge only with the tank isolated and empty of water.

Some smaller Amtrol inline models carry an indicator cap that discolors when the tank needs replacement. Amtrol says a discolored cap means the tank must be replaced, and also says not to rely on the cap as the only way to judge the tank’s condition.

If the relief valve begins dripping again on an older installation that used to be quiet, a waterlogged expansion tank is a strong suspect, alongside the valve and thermostat causes on the dripping-valve page. Do not lift the relief valve lever to see whether it stops. The reasons are in how to test a relief valve.

What an expansion tank does not do

An expansion tank does not replace the temperature-and-pressure relief valve. Amtrol’s own sheet requires a relief valve to prevent pressure above the local code limit or the tank’s working pressure, whichever is less, and says a licensed professional should inspect the heater’s relief valve at least every three years or whenever discharge appears.

It also does not lower a high street pressure, stop a failed thermostat from overheating the tank, or fix a valve seat damaged by scale. It handles one job: absorbing the swell of heated water in a system that cannot push it back to the main.

When you replace a heater, ask whether the new installation includes expansion control, and write down the date on the tank or in the manual. The rest of a correct installation, including the pan and the discharge pipe rules, is collected in permits, pans, and strapping.

Sources

• International Residential Code, Sections P2903.4, P2903.4.1, P2903.4.2 (thermal expansion control) and P2903.3.1 (80 psi maximum static pressure), as reproduced by UpCodes and in local adoptions. Check the edition and amendments your jurisdiction has adopted.
• Arapahoe County, Colorado, “Clarification statement on the installation of expansion tanks on water heater installations” (PDF), for the closed-system explanation and the 2015 IRC commentary excerpt.
• International Code Council, “CodeNotes: Water Heater Safety in the I-Codes” (iccsafe.org), for the point that a sound system should produce no relief-valve discharge.
• Amtrol, “Thermal Expansion Tanks for Potable Water: Installation & Operation Instructions,” part 9015-942 (PDF), for location, precharge, mounting and support, startup, the sizing chart’s assumptions, maintenance, and the indicator cap.
• Rheem Technical Service Bulletin TB-1235, “Installation of an Expansion Tank” (PDF), for measuring static pressure, setting precharge at zero system pressure, the 80 psi caution, support, and startup.
• Watts, expansion tank installation instructions (PDF), for the 80 psi limit and what to do when system pressure is higher.