Water Heaters · Guide

Water Heater Expansion Tank: When You Need One and How to Size It

By Marcus Reid Updated Documentation-based guide
Systems covered in this article
Amtrol ST-5 (Amazon)Watts PLT-12 (Amazon)

Water heater expansion tanks do not come up in casual conversation about plumbing, but they are required by code in most new water heater installations — not as an upgrade, as a response to a specific, unavoidable physics problem: heated water takes up more space than cold water, and in a modern closed plumbing system, that extra volume has nowhere to go.

Why closed systems create the problem

Water expands by roughly 2% in volume when heated from cold to typical water heater temperature. In an older, open plumbing system, that expanding water simply pushes back toward the municipal main or the well supply — no buildup, no problem.

Modern systems are usually closed. A backflow preventer, a check valve on the main, or a pressure-reducing valve with a built-in check all block that path, sealing the home's plumbing off from the supply side. Once that seal exists, expanding water has nowhere to go but into the existing pipes — and pressure climbs every time the water heater fires, cycling repeatedly through the system.

How to tell if your system is closed

Any of the following makes your system closed:

  • A backflow preventer is installed on the main (required by most modern codes)
  • A check valve sits on the main line
  • A pressure-reducing valve (PRV) with a built-in check is installed
  • Your water meter has a built-in check valve

A direct test: let the water heater reach full temperature with no fixtures running, and watch a pressure gauge on a hose bib or washing machine valve. If pressure climbs noticeably — commonly cited as rising above 80 PSI — during that heating cycle with no water flowing, that is thermal expansion confirming a closed system.

What happens without one

Left unaddressed, thermal expansion does not usually cause a dramatic failure on day one. It is a slow, repeated stress: pressure spikes with every heating cycle, gradually straining valves, fittings, and the water heater's own tank seams. A pressure-relief valve on the water heater may start cycling more frequently as it repeatedly vents the excess — itself a sign something upstream is not managing the pressure.

There is also a practical warranty angle: many water heater manufacturers require an expansion tank on a closed system as a condition of coverage, on the reasoning that constant pressure cycling shortens the heater's life. Skipping the tank can void coverage on the appliance it is meant to protect.

How the tank works

Inside the tank, a rubber bladder or diaphragm separates two chambers. One side is pre-charged with air, typically to match the home's static water pressure. The other side connects to the cold water supply line near the water heater.

When the heater raises water temperature and the expanding volume pushes into the tank, it compresses the air-charged side instead of spiking pressure through the rest of the plumbing — absorbing the expansion, then slowly releasing it back into the system. It functions as a shock absorber for pressure, not as storage in the way a well pressure tank does.

Sizing and pre-charge

Expansion tanks come in a range of sizes, and the right one depends on the water heater's capacity and the home's incoming static pressure — undersizing means the tank fills completely and stops cushioning anything; oversizing wastes money but is not harmful.

Most tanks ship pre-charged to 40 PSI. If your home's static pressure is higher, the pre-charge needs to be adjusted — through the tank's Schrader valve, the same type of valve used on a bicycle tire — to match. A tank pre-charged below your home's actual static pressure will not function correctly regardless of how well it is sized.

Reading the pre-charge accurately requires the tank to be isolated and the water side depressurized first; checking it under system pressure gives a false high reading.

Tankless and well-system exceptions

Tankless water heaters generally do not need an expansion tank. They do not store a large standing volume of hot water, so thermal expansion is minimal — though a tankless unit on a closed system with other pressure issues can still benefit from one.

Homes on a private well already have a pressure tank near the pump, and in some configurations that tank can absorb thermal expansion from the water heater too. But if a check valve sits between the pump's pressure tank and the water heater, the heater side becomes its own closed loop regardless of the well tank upstream — and needs its own dedicated expansion tank. The deciding factor is whether expanded water can actually reach the well's pressure tank, not simply whether a pressure tank exists somewhere in the house.

If you are on a well and the pump is also behaving oddly, the pre-charge check here is the same one that appears in well pump short cycling — for a different tank, with a different correct value. Thermal expansion and pressure are covered across Water Heaters.

Code and certification

The International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC) both address thermal expansion control on closed systems, and most manufacturers set warranty terms assuming the heater operates within its rated pressure — which is part of why code adoption in this area is broad, though the specific requirement still varies by local jurisdiction. Confirm with your local building authority before doing the work, not after.

Tanks intended for domestic hot water — potable systems — are a different product from tanks used in hydronic heating loops. Potable-rated tanks are approved under NSF/ANSI 61, which governs materials in contact with drinking water; non-potable tanks look similar but are not approved for that use and are not interchangeable.

Common tank sizes

Most residential tank-type water heaters on a closed system use one of two sizes. Run the sizing check above against your heater volume and static pressure before you buy:

  • About 2 gallons (for example, the Amtrol Therm-X-Trol ST-5) is a common match for many residential heaters in the 40–50 gallon range at typical household pressure.
  • About 4.5 gallons (for example, the Watts PLT-12) is used for larger heaters or higher static pressure.

Whichever you choose, set the pre-charge to your home's static pressure before installing.

Amtrol ST-5 2-gallon expansion tank on Amazon
Watts PLT-12 4.5-gallon expansion tank on Amazon

Before you buy

  1. Confirm your system is actually closed — check for a backflow preventer, check valve, or PRV, or run the pressure-gauge test above.
  2. Know your home's static water pressure before buying, so the tank can be pre-charged to match.
  3. Confirm you are buying a potable-rated tank for a domestic hot water line, not a hydronic (heating loop) tank.
  4. Check your local code for the specific requirement in your jurisdiction, and check your water heater's warranty terms — some require the tank explicitly.

While you have the main line open

An expansion tank sits on the cold supply near the heater, which is the same stretch of pipe a point-of-entry filter goes on — and the filter belongs upstream of the heater for the same reason the heater needs protecting. If both jobs are on your list, doing them in one visit saves shutting the main down twice. Placement, sizing and clearance are covered in whole house water filter installation, and which treatment category your water actually calls for in our breakdown of the six technologies. Both sit in Whole-House Filtration.

Frequently asked questions

Do I need an expansion tank on my water heater?
If your plumbing is a closed system — a backflow preventer, a check valve on the main, a pressure-reducing valve with a built-in check, or a meter with a built-in check — then thermal expansion has nowhere to go and a tank is normally required. Requirements are set locally, so confirm with your building authority.
How do I know if my plumbing is a closed system?
Look for a backflow preventer, check valve or PRV on the main. A direct test: let the heater reach full temperature with no fixtures running and watch a pressure gauge on a hose bib. If pressure climbs noticeably with no water flowing, the system is closed.
What happens if you do not install an expansion tank?
Not usually a dramatic failure on day one. It is repeated stress: pressure spikes with every heating cycle, gradually straining valves, fittings and the heater's tank seams. A relief valve that starts cycling more often is a sign something upstream is not managing pressure.
What pressure should a water heater expansion tank be pre-charged to?
To match the home's static water pressure. Most ship pre-charged to 40 PSI, which has to be adjusted through the Schrader valve if your static pressure is higher. A tank pre-charged below actual static pressure will not work correctly regardless of sizing.
Do tankless water heaters need an expansion tank?
Generally not. They do not store a large standing volume of hot water, so thermal expansion is minimal — though a tankless unit on a closed system with other pressure issues can still benefit from one.
Does a house on a well need a water heater expansion tank?
It depends on whether expanded water can actually reach the well's pressure tank. If a check valve sits between the pump's pressure tank and the water heater, the heater side is its own closed loop and needs a dedicated expansion tank regardless of the well tank upstream.