Short answer
You need thermal expansion control when your water system is closed: a check valve, backflow preventer or pressure reducing valve on the supply stops heated water flowing back to the main. The North Carolina fire marshal’s office notes this applies to water heater replacements too. The usual fix is a small potable expansion tank on the cold line. Set its air pre-charge to match your house water pressure; Watts says never exceed 80 psi.
Why closed systems need it
When a heater warms a full tank, the water grows in volume. If a check valve, backflow preventer or pressure reducing valve stops it pushing back to the main, pressure in the house pipes rises until something gives, often the T&P relief valve. Code calls this thermal expansion control; the IRC covers it in section P2903.4 and its subsections on pressure reducing valves and check valves (Sioux Chief summary). The North Carolina Office of State Fire Marshal applies the rule when a water heater is replaced (NC OSFM).
How it works
The tank has two chambers split by a rubber diaphragm or bladder. Air is pre-charged on one side; house water is on the other. As heated water expands, it pushes into the tank and compresses the air, so pressure in the pipes rises only a little.
Sizing
Expansion tank size depends on the heater’s gallons, your house pressure, and the relief valve setting. Maker charts commonly put a 40 or 50 gallon heater at normal house pressure on the smallest residential tank, about 2 gallons. Above that, use the maker’s calculator (Watts sizing tool).
Expansion tank sizing, starting points
| Heater | Typical tank | Check |
|---|---|---|
| Up to about 50 gallons, normal pressure | About 2 gallon potable tank | Maker chart at your measured pressure |
| 50 gallons, high pressure | Next size up (about 4.4 to 4.5 gallon) | Maker chart |
| 75 to 80 gallons | Larger tank per chart | Maker chart |
Setting the pre-charge
- Measure your static water pressure with a gauge on a hose bib, with no water running.
- Before installing (or with the tank isolated and drained), check the air pre-charge at the tank’s air valve with a tire gauge.
- Add or release air until it matches your house pressure. Watts: adjust it to match initial pressure, and never exceed 80 psi (Watts).
What you need
Expansion tank types
-
01Most homes
Potable thermal expansion tank, about 2 gallons
Best for: a 40 or 50 gallon heater at normal house pressure
- Connection
- Usually 3/4 in male thread
- Pre-charge
- Factory set, adjust to house pressure
- Rated
- Potable water
- The size maker charts often list for 40 and 50 gallon heaters
- Watts lists a 20 psi factory pre-charge on its PLT-5 in its literature; set yours to house pressure
- Mount it on the cold line, supported
-
02Bigger jobs
Potable expansion tank, about 4.5 gallons
Best for: larger heaters or higher house pressure
- Connection
- Usually 3/4 in
- Use
- Per maker sizing chart
- Rated
- Potable water
- Heavier when full: support it with a strap or bracket
- Check the chart before upsizing
- Same pre-charge rule
Installing one is a short plumbing job: how to install an expansion tank.
Questions people ask
How do I know if my system is closed?
Look for a pressure reducing valve near the main shutoff or meter, a backflow preventer, or a check valve on the incoming line. Some water meters include a check valve; your water utility can tell you. A relief valve that drips after the heater runs is a common sign of thermal expansion.
What size expansion tank do I need for a 50 gallon water heater?
At normal house pressure, maker sizing charts commonly put 40 and 50 gallon heaters on the smallest residential tank, about 2 gallons (for example Amtrol ST-5 or Watts PLT-5). Higher pressure or a larger heater moves you up a size. Use the maker’s sizing chart or calculator.
What should the expansion tank pressure be?
Set the air pre-charge to match your static house water pressure, measured with a gauge on a hose bib. Watts says the pre-charge should match the initial pressure and never exceed 80 psi.
How do I know if my expansion tank is bad?
Tap it: a good tank sounds hollow on the top half. Press the air valve: water coming out means the diaphragm has failed. A tank that is heavy all over or a T&P valve that starts dripping are other signs.
Can I use a heating (boiler) expansion tank?
No. Use a tank rated for potable water. Hydronic heating tanks are not made for drinking water.
Sources
Last reviewed October 29, 2026. Models, codes and listings change; check your heater’s manual and your local code, and follow the maker’s instructions.