Suggested tank: —
| Water heater | Expansion volume | Typical tank |
|---|---|---|
| 30 gal | ~0.5 gal | ~2 gal (e.g. ST-5 / PLT-5) |
| 40 gal | ~0.7 gal | ~2 gal (e.g. ST-5 / PLT-5) |
| 50 gal | ~0.8 gal | ~2 gal (e.g. ST-5 / PLT-5) |
| 75 gal | ~1.3 gal | ~4.4 gal (e.g. ST-8 / PLT-12) |
| 80 gal | ~1.4 gal | ~4.4 gal (e.g. ST-8 / PLT-12) |
What Size Expansion Tank Do I Need?
Expansion tank sizing depends on three things: how much water heats up (your water heater or system volume), how much that water expands (the temperature rise), and the pressure window between your normal supply pressure and the relief valve setting. As water is heated it expands by roughly 2 percent of its volume; in a closed system that extra volume has nowhere to go, so pressure climbs until the relief valve opens. A correctly sized tank absorbs the expanded water and keeps pressure below the relief setting.
The calculator above estimates the thermal expansion volume from your inputs, then divides by the tank acceptance factor (set by the gap between precharge and relief pressure) to give a recommended tank size. For most residential 40 to 50 gallon water heaters, a small 2 gallon tank is enough; 75 to 80 gallon heaters usually move up to a 4.4 gallon tank.
Expansion Tank Precharge Pressure
A diaphragm tank works only when its air precharge matches the static water pressure of your system. If the precharge is too low the tank fills with water at rest and has no room left to absorb expansion; too high and it never engages. Set the precharge (measured at the tank air valve with the tank empty of system pressure) equal to your supply or PRV-reduced pressure, commonly 40 to 60 PSI. Check it with a tire gauge at install and once a year afterward.
Closed Plumbing System Explained
A closed system is any plumbing where water cannot flow back into the city main. A check valve, backflow preventer, or pressure-reducing valve on the incoming line all create a closed system. In an open system, thermal expansion can push back into the municipal supply, so an expansion tank may not be strictly required. In a closed system that path is blocked, expansion is trapped, and a thermal expansion tank becomes necessary to protect the water heater, fixtures, and relief valve. Many municipalities now require both a backflow device and an expansion tank by code.
Frequently Asked Questions
Where does the expansion tank go? On the cold water supply line near the water heater inlet, downstream of any check valve or PRV.
What precharge should I set? Match your static supply pressure, typically 40 to 60 PSI. Set it before the tank sees system pressure.
Do I need one on an open system? Not always required, but recommended, since adding a meter check valve or PRV later turns it into a closed system.
How do I know my tank failed? Water sputters from the air valve, the relief valve drips after heating cycles, or pressure spikes. Re-check precharge or replace the tank.