Expansion tank calculator
Water expands when you heat it and it cannot compress. On an open system the expansion pushes back into the main and disappears. Put a check valve, a backflow preventer or a pressure-reducing valve in the line and the system is closed — the expansion has nowhere to go, and the relief valve becomes the pressure control. That is what an expansion tank prevents.
Pre-filled with a 50 gallon heater at 140 °F on a 60 psi supply.
Set the tank pre-charge to this figure.
Minimum tank volume
- Expansion factor
- 1.62%
- Water expanded
- 0.81gal
- Acceptance fraction
- 21.1%
- Pre-charge to
- 60psi
A #12 (4.4 gal) tank covers it. Pre-charge to 60 psi with the system drained.
| Tank | Volume | Covers |
|---|---|---|
| #2 (2.1 gal) | 2.1 gal | no |
| #5 (2.5 gal) | 2.5 gal | no |
| #12 (4.4 gal) | 4.4 gal | yes |
| #15 (4.5 gal) | 4.5 gal | yes |
| #30 (14.0 gal) | 14.0 gal | yes |
How it sizes
Two quantities decide the answer. The first is how much the water expands: the density of water falls about 2.2% between 55 °F and 140 °F, so fifty gallons becomes about 1.1 gallons more than it was. The second is how much of the tank is usable — the acceptance fraction — which depends on the gap between your supply pressure and the pressure you are willing to reach.
A wide gap means a small tank; a narrow gap means a large one. That is why a house on 80 psi needs a bigger tank than the same house on 50 psi, and why raising the pre-charge above the supply pressure quietly makes the tank useless.
Pre-charge is not optional
The symptom that sends you here
A relief valve that drips a cup or two after a shower and then stops is almost never a faulty valve. It is thermal expansion on a closed system with no tank, or with a tank that has lost its charge. Replacing the relief valve fixes it for a week.
Check it with a gauge on a hose bibb: read static pressure cold, then read it again after a heating cycle with no water drawn. If the second number is 30 or 40 psi higher, the expansion has nowhere to go.
Code notes
IPC 607.3 Where a storage water heater supplies a closed system, a device for controlling thermal expansion is required. IPC 604.8 is what usually creates the closed system in the first place: above 80 psi a pressure-reducing valve is mandatory, and a PRV with a check closes the system. The two clauses travel together on almost every job.