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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.

Expansion tank volumeIPC 607.3

Pre-filled with a 50 gallon heater at 140 °F on a 60 psi supply.

gal
°F
°F
psi

Set the tank pre-charge to this figure.

psi
3.9gallons

Minimum tank volume

Expansion factor
1.63%
Water expanded
0.82gal
Acceptance fraction
21.1%
Pre-charge to
60psi

A 4.4 gal tank covers it. Pre-charge to 60 psi with the system drained.

TankVolumeCovers
2.1 gal2.1 galno
2.5 gal2.5 galno
4.4 gal4.4 galyes
4.5 gal4.5 galyes
14.0 gal14.0 galyes

How it sizes

Two quantities decide the answer. The first is how much the water expands: the density of water falls about 1.6% between 55 °F and 140 °F, so fifty gallons becomes about 0.8 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

Set the air charge to the measured supply pressure before you hang the tank, with no water in it. A tank left at a factory charge below the supply pressure takes on water the moment it is connected, and that water is capacity it no longer has for expansion.

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. Skip it and the tank warranty goes with it: both warranties read on the best water heater brands page exclude a closed system without one. 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.

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