What size expansion tank do you need?

Common questions

What size expansion tank do I need for my water heater?

Size it from the water volume, the temperature rise and the pressures. The water grows by an expansion factor (about 1.7% for a 50 °F to 140 °F rise), and only part of a pre-charged tank is free to accept that water. For a typical 40–50 gallon heater at 50 psi supply and a 150 psi relief valve the minimum works out around 1.4 gallons, which is why manufacturers commonly rate a 2-gallon thermal expansion tank for heaters up to about 50–60 gallons. Enter your own numbers above for an exact minimum.

How is expansion tank size calculated?

In three steps. First the expansion volume Ve = system volume × expansion factor, where the expansion factor is the density of the cold water divided by the density of the hot water, minus one. Second the acceptance factor AF = 1 − (fill pressure ÷ max pressure), using absolute pressures (gauge + atmospheric). Third the minimum tank volume Vt = Ve ÷ AF. The calculator does all three from your inputs.

Do I need an expansion tank on my water heater?

You need one on a closed system — where a check valve, backflow preventer or pressure-reducing valve stops heated water pushing back into the main. In a closed system the extra volume has nowhere to go, so pressure spikes every heating cycle and the relief valve may weep. Most plumbing codes require thermal expansion control on closed systems; check with your local authority, because whether your system is closed depends on your specific valves and meter.

What pressure should an expansion tank be set to?

Pre-charge the empty tank to match the system fill pressure — the same number you enter as the fill pressure above (often your incoming water pressure, commonly 40–60 psi). If the pre-charge is lower than the fill pressure the tank starts partly full of water and cannot accept the expansion; if it is much higher the diaphragm will not move. Set it with a tyre gauge before the tank is connected to water.

What happens if the expansion tank is too small?

A tank that is too small fills with the expansion water before the heating cycle ends, so pressure keeps climbing and the temperature-and-pressure relief valve opens to dump the excess — you see water dripping from the relief line and the system runs at high pressure. Sizing to at least the minimum the calculator gives, then choosing the next size up, leaves a margin so the relief valve stays closed.