TradesHow this works
Water expands as it heats. On a closed system that extra volume has nowhere to go, so pressure climbs until the relief valve weeps or a fitting gives. An expansion tank gives the water a cushion of air to push into. This tool figures the expansion volume from your heater size and temperature rise, then applies the standard diaphragm-tank formula across your fill and maximum pressure to get the smallest tank that holds pressure in check.
Watch the fill pressure. When it sits close to the maximum, the tank has to grow fast, and a pressure-reducing valve is often the cheaper, whole-house fix. Set the tank pre-charge to the fill pressure before you install it.
Common questions
Do I even need an expansion tank?
You need one on a closed system, where a check valve, backflow preventer, or pressure-reducing valve stops heated water from pushing back into the main. On an open system the expansion relieves itself into the supply. Most homes with a PRV or backflow device on the service are closed systems and need one.
What pre-charge should the tank have?
Set the air pre-charge to match your supply (fill) pressure before you install it, and check it with a gauge. If the pre-charge is wrong, the tank accepts the wrong amount of water and the sizing no longer holds.
Why does higher water pressure need a bigger tank?
The tank only accepts water in the gap between your fill pressure and the maximum. The closer the fill pressure sits to the max, the smaller that gap, so the tank has to be larger to absorb the same expansion. Lowering fill pressure with a PRV is often a better fix than a big tank.
Sources: Diaphragm expansion tank formula Vt = Ve / (1 - Pi/Pf) (absolute pressures), ASHRAE / ASPE; water density vs temperature. Reproduces Amtrol Therm-X-Trol ST-series sizing.. Engine version 1.0.0. Risk tier 2.