What formula sizes the tank?

It uses the ASME bladder-tank relation Vt = Ve / (1 − Pi/Pf), where Ve is the acceptance (expansion) volume equal to system volume times (v2/v1 − 1), and Pi and Pf are the absolute fill and maximum operating pressures. Pressures are converted to absolute by adding 14.7 psi to the gauge value.

Why must I use the cold and hot temperatures?

Water expands as it heats. The ratio of its specific volume at the hot operating temperature to its specific volume at the cold fill temperature gives the fractional expansion. A 50 to 180 F swing expands the water by roughly 3 percent, which is the volume the tank must absorb.

What precharge should the tank have?

Set the tank air precharge equal to the system fill pressure before connecting it to the wet side. If the precharge is wrong the tank either has no room to accept expansion or sits waterlogged, and the relief valve weeps on every heat cycle.

What happens if the tank is too small?

Undersizing means the system pressure climbs past the relief setpoint as the water heats, so the relief valve discharges and the system slowly loses water and gains air. Oversizing wastes a little money but is otherwise harmless.

Does this work for domestic hot water?

This sizing is for closed hydronic heating loops. Potable domestic-hot-water thermal-expansion tanks use a similar idea but a different acceptance basis and a potable-rated bladder. Use a potable expansion tank product and its own sizing chart for water-heater thermal expansion.

Hydronic Expansion Tank Sizing Calculator

A closed hydronic system has nowhere for heated water to expand, so it needs an expansion tank to absorb the volume change and keep pressure under the relief setpoint. This calculator applies the ASME bladder-tank formula to size that tank from your system volume, temperatures, and pressures.

How it works

Water expands as it heats; the tank must accept that extra volume while staying below the relief valve setpoint:

Ve = Vs × (v2/v1 − 1)          acceptance (expansion) volume
Vt = Ve / (1 − Pi/Pf)          required tank volume
Pi = fill gauge psi + 14.7     absolute fill / precharge pressure
Pf = max gauge psi + 14.7      absolute maximum operating pressure

v1 and v2 are the specific volumes of water (ft³/lb) at the cold fill and hot operating temperatures, interpolated from steam tables. The bigger the pressure window between fill and relief, the smaller the tank can be.

Example and notes

A 40-gallon system filled at 12 psig and operating up to 25 psig, swinging from 50 to 180 F, expands by about 3 percent — roughly 1.2 gallons of acceptance volume — and needs a tank of a few gallons once the pressure factor is applied. Always precharge the tank air side to the fill pressure before connecting it, and round up to the next standard tank size. Estimate system volume generously: piping and cast-iron emitters hold more water than people expect.