EXPANSION TANK SIZE

Enter the water heater's nominal capacity in gallons. Set the supply pressure (whatever your PRV is set to, typically 50-65 PSI). Set the maximum allowable system pressure (80 PSI is the IPC ceiling for fittings; T&P relief valves typically pop at 150 PSI but you don't want to ride near that line).

Enter the heating temperature rise — the difference between cold inlet and stored hot water temp. Typical: 50°F inlet rising to 140°F storage = 90°F rise. Cold inlet (40°F in winter) to higher storage (160°F for legionella protection) = 120°F rise. The calculator returns the minimum tank size and rounds up to a standard product (2, 4.4, 6, 14, 20, 32, or 44 gallons).

For a typical 50-gallon water heater with 60 PSI supply, 80 PSI max, 90°F rise:

Water expansion: 50 × 0.00041 × 90 = 1.85 gallons. Acceptance factor: 1 - (60 ÷ 80) = 0.25. Tank size: 1.85 ÷ 0.25 = 7.4 gallons.

Next standard size: 14 gallons (Watts ETX-30, Amtrol THERM-X-TROL ST-12, or Zilmet AC-15). At ~$80-$150 retail, plus $50-$150 in install (tee, isolation valve, mounting bracket).

For an 80-gallon heater (large family): 80 × 0.00041 × 90 = 2.95 gallons. Same acceptance factor (0.25): tank = 11.8 gallons → 14-gallon tank still works.

For a high-pressure system (75 PSI supply, 80 PSI max, 50-gal heater, 90°F rise): acceptance factor: 1 - (75 ÷ 80) = 0.0625. Tank: 1.85 ÷ 0.0625 = 29.6 gallons → 32-gallon tank. High supply pressure dramatically increases required tank size — strong argument for setting the PRV lower.

For a tankless heater: usually no expansion tank needed because there's no stored water heating up. Add a small 2-gallon tank only if the system has a hot-water storage buffer.

Skipping the tank entirely. Closed systems without expansion tanks WILL fail — pressure spikes during every heating cycle eventually break a fitting, rupture the heater, or pop the T&P. Required by code (IPC 607.3.1, UPC 608.3) on any closed system.

Not pre-charging the tank to match supply pressure. Out of the box, expansion tanks are typically pre-charged to 40 PSI. If your supply is 60 PSI, you need to pump the tank to 60 PSI before installation — otherwise it never properly absorbs expansion.

Mounting the tank wrong. Bladder tanks must be installed on the cold supply ABOVE the water heater, with the bladder oriented per manufacturer spec. Wrong orientation reduces capacity by 30-50%.

Ignoring high supply pressure. Above 70 PSI supply, expansion tank sizing balloons because the acceptance factor shrinks. Lowering the PRV to 50-60 PSI usually saves 1-2 tank sizes — worth doing if static pressure is high.

Required on any closed system: backflow preventer, PRV, or check valve between meter and heater triggers the requirement.

Water expansion ≈ 0.00041 × heater_volume × temperature_rise.

Tank size = expansion_volume ÷ (1 − supply_PSI ÷ max_PSI).

Standard sizes: 2, 4.4, 6, 14, 20, 32, 44 gallons. Round up.

Lower supply pressure = smaller tank. Above 70 PSI supply, tank sizes balloon — set PRV to 55-60 PSI.

Pre-charge the tank to match supply pressure before install — out of the box it's typically 40 PSI.

Mount on cold-water side, above the water heater, per manufacturer's orientation spec (usually bladder up).

Tankless heaters generally don't need expansion tanks — no stored water means no thermal expansion volume.