Expansion tank sizing calculator
On a closed plumbing system, heated water has nowhere to expand — so pressure spikes and the relief valve weeps. A thermal-expansion tank absorbs it. Size the acceptance volume from your tank gallons and your supply and relief pressures.
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A closed system needs a thermal-expansion tank so heated water doesn’t spike pressure and dump through the relief valve. From your supply (60 psi) and relief (150 psi) pressures the acceptance volume is about 1.67 gal → a 2-gallon tank. A plumber and local code have the final word.
If a backflow preventer, check valve or pressure-reducing valve sits on your supply, your home is a closed system. When the heater fires, water expands with nowhere to go, pressure climbs, and the temperature-&-pressure relief valve starts dripping — or, worse, the pressure stresses the tank and fixtures. A small thermal-expansion tank gives that expanded water a cushion of air to push into.
Sizing it is a two-step job: estimate how much the water expands when heated (about 2% of the tank), then size the acceptance volume using the ratio between your supply pressure and the relief setting. Get it wrong on the small side and the tank fills and stops absorbing; this tool rounds you to the next standard size.
Formula
Thermal expansion, then acceptance volume (Boyle’s law on the air side):
Ve = 0.02 × tank_gallons
Vt = Ve ÷ (1 − supply_psi ÷ relief_psi)
Pre-charge the expansion tank to match your supply pressure. The 2% expansion factor is a LABELED planning typical for a normal temperature rise; a plumber and local code have the final word on whether a device and its discharge are required.
Worked example
A 50-gallon tank, 60 psi supply, 150 psi relief:
Ve = 0.02 × 50 = 1.0 gal
Vt = 1.0 ÷ (1 − 60÷150) = 1.0 ÷ 0.60 = 1.67 gal
That points to a standard 2-gallon expansion tank. Watch the pressure ratio: raise the supply to 80 psi and the denominator shrinks to 0.467, so the required acceptance volume jumps to about 2.14 gallons — a good reason to fit a pressure-reducing valve if your street pressure is high.
Measure first, avoid a wrong size
Higher supply pressure needs a bigger tank. The closer your supply is to the relief setting, the less room the air side has to work, so the acceptance volume climbs fast — another reason to tame high street pressure first.
- Pre-charge to the supply. Set the expansion tank’s air pre-charge to your measured supply pressure before installing, or it will not absorb correctly.
- Measure real pressure. A cheap gauge on a hose bib gives your static supply psi; do not guess. Check the T&P valve rating (usually 150 psi) for the relief figure.
- A weeping relief valve is the symptom. If your T&P valve drips only after the heater runs, thermal expansion on a closed system is the usual cause — and the fix is an expansion tank, not a new valve.
- Code and discharge are a plumber’s call. Whether an expansion tank is required, and how the relief discharge must be piped, is set by local code and the manufacturer’s instructions.
Reference table
Acceptance volume for a 50-gallon heater at a 150 psi relief setting, by supply pressure.
| Supply pressure | Acceptance volume (gal) | Standard tank |
|---|---|---|
| 40 psi | 1.36 | 2-gallon |
| 50 psi | 1.50 | 2-gallon |
| 60 psi | 1.67 | 2-gallon |
| 70 psi | 1.88 | 2-gallon |
| 80 psi | 2.14 | 3-gallon |