The short answer
A 50 gallon water heater at 120°F on a 60 psi supply needs an expansion tank of about 1.1 gallons, keeping pressure under 125 psi. The water grows by 0.5 gallons as it heats from 40°F. Higher water pressure or a hotter tank needs a bigger one.
Tank size by water heater and pressure
Tank gallons at 120°F with a 125 psi limit, by water heater size and house water pressure.
| Heater | 40 psi | 50 psi | 60 psi | 70 psi | 80 psi |
|---|---|---|---|---|---|
| 30 gal | 0.5 | 0.6 | 0.7 | 0.8 | 1 |
| 40 gal | 0.7 | 0.8 | 0.9 | 1.1 | 1.3 |
| 50 gal | 0.9 | 1 | 1.1 | 1.3 | 1.6 |
| 65 gal | 1.1 | 1.3 | 1.4 | 1.7 | 2.1 |
| 80 gal | 1.4 | 1.5 | 1.8 | 2.1 | 2.5 |
| 100 gal | 1.7 | 1.9 | 2.2 | 2.6 | 3.2 |
How it works
The formula
Heated water takes up more room. In a closed system it has to go somewhere, so the tank's air cushion compresses to take it. By Boyle's law, the less the pressure may rise, the bigger the tank.
pressure factor = (highest pressure + 14.7) ÷ (highest pressure − supply pressure)
tank = expanded water × pressure factor
Assumptions
- Method, expansion factors and pressure factor from AMTROL's Therm-X-Trol sizing guide; its worked example (240 gal at 150°F, 60 psi, 125 psi limit) comes to 9.0 gallons with the pressure factor rounded to 2.1; the exact factor, 2.15, gives 9.2, which we use. Sizes are rounded up. Expansion factors from 40°F incoming water: 100°F 0.6%, 120°F 1%, 130°F 1.2%, 140°F 1.5%, 150°F 1.8%, 160°F 2.1%, 170°F 2.4%, 180°F 2.8%; we interpolate between them.
- The tank's air charge equals the supply pressure, as AMTROL requires.
- The result is the total tank volume; also check the tank's rated acceptance volume covers the expanded water.
Worked example: 50 gal at 120°F, 60 psi
- 50 × 0.01 = 0.5 gal of expansion
- (125 + 14.7) ÷ (125 − 60) = 2.15
- 0.5 × 2.15 = 1.1 gallons
Too big or too small
- Too small: a dripping relief valve. Pressure still climbs past the limit on every heating cycle.
- Wrong air charge: a too-small tank in disguise. Set the charge to the house pressure before installing.
- Very high water pressure? Above about 80 psi, a pressure-reducing valve protects fixtures and shrinks the tank you need.
Questions people ask
What size expansion tank do I need for a 50 gallon water heater?
At 120°F, a 60 psi supply and a 125 psi limit: about 1.1 gallons of tank. The water grows by 0.5 gallons as it heats, and the tank's air cushion can only take part of its volume before the pressure reaches the limit. At 140°F it's 1.7 gallons; with an 80 psi supply, 1.6.
Do I need an expansion tank?
You need one when the house has a closed system: a check valve, backflow preventer or pressure-reducing valve keeps expanding hot water from pushing back into the main. Without somewhere to go, each heating cycle spikes the pressure and can open the relief valve. Many codes require a tank on any closed system.
What should I set the expansion tank pressure to?
Match the house water pressure, measured at a hose bib with no water running, before the tank is installed. A tank left at its factory charge below the supply pressure fills with water before any expansion happens.
Why does higher water pressure need a bigger tank?
The air in the tank starts compressed to the supply pressure, so it has less room to shrink before reaching the limit. At 60 psi the factor is 2.15; at 80 psi it's 3.1.