Thermal Expansion Tank Sizing
Get a starting thermal-expansion-tank size for your water heater from its gallon capacity, using a labeled planning band.
Calculator
A 50-gallon water heater on a closed system typically pairs with a roughly 2.0-gallon thermal-expansion tank (a labeled planning band). The exact size depends on your incoming and relief pressures — confirm with a licensed plumber and local code.
On a closed plumbing system — one with a backflow preventer, check valve or pressure-reducing valve on the supply — water heated in the tank has nowhere to expand. A thermal expansion tank absorbs that pressure rise so the T&P relief valve is not forced to weep. This tool gives a starting size from a labeled band.
Formula
Expansion-tank size scales with the water-heater volume (and the incoming-to-relief pressure spread). A common planning band:
water heater ≤ 60 gal → ~2-gallon expansion tankwater heater ≤ 80 gal → ~4.4-gallonwater heater ≤ 120 gal → ~5-gallon
These are labeled planning figures. The exact size depends on your incoming (fill) pressure and the relief setting; higher street pressure needs a larger tank. A licensed plumber sizes and precharges it to your actual supply pressure.
Worked example
A common 50-gallon residential water heater falls in the first band:
50 gal ≤ 60 → ~2-gallon expansion tank
So a 2-gallon expansion tank is the usual starting point. Precharge it to match your measured static supply pressure before install, and confirm the final choice against your local code.
Background & practice
Signs you need an expansion tank: the T&P valve drips after the burner runs, or a water-pressure gauge climbs well above street pressure after a hot draw. Most jurisdictions now require one on closed systems at water-heater replacement — it is one of the code upgrades that can surprise a replacement budget.
The tank is precharged with air to your incoming water pressure so its bladder sits ready to take up expansion. If your street pressure is high, add a pressure-reducing valve and check the number with the water-pressure tool. Sizing here is a labeled band, not a code citation — confirm with a licensed plumber.
Reference table
| Water heater (gal) | Expansion tank (gal, labeled band) |
|---|---|
| up to 60 | 2.0 |
| 60–80 | 4.4 |
| 80–120 | 5.0 |
Labeled planning band — the exact size depends on your incoming and relief pressures; a licensed plumber sizes and precharges it to code.
Frequently asked questions
What size expansion tank for a 50-gallon water heater?
A 2-gallon expansion tank is the usual starting point for a 50-gallon (or smaller) heater on a closed system. Higher supply pressure can call for a larger one — confirm with a licensed plumber.
Do I actually need a thermal 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. Most codes now require it at water-heater replacement.
How do I know my system is "closed"?
If there is a pressure-reducing valve or backflow preventer on your incoming line, or the water meter has a check valve, the system is closed. A dripping T&P valve after the heater runs is a classic symptom of thermal expansion with nowhere to go.
What pressure should the expansion tank be set to?
Precharge it to match your static supply pressure (measure it, or estimate with the water-pressure tool). Setting it wrong defeats the purpose — a plumber sets it at install.