Do I need a thermal expansion tank?

You need one if your supply is a closed system, which it is whenever a check valve, a pressure reducing valve or a backflow preventer sits between the street and the house. Heated water has nowhere to expand back into, so pressure rises until the relief valve opens. If your relief valve weeps after the heater runs, that is the symptom and an expansion tank is the fix.

What size expansion tank for a 50 gallon water heater?

A 2 gallon tank covers most 50 gallon installations at ordinary supply pressures. A 50 gallon tank heated from 50 F to 140 F produces about 0.84 gallons of expansion, and at 60 psi supply against a 150 psi relief the acceptance relation calls for about 1.5 gallons of tank. Higher static supply pressure needs a larger tank, because less of its volume is available to accept water.

What should the expansion tank pre-charge be set to?

Equal to your static supply pressure, measured with a gauge on a hose bibb with nothing running. Tanks ship pre-charged at around 40 psi, which is right for very few houses. Check and set the pre-charge with a tyre gauge while the tank is empty and off the system, because a tank charged wrong performs as if it were not there at all.

Can I fit a hammer arrestor instead?

No. They look similar and some listings use both terms, but a hammer arrestor is a tiny chamber sized to absorb the pressure spike from a valve closing quickly, not the sustained volume of a whole tank of water expanding. A half inch arrestor holds a fraction of the gallon or so a 50 gallon heater produces, so it will not control the pressure rise.

Install calculator

Thermal Expansion Tank Size Calculator

Quick answer

Water expands about 1.7 percent heating from 50 F to 140 F, so a 50 gallon tank makes roughly 0.84 gallons of expansion. At 60 psi supply against a 150 psi relief setting, the acceptance relation gives a required tank of about 1.5 gallons, which is a standard 2 gallon expansion tank.

Result

Required tank volume 1.54 gal Published standard

Buy this size 2 gal Convention

Water expansion 0.84 gal Published standard

Set the pre-charge to 60 psi Published standard

Check the pre-charge with a tyre gauge on the schrader valve with the tank empty and off the system, and set it to your static supply pressure. A tank fitted at its factory 40 psi onto a 70 psi supply is already fully compressed and accepts nothing.

The formula

Expansion volume = tank gallons x (cold density / hot density - 1). Required tank = expansion volume / (1 - (supply pressure + 14.7) / (relief pressure + 14.7)).

The expansion fraction is computed from the published density of water at each temperature rather than from a "water expands two percent" rule. The pre-charge on the expansion tank must be set to match your static supply pressure before it goes on, or the tank accepts almost nothing and the whole exercise is wasted.

The figures above are computed with these exact inputs: Water heater capacity 50 gal, Static supply pressure 60 psi, T&P relief setting 150 psi, Cold inlet temperature 50 F, Stored temperature 140 F.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Expansion tank acceptance relation Vt = Ve / (1 - Pi/Pf) with absolute pressures	Published standard	Expansion tank acceptance relation Vt = Ve / (1 - Pi/Pf) with absolute pressures
Published density of liquid water against temperature	Published standard	Published density of liquid water against temperature, used to compute thermal expansion directly rather than from a percentage
Relief Setting: 150	Published standard	ASME rated residential T&P valves are commonly set at 150 psi and 210 F. Confirm against your own tank data plate

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Beginner

Eastman

Eastman 2 Gallon Thermal Expansion Tank

$34.98

A 2 gallon potable expansion tank with a 3/4 inch MIP connection, which is the size and the fitting that suits almost every 40 to 50 gallon residential tank.

Best for: The standard 40 or 50 gallon tank on a closed system

Check price on Amazon

Beginner

GE 2.1 Gallon Expansion Tank

$42.99

Pre-charged to 40 psi from the factory. That figure matters: the pre-charge has to be adjusted to match your static supply pressure before the tank goes on.

Best for: A supply near 40 psi, or anyone with a gauge and a bike pump

Check price on Amazon

Beginner

EFIELD

EFIELD 2.1 Gallon Expansion Tank

$39.98

The budget end of the 2.1 gallon size. An expansion tank is a bladder in a steel shell, and the failure mode is a waterlogged bladder rather than a leak.

Best for: A budget replacement of a failed tank

Check price on Amazon

Beginner

TT FLEX

TTFLEX 2.1 Gallon Expansion Tank

$39.99

Another 2.1 gallon option with the same 3/4 inch male thread, useful when the others are out of stock, which happens often in this category.

Best for: A straightforward 2.1 gallon replacement

Check price on Amazon

Intermediate

Amtrol

Amtrol ST-5 2 Gallon

$77.99

The Amtrol ST series is the potable line, as distinct from the Extrol hydronic line. Getting that distinction wrong puts a non-potable bladder in a drinking water system.

Best for: Anyone who wants the name plumbers recognise

Check price on Amazon

Beginner

Amtrol

Amtrol THERM-X-TROL ST-1

$50.43

The smallest tank in the Amtrol potable range, which suits a small tank on a low static pressure supply and nothing larger.

Best for: A 30 gallon tank at low supply pressure

Check price on Amazon

Frequently asked questions

Do I need a thermal expansion tank?: You need one if your supply is a closed system, which it is whenever a check valve, a pressure reducing valve or a backflow preventer sits between the street and the house. Heated water has nowhere to expand back into, so pressure rises until the relief valve opens. If your relief valve weeps after the heater runs, that is the symptom and an expansion tank is the fix.
What size expansion tank for a 50 gallon water heater?: A 2 gallon tank covers most 50 gallon installations at ordinary supply pressures. A 50 gallon tank heated from 50 F to 140 F produces about 0.84 gallons of expansion, and at 60 psi supply against a 150 psi relief the acceptance relation calls for about 1.5 gallons of tank. Higher static supply pressure needs a larger tank, because less of its volume is available to accept water.
What should the expansion tank pre-charge be set to?: Equal to your static supply pressure, measured with a gauge on a hose bibb with nothing running. Tanks ship pre-charged at around 40 psi, which is right for very few houses. Check and set the pre-charge with a tyre gauge while the tank is empty and off the system, because a tank charged wrong performs as if it were not there at all.
Can I fit a hammer arrestor instead?: No. They look similar and some listings use both terms, but a hammer arrestor is a tiny chamber sized to absorb the pressure spike from a valve closing quickly, not the sustained volume of a whole tank of water expanding. A half inch arrestor holds a fraction of the gallon or so a 50 gallon heater produces, so it will not control the pressure rise.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.