Does a water heater need an expansion tank?

Required by code on closed systems (with a check valve or PRV).

What pre-charge should I use?

Match the static water pressure.

How much does water expand?

About 1.6% from 50 to 140 °F and 2.8% to 176 °F.

Where does a water heater expansion tank go?

On the cold supply between the check valve and the heater.

How do I know if it's failed?

It sounds full when tapped or water comes out the air valve.

Same tank for potable water and heating?

No — potable tanks are rated for drinking water; hydronic ones aren't.

Expansion tank calculator

System water volume, minimum and maximum temperature, and initial and maximum pressure: you get water expansion and the stock expansion tank size.

Purchase quantity

8 l

Net measurement: 5.65 l

Stock expansion tank8 l
Calculated tank volume5.65 l
Water expansion2.42 l

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How it's calculated (visible formula)

expansion = V × (e(Tmax) − e(Tmin)); tank = expansion ÷ (1 − P_initial,abs ÷ P_max,abs)

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Work out system water volume (heater or loop).
Set minimum (cold water) and maximum working temperature.
Work out water expansion between them.
Enter initial pressure and relief valve setting.
Size the tank and pick the next larger stock size.

Worked example: 40 gal (150 l) heater, 50 to 140 °F, 43 psi supply, 87 psi relief

Expansion: 150 × (0.0161 − 0) = 2.42 l
Tank: 2.42 ÷ (1 − 4/7) = 5.65 l

→ A 2-gallon (8 l) expansion tank.

Common mistakes

Not setting the tank pre-charge to supply pressure.
Sizing hydronic tanks without radiator volume.
Isolating the tank behind a closed valve.

Pro tips

Set the tank pre-charge with a gauge before installing.
Install on the cold supply with no valve isolating it.
Check pre-charge yearly.
If the relief valve drips when heating, the tank is often waterlogged.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

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FAQs

Does a water heater need an expansion tank?: Required by code on closed systems (with a check valve or PRV).
What pre-charge should I use?: Match the static water pressure.
How much does water expand?: About 1.6% from 50 to 140 °F and 2.8% to 176 °F.
Where does a water heater expansion tank go?: On the cold supply between the check valve and the heater.
How do I know if it's failed?: It sounds full when tapped or water comes out the air valve.
Same tank for potable water and heating?: No — potable tanks are rated for drinking water; hydronic ones aren't.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.