Expansion Tank Calculator
This expansion tank calculator sizes the diaphragm expansion tank for a closed heating system or a hot water cylinder. Enter the water volume, the fill and maximum temperatures, the tank pre-charge pressure, and the relief valve setting, and the expansion tank calculator returns the expansion volume, the acceptance factor, the minimum tank volume, and the next standard size in liters and gallons.
Results
The Expansion Tank Calculator is part of our Plumbing Calculators collection. Browse more plumbing calculators to size, convert, and check your work alongside this tool.
What the expansion tank calculator does
Water expands as it heats: from 10 °C to 70 °C its volume grows by about 2.2%. In a closed system that extra water has nowhere to go, so the pressure climbs until the relief valve opens. A diaphragm expansion tank holds a cushion of air or nitrogen that compresses to take the extra water. The expansion tank calculator finds that extra volume from the density of water, then scales it by how much of the tank can fill between the pre-charge and the maximum pressure.
Expansion tank formula
The expansion volume comes from the density of water at the two temperatures:
Pressures are gauge pressures in bar. The final pressure pe is taken 0.5 bar below the relief valve setting, or 10% below for valves above 5 bar, so the valve does not weep. The acceptance factor (pe − p0) ÷ (pe + 1) is the share of the tank that fills with water, which follows from Boyle's law for the gas cushion. The water seal reserve of EN 12828 adds 0.5% of the system volume, at least 3 liters, so the tank still holds water when the system cools and loses a little.
Expansion tank calculator example
A heating system with 200 L of water, filled at 10 °C and running up to 70 °C, a pre-charge of 1.0 bar, and a 3 bar relief valve:
A 300 L hot water cylinder heated from 10 °C to 60 °C on a 3.5 bar supply with a 6 bar relief valve needs 17 L, so an 18 L potable water tank.
How to use the expansion vessel calculator
Enter the total water volume of the system, including the boiler, pipes, radiators or floor heating, and any buffer tank. Enter the fill temperature and the highest water temperature the system reaches. Set the tank pre-charge pressure, usually the static height of the system in bar plus 0.2 bar, and the relief valve setting. Leave the water seal reserve on for heating systems built to EN 12828. Press Calculate.
Reading the expansion tank size
Choose the next standard tank at or above the minimum. A tank that is too small lets the pressure rise to the relief valve setting on every heat-up, so the valve drips and the system loses water. A larger tank does no harm. The pre-charge matters as much as the volume: set it with the system cold and empty of pressure on the water side, and match it to the static head plus a margin, otherwise the tank starts the cycle already part full.
When to use an expansion tank sizing tool
- Sizing a boiler expansion tank for a new heating system
- Checking the vessel after adding radiators or a buffer tank
- Sizing a thermal expansion tank for a hot water cylinder
- Setting the pre-charge for a tall building
- Fixing a relief valve that drips when the system heats up
Common expansion tank calculator mistakes
- Leaving the factory pre-charge. Set it to the static head plus margin.
- Forgetting the buffer tank volume. Count all the water.
- Using the relief setting as the final pressure. Stay 0.5 bar below.
- Using a heating tank on drinking water. Use a potable tank.
Tips for expansion tank sizing
- Check the pre-charge once a year with the system depressurized.
- Round up to the next standard size.
- Fit the tank on the return side, near the boiler or pump suction.
Boiler expansion tank and system volume
The system volume is the hardest input to know. As a rough guide, radiator systems hold about 10 to 15 liters per kW of boiler output, floor heating 15 to 20 liters per kW, and old cast-iron systems more. For an exact figure, add the boiler water content from its data sheet, the radiator contents, and the pipework, which the pipe volume calculator gives per meter of each pipe size. Heat pumps with a buffer tank add the buffer volume, and the tank volume calculator measures one from its dimensions.
Thermal expansion tank size for hot water
When a check valve or pressure reducer stops heated water from flowing back into the mains, a hot water cylinder needs its own potable water expansion tank with a food-grade membrane. The same formula applies with the cylinder volume, the cold inlet and storage temperatures, the supply pressure as pre-charge, and the relief valve setting, typically 6 bar in Europe and 150 psi in the US. Size the cylinder first with the water heater size calculator.
Water expansion factor by temperature
| Maximum temperature | Expansion from 10 °C | Per 100 L of water |
|---|---|---|
| 40 °C | 0.75 % | 0.75 L |
| 50 °C | 1.18 % | 1.18 L |
| 60 °C | 1.68 % | 1.68 L |
| 70 °C | 2.24 % | 2.24 L |
| 80 °C | 2.87 % | 2.87 L |
| 90 °C | 3.56 % | 3.56 L |
Expansion tank size chart: heating systems with a 3 bar relief valve
Standard tank size including the EN 12828 water seal reserve, fill at 10 °C, by maximum temperature and pre-charge:
| System volume | 70 °C, 1.0 bar | 70 °C, 1.5 bar | 90 °C, 1.5 bar |
|---|---|---|---|
| 100 L | 18 L | 25 L | 25 L |
| 150 L | 18 L | 25 L | 35 L |
| 200 L | 18 L | 35 L | 50 L |
| 300 L | 25 L | 35 L | 50 L |
| 500 L | 35 L | 50 L | 80 L |
| 1000 L | 80 L | 100 L | 200 L |
Expansion Tank Calculator FAQ
How do you size an expansion tank?
Multiply the system volume by the water expansion factor, add a small reserve, and divide by the acceptance factor (pe − p0) ÷ (pe + 1). A 200 L system heated to 70 °C with 1 to 3 bar needs about 18 L.
What should the expansion tank pre-charge be?
For heating, the static height of the system in meters divided by 10, plus about 0.2 bar. For a hot water cylinder, the incoming water supply pressure.
How much does water expand when heated?
About 1.7% from 10 °C to 60 °C, 2.2% to 70 °C, and 3.6% to 90 °C.
What happens if the expansion tank is too small?
The pressure reaches the relief valve setting as the system heats, the valve opens, and the system loses water and needs topping up.