Heating Expansion Vessel Sizing Calculator
Volume of the expansion vessel for a closed heating or chilled-water circuit, from the system water volume, the fill and maximum temperatures and the pressures — with the water's expansion worked out from its density.
Results
The dashed line marks the value you entered.
What this tool does
Water expands when it warms: about 2.9 % between 10 °C and 80 °C and 4.3 % by 100 °C, so a 200 L system must find room for nearly 6 litres. The vessel is a gas-filled cushion, and only part of its volume is usable because the gas is compressed from the pre-charge pressure to the maximum pressure by Boyle's law: usable share = 1 − (p_pre + 1) ÷ (p_max + 1) with pressures in bar gauge. A small water reserve keeps the vessel from being emptied as the system cools. Glycol mixtures expand more than water, and the standards add margins — this is an estimate, so check the maker's sizing.
Formula
expansion = ρ(t_fill) ÷ ρ(t_max) − 1 · V_vessel = (V_system × expansion + V_reserve) ÷ (1 − (p_pre + 1) ÷ (p_max + 1)) (pressures gauge, +1 bar for atmosphere)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
vs | Water in the whole system | L |
t1 | Water temperature when filled | °C |
t2 | Highest water temperature | °C |
pa | Vessel pre-charge (gauge) — about the static pressure plus a margin | bar |
pe | Highest pressure allowed (gauge), a bit under the safety valve | bar |
rv | Water reserve to keep in the vessel | L |
VN | Expansion vessel volume needed | L |
EX | Expansion of the water | % |
VE | Extra volume of water | L |
UF | Share of the vessel that is usable | % |
Worked example
- Water in the whole system200 L
- Water temperature when filled10 °C
- Highest water temperature80 °C
- Vessel pre-charge (gauge) — about the static pressure plus a margin1 bar
- Highest pressure allowed (gauge), a bit under the safety valve2.5 bar
- Water reserve to keep in the vessel3 L
- Expansion vessel volume needed20.4 L
- Expansion of the water2.87 %
- Extra volume of water5.74 L
- Share of the vessel that is usable43 %
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.