Heating expansion vessel calculator

System volume

Estimate is a trade allowance, not measured content

System water content (litres)

Domestic or commercial. Up to 20,000 L.

PRV setting (bar)

Typical boiler 3.0. Must stay above cold fill.

Maximum flow temperature 80 °C

Static height 5 m · 0.50 bar

Vessel to highest pipe, 0.1 bar per metre. Pre-charge is set from this, not left at the usual 0.75 bar factory charge.

Fit this vessel

12 L

Next stock size from CIBSE Table 5.15

0.50 bar

Factory is usually 0.75 bar — drop to this, system empty

CIBSE minimum

8.3 L

Before rounding up

Cold fill

1.00 bar

Static + 0.5 bar once the system is full

Zilmet / EN 12828

4.6 L

Water only, no glycol

System 100 L e 0.0287 CIBSE k 0.0833 Acceptance 63%

Method

Static pressure = height (m) ÷ 10 (0.1 bar per metre). Set the vessel pre-charge to that pressure, not below 0.5 bar, on the Schrader with the system empty. New vessels are usually factory-charged to 0.75 bar (right only for about 7.5 m of static head).
Cold fill = static + 0.5 bar (BS EN 12828 water reserve when cold).
Vessel volume for fitting is from CIBSE Domestic Heating Design Guide Table 5.15: system litres × multiplying factor for the safety-valve setting and vessel charge / initial pressure. Example: 100 L, 3.0 bar valve, 0.5 bar charge → 8.3 L, so fit 12 L.
Below 80 °C the Table 5.15 result is scaled by e(T) / e(80 °C), because the CIBSE table is for conventional boiler temperatures. Heat-pump flow of 45 °C expands much less water.
e is the volume change of water from 10 °C to the flow temperature you set.
Acceptance = 1 − (p₀ + 1) / (PRV + 1). The Zilmet check is V = e × Vs / acceptance (EN 12828 / VDI 4708, water only). Same example: about 4.6 L, stock 8 L. CIBSE is what we size to. Stock vessels 8–900 L. Glycol needs a larger vessel.