[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

We are sizing the membrane expansion vessel for a closed radiator heating system in a small office building. The system water volume is estimated at about 800 litres, the maximum flow temperature is 80 °C, the highest point of the system is 15 m above the vessel and the boiler safety valve is 3 bar.

What is the step-by-step method (EN 12828 or ASHRAE), which pressures should be used for the pre-charge and the final pressure, and why do vessel sizes come out so different between designers?

[MEP Center Engineering]

The vessel must accept the expansion volume of the water plus a small water reserve between the pre-charge pressure (set by the static height) and the final pressure (set by the safety valve). Nominal volume = (expansion + reserve) × (p_e + 1) / (p_e − p₀), with pressures in bar gauge. The pressure window, not the water volume, is what usually drives the size.

Step by step (EN 12828 method)

1. System volume V_s: boiler, pipes, emitters, buffer tanks. 800 l in this example.
2. Expansion coefficient e from the cold fill temperature (about 10 °C) to the maximum temperature: water density 999.7 kg/m³ at 10 °C and 971.8 kg/m³ at 80 °C gives e ≈ 0.029.
3. Expansion volume: V_e = 800 × 0.029 = 23.2 l.
4. Water reserve: V_wr ≥ 0.5 % of V_s, minimum 3 l → 4 l.
5. Pre-charge p₀: static height 15 m = 1.5 bar, plus about 0.3 bar margin so the highest point stays under positive pressure → p₀ = 1.8 bar.
6. Final pressure p_e: for safety valves up to 5 bar, p_e = p_sv − 0.5 bar = 3.0 − 0.5 = 2.5 bar.
7. Nominal volume: V_n = (23.2 + 4) × (2.5 + 1) / (2.5 − 1.8) = 27.2 × 3.5 / 0.7 = 136 l → select the next standard size, for example 140 or 150 l.

Why sizes differ so much

The factor (p_e + 1) / (p_e − p₀) is very sensitive. If the safety valve can be 4 bar (check the rating of the boiler and every component), p_e = 3.5 bar and V_n = 27.2 × 4.5 / 1.7 ≈ 72 l, about half. In a tall building the static height eats the pressure window; a pressure-maintaining unit (pump or compressor type) is then often smaller and more stable than a very large membrane vessel.

ASHRAE form

For a diaphragm tank: V_t = V_s × [(v₂/v₁ − 1) − 3αΔt] / (1 − P₁/P₂), with absolute pressures. The pipe expansion term 3αΔt is small for steel, and 1 − P₁/P₂ equals (p_e − p₀)/(p_e + 1) in gauge bar, so both methods give the same answer for the same inputs.

Installation points

• Connect the vessel on the pump suction side, near the boiler or heat source, so the pump does not lower the pressure at the high points.
• Use a lockable isolating valve with a drain between vessel and system for servicing; the vessel must never be isolated in operation.
• Fill pressure when cold: slightly above p₀ (typically +0.2 to 0.3 bar) so the water reserve is in the vessel.
• Glycol systems, solar circuits and heat pumps with large buffers need their own expansion coefficients and temperature ranges.

Common mistakes

• Pre-charge left at the factory value (often 1.5 bar) regardless of static height.
• Forgetting buffer tanks in the system volume.
• Using the operating temperature instead of the maximum the system can reach.
• No water reserve, so the system loses pressure after the first venting.

Related: Expansion Vessel Sizing for Heating Systems. For deeper study: HVAC Systems Design Course.

Check the edition of the code that applies to your project.

[Author]

Posts