Expansion Vessel Calculator

Heating system
System water volume
Pressures
Final pressure below the valve setting
Available vessel sizes
Vessel sizes on offer (l)
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EN 12828 Annex D is informative guidance. Set the vessel's pre-charge to p₀ before filling, and follow the boiler and vessel manufacturers' instructions.

Vessel to buy18.00L
Minimum nominal volume16.80L
Pre-charge pressure p₀0.79bar
Volumes
System water content V_s150.00 L
Temperature row used (Table D.2)90 °C
Expansion percentage e3.47 %
Expansion volume V_e = e × V_s5.21 L
Water reserve V_WR3.00 L
Pressures
Static pressure p_st0.49 bar
Pre-charge p₀ = p_st + 0.3 bar (≥ 0.7 bar)0.79 bar
Final pressure p_e2.50 bar
Pressure factor (p_e + 1) / (p_e − p₀)2.047
Selected vessel — fill pressure
Selected vessel18.00 L
Cold fill pressure, at least1.15 bar
Cold fill pressure, at most1.24 bar

About this calculator

Water expands as it heats up. In a sealed (closed) heating system that extra volume goes into a diaphragm expansion vessel: a steel tank split by a rubber membrane, with nitrogen on one side and system water on the other. Too small a vessel and the pressure climbs to the safety valve every time the boiler heats up; too low a pre-charge and the top radiators run dry of pressure.

This calculator sizes the vessel by EN 12828 Annex D (in Lithuania LST EN 12828): it works out the expansion volume, adds the water reserve the standard asks for, sets the pre-charge from the height of the building and the final pressure from the safety valve, and picks the next vessel from a list of common sizes you can edit.

Formula

V_e  = e × V_s / 100                         expansion volume
V_WR = max(0.5 % × V_s, 3 l)                 water reserve (vessel over 15 l)
     = 20 % × V_n                            water reserve (vessel up to 15 l)
p_st = 0.0981 × h                            static pressure, bar (h in m)
p₀   = max(p_st + 0.3, 0.7)                  pre-charge, bar
p_e  = p_SV − 0.5   (or 0.9 × p_SV)          final pressure, bar
V_n  = (V_e + V_WR) × (p_e + 1) / (p_e − p₀)
  • e — expansion percentage from EN 12828 Table D.2, for the maximum temperature the water can reach (the boiler's limit thermostat), filling at 10 °C: 2.22 % at 70 °C, 2.81 % at 80 °C, 3.47 % at 90 °C, 4.21 % at 100 °C, 5.03 % at 110 °C. A temperature between rows is taken at the next hotter row.
  • p₀ — the static height pressure plus a practical margin of 0.3 bar, which Annex D allows instead of the water's vapour pressure; the standard says p₀ should normally be at least 0.7 bar.
  • p_e — must stay below the safety-valve setting by the valve's closing difference. EN 12828 gives "typically 10 %" of the setting; vessel manufacturers use p_SV − 0.5 bar, which is stricter for valves under 5 bar. The calculator defaults to the stricter p_SV − 0.5 bar; the 10 % rule can be selected.
  • (p_e + 1) / (p_e − p₀) — Boyle's law for the gas cushion, with absolute pressures (gauge + 1 bar).

The calculator also gives the cold fill pressure window for the chosen vessel: high enough that the water reserve is already in the vessel when cold, low enough that the hot system does not exceed p_e.

If you don't know the system's water content, estimate it from the installed heat output. The default 20 l per kW is a rough rule of thumb from vessel manufacturers' planning guides, not a standard. Panel-radiator systems usually hold less, cast-iron radiators and underfloor heating more, and a buffer tank adds its full volume.

Worked example

150 l radiator system, boiler limit 90 °C, highest radiator 5 m above the vessel, 3 bar safety valve:

e     = 3.47 %  → V_e = 3.47 × 150 / 100 = 5.21 l
V_WR  = max(0.75, 3) = 3 l
p_st  = 0.0981 × 5 = 0.49 bar  → p₀ = 0.49 + 0.3 = 0.79 bar
p_e   = 3 − 0.5 = 2.50 bar
V_n   = (5.21 + 3) × (2.50 + 1) / (2.50 − 0.79) = 8.21 × 2.05 = 16.8 l

Next size up from the list: 18 l, pre-charged to 0.8 bar. Fill the cold system to between 1.15 and 1.24 bar — a narrow window, because 18 l is only just above the 16.8 l minimum; a 25 l vessel would leave more room.

FAQ

Which temperature should I enter — the flow temperature? No — the highest temperature the water can reach, usually the boiler's safety/limit thermostat (often 90–95 °C for a gas or solid-fuel boiler). For a heat pump that never exceeds 60–65 °C you may enter that. Higher temperature, more expansion, bigger vessel.

What is the static height? The height of the system's highest point (top radiator, air vent) above the expansion vessel. Each metre of water is about 0.1 bar. The vessel's gas side must be pre-charged above that pressure, or the top of the system would go under-pressure and draw in air.

Why is a 3 l water reserve added? Every system slowly loses a little water (venting, small leaks). Annex D asks for a reserve already pushed into the vessel when the system is cold, so a small loss doesn't immediately drop the pressure below p₀. That is also why the fill pressure must be a little higher than the pre-charge.

My boiler has a built-in vessel. Is it enough? Compare its volume and pre-charge with the result. Built-in vessels are usually 6–12 l; a large radiator, underfloor or buffer system often needs an additional vessel.

The standard says 0.3 bar; the manufacturer says pre-charge + 0.2 bar. Which is right? EN 12828:2003 adds 0.3 bar to the static pressure instead of the vapour pressure. Some manufacturers add the vapour pressure and 0.2 bar, which for water below 100 °C comes to less (the gauge vapour pressure is negative there). This calculator follows the standard.

Assumptions and limits

  • Follows the 2003 edition of EN 12828 Annex D. The 2012 edition revised Annex D; the revised text could not be checked, so small differences to the current edition are possible.
  • Water only. Glycol (antifreeze) expands more and changes Table D.2 — ask the glycol supplier.
  • The vessel is assumed to sit on the return near the boiler, at the pump's suction side, as Annex D recommends; the pump head is not added to p₀.
  • The standard vessel sizes are a typical catalogue range, not a standard — edit the list to match your supplier.
  • Pressure maintenance with pumps or compressors, degassing units and systems above 105 °C are out of scope.
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