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Expansion Vessel

Quick Guide

Water’s volume, when heated, increases. Water expands by approximately 9% when it is heated from room temperature to its boiling point.

Installations need to be designed so that we control the maximum temperature that the system reaches, but we also need to have adequate management in place should this maximum be exceeded.

In a sealed system, the thermal expansion of water must be compensated for, this is normally achieved with an expansion vessel. Expansion vessels are pre-charged with air that pushes down on a diaphragm, in turn, water enters the vessel on the other side of the diaphragm and begins to compress the air. Vessels are sized according to the volume of the system and the maximum temperature that the system generates, we also must consider the static head above the vessel.

Sizing Guide:

For systems with 82℃ / 71℃ flow and return and cold fill pressure up to 1.5 bar, a good rule of thumb is, 1 litre vessel capacity minimum per KW boiler output – round up to larger vessel capacity (e.g.160 KW boiler may require 200 litre vessel).

Please contact us with your values.

A chilled system would require minimal expansion due to the lower temperatures. Underfloor heating similarly would require less expansion than a radiant system – please contact us.

Below is a guide to vessel sizing based on a radiant system with a flow and return of 82℃ / 71℃ and a static head of 10 meters. We can provide you with a more accurate calculation based on more system parameters: static head (m), flow and return temps (c), boiler output (kw), working pressure (bar) – Please call us.


    System Volume (L)   
    Expansion Vessel Size (L)   
    Boiler Output (kW)   
50 4 4
100 8 8
200 18 17
300 24 25
400 35 33
500 50 42
750 80 63
1000 100 83
1500 140 125
2000 200 167
3000 250 250
5000 400 417
7500 600 625
10000 800 833

Expansion vessels & pressurisation units.