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Expansion Vessel

Expansion Vessel Pressure Setup

Expansion vessels are essential for keeping heating and chilled water systems stable and safe. When the pressure isn’t set correctly, you’ll often see issues like relief valves discharging, system pressure dropping, or air entering the circuit.

Here’s a quick refresher on how to set, check and maintain the expansion vessel pressure correctly and what to look out for during installation and servicing.

What an expansion vessel does

In any sealed heating or chilled water system, the water expands as it heats up. The expansion vessel gives that water somewhere to go, preventing overpressure and protecting the system from damage.

It’s divided into two chambers by a flexible diaphragm:

  • one side contains air or nitrogen, pre-charged to a set pressure;
  • the other connects to the system and fills with water as it expands.

If the vessel pressure is wrong, the diaphragm won’t work as intended and the system pressure will rise too high and open the safety valve, or drop too low when cooling, leading to air ingress.

 Setting the pre-charge pressure

The pre-charge pressure is the air or nitrogen pressure on the dry side of the vessel. It must be set correctly before filling the system.

For most sealed heating systems:

  • the pre-charge should match the cold fill pressure of the system,
  • typically 0.2 bar below the system’s fill pressure, to allow for small fluctuations.

Example:

If the system’s cold fill pressure is 1.5 bar, set the vessel pre-charge to 1.3 bar.

For chilled water systems, pre-charge pressures are generally lower, depending on system design. Always follow the manufacturer’s data and BS EN 12828.

 How to check and adjust the pressure

  1. Isolate the vessel from the system using the service valve.
  2. Release any water pressure from the vessel’s water side (it must be at zero).
  3. Check the pre-charge using a reliable pressure gauge on the Schrader valve.
  4. Adjust as needed using a nitrogen bottle or air pump, but avoid over pressurising.
  5. Reopen isolation valves and bring the system back up to pressure.

Tip: Always use nitrogen where possible as it reduces permeability and prolongs the diaphragm’s life compared to air.

Common signs the pressure isn’t set correctly

  • The pressure relief valve discharges water after heat-up.
  • The system pressure drops when it cools.
  • The vessel sounds “solid” when tapped (indicating waterlogged).
  • The diaphragm has failed due to incorrect pre-charge or age.

If the vessel is full of water or can’t hold pressure, replacement is the only option.

 Sizing and positioning reminders

  • Always size the expansion vessel according to system volume, operating temperature, and pressure.
  • Fit it on the return side of the system, close to the pump suction point for stable operation.
  • Check that isolation valves, drain points and a gauge are fitted to make future servicing easier.

LoCO₂ Heat stocks a range of BOSS expansion vessels for both heating (HTG) and chilled water (CHW) systems for all your system requirements

Note: This article is intended as general guidance for qualified heating engineers. Always follow the manufacturer’s instructions, site-specific requirements, and relevant standards such as BS EN 12828 when commissioning or servicing expansion vessels.

Visit our expansion vessel page for more information