Onyx 5L Red Heating Vessel 3/4 1.5 Bar ONYX5LHEAT

The Onyx 5L Red Heating Vessel is a diaphragm expansion vessel for sealed heating systems, supplied with a 3/4 inch connection and a 1.5 bar pre-charge. It absorbs the expansion of heated water in a closed circuit, limiting pressure rise and protecting the boiler and system components. The red casing identifies it as a heating-side vessel rather than a potable water unit.

• The boiler pressure rises sharply as the system heats up and falls again once it cools
• Water discharges from the pressure relief valve during normal operation
• The vessel sounds full of water when tapped, indicating a failed or perforated diaphragm
• System pressure needs topping up frequently with no visible leak elsewhere
• The pressure gauge reads erratically with no change in system temperature

• Confirm the connection size on the existing vessel is 3/4 inch
• Check the required pre-charge pressure against the system design pressure
• Verify that 5 litres is adequate for the system volume
• Measure the available space, as the replacement must fit the existing mounting position