Onyx 18L Red Heating Vessel 3/4 1.5 Bar ONYX18LHEAT

The ONYX 18L red heating vessel is a diaphragm expansion vessel for sealed heating systems, manufactured by Onyx. It provides the expansion volume that absorbs the increase in water volume as the system heats up, keeping system pressure within the range the boiler and its safety devices are designed for. This is the heating-side (red) vessel, with a 3/4 connection and a 1.5 bar rating.

• System pressure climbs sharply once the heating is running, then falls again when it cools
• The pressure relief valve discharges during normal heating cycles
• Water, rather than air, escapes when the air valve on the vessel is depressed
• The vessel feels heavy or sounds full of water when tapped
• The boiler locks out or cuts out on low pressure and needs frequent topping up

• Confirm the connection size and thread on the existing vessel match the 3/4 connection on this one
• Check the pressure rating suits the system's design pressure and the boiler manufacturer's requirement
• Measure the available space and confirm the vessel's physical size and mounting arrangement will fit
• Confirm the vessel is for the heating circuit, not the domestic hot water supply