Baxi 720753901 Expansion Vessel

The Baxi 720753901 is an 8 litre expansion vessel manufactured by Baxi. It is a sealed-system component that provides a pressurised air cushion against the water in the heating circuit, absorbing the volume increase as the system heats up and giving back pressure as it cools. Keeping that cushion intact is what allows the appliance to hold a steady operating pressure instead of swinging between high and low readings.

• System pressure climbs quickly when the appliance heats up and drops again once it cools, with the gauge moving further than it should.
• The boiler loses pressure over days or weeks and needs regular topping up through the filling loop.
• Water discharges from the pressure relief valve during normal heating cycles.
• The vessel feels heavy or full of water when the connection is opened, rather than sounding hollow when tapped.
• Pressure rises sharply as soon as the pump runs, suggesting the air charge has been lost or the internal diaphragm has failed.

• Confirm the appliance model and GC number from the data badge, then cross-check them against the compatibility information for this part.
• Check the part number 720753901 against the number printed on the existing vessel or listed in the appliance's spare parts documentation.
• Inspect the connection and any sealing washer or O-ring at the vessel inlet, and replace it if it is compressed or perished.
• Work on a sealed heating system should follow the appliance manufacturer's instructions, including isolation, draining and correct recharging of the system.