Ideal Expansion Vessel 7.5 Litre HIU 177640

The Ideal 177640 is a 7.5 litre expansion vessel supplied for Ideal Commercial heat interface units (HIU). It is a sealed diaphragm vessel that absorbs the expansion of water as the heating circuit heats up, keeping system pressure stable across the operating range. The vessel is charged on the air side of the diaphragm and connects into the primary or secondary circuit pipework of the HIU.

• System pressure drops repeatedly and needs frequent topping up, with no visible leak on the pipework or radiators.
• Water discharges from the Schrader valve on the air side of the vessel when the valve core is depressed.
• The vessel feels heavy or full of water when shaken, indicating a ruptured or perished diaphragm.
• Pressure rises sharply and quickly as the circuit warms, then falls again once it cools.
• The pressure relief valve weeps or discharges during normal operation because the vessel can no longer absorb expansion.

• Confirm the part number 177640 against the appliance data plate or the original component before ordering.
• Note the alternative number 1782912, which may appear on the existing vessel or in older documentation.
• Check the connection size and mounting arrangement on the existing vessel match the replacement before fitting.
• Isolate and depressurise the HIU before removing the vessel, and re-charge the system to the correct cold fill pressure afterwards.