Worcester Bosch Mixing Unit Assembly 871860900A0

The Worcester Bosch mixing unit assembly, part number 871860900A0, is a factory-built component used inside Worcester Bosch Greenstar gas boilers. It sits in the appliance's water circuit and blends flows to hold a stable delivery temperature, which supports consistent domestic hot water and heating performance.

It is supplied as a complete assembly rather than as individual internal components, so it is the part an engineer replaces when the mixing function inside the unit stops behaving correctly. The assembly is listed under spare part type "mixing unit" and carries the Worcester Bosch part number 871860900A0.

• Domestic hot water temperature drifts away from the setting, or fluctuates between hot and lukewarm during a draw-off.
• The boiler struggles to hold a steady flow temperature, with noticeable swings at the tap or radiator.
• Heating and hot water demand are not handled cleanly, with the appliance failing to respond as expected when the mode changes.
• Fault codes or lockouts appear that point to the water circuit rather than to ignition or gas supply.
• Weeping, staining or scale build-up is visible around the mixing unit body or its connections.

• Check the appliance GC number and model against the compatibility list before ordering, as this assembly is used across a large number of Greenstar variants.
• Quote part number 871860900A0, or the alternative number 1030951, when confirming the correct item.
• This part is fitted to a gas appliance; installation must be carried out by a Gas Safe registered engineer.