Worcester Bosch Mixing Unit 21 30 8716117257

The Worcester Bosch mixing unit is a hydraulic component used in Greenstar i ErP system and combi boilers. It blends flow and return water so the boiler can hold a stable temperature across the heating circuit instead of cycling between hot and cold. The unit is supplied as a complete assembly under part number 8716117257, with the alternative code 1031223 also used to identify it.

• Heating flow temperature swings between hot and cold rather than holding a steady set point
• Radiators heat unevenly, with some circuits slow to reach temperature
• The boiler short-cycles or locks out during a heating demand
• Fault codes relating to temperature control or flow and return sensing appear on the display
• Visible scale, debris or seizure inside the mixing body when the assembly is inspected

• Check the GC number on the appliance data badge against the compatibility list before ordering, as the same boiler range uses several mixing unit variants
• Confirm the alternative number 1031223 matches the part currently fitted
• Inspect the sealing washers and O-rings on the mixing body and replace them if they are compressed or perished
• This part is fitted to a gas appliance; installation must be carried out by a Gas Safe registered engineer.