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Boiler Diverter valve: What It Does and Signs It's Failing

A combi boiler has a diverter valve built in because it heats tap water on demand instead of storing it. It sits low in the boiler on the water side and decides whether heated water goes to the radiators or to the plate heat exchanger that warms your taps. When a hot tap opens, the taps get priority until it closes.

How it works

A motor on top of the valve moves a paddle or piston between two ports. A flow sensor on the cold inlet tells the board a tap is open, and the board drives the motor across. Scale, wear or a failed motor can leave it stuck in one spot or halfway.

Signs it is failing

• Radiators get hot when you run a hot tap
• Hot water but no heating
• Heating but no hot water
• Lukewarm hot water that never fully heats
• Clicking or buzzing from the motor with no change

Overheat and flow-return gap codes can come from a diverter that's stuck. Ideal's fault-finding ends no-heating cases with a new diverter valve and no-hot-water cases with the diverter motor.

Can you replace it yourself?

The valve is inside the casing and replacing it means draining the boiler, so it's engineer work. Before you book, check the timer or thermostat is actually calling for heating, and that the pressure is right. Describe exactly which mode fails, since that helps the engineer decide between the motor and the valve body.

Buying the right part

Motors and valve bodies are often sold separately, and the engineer may only need one. Both are specific to the boiler model.

Error codes linked to the diverter valve

• F.20Overheat safety cut-out has trippedVaillant
• F.23Too big a gap between flow and return temperatureVaillant

Last reviewed 2026-10-21.