Diverter Valves in Combi Boilers Explained
The diverter valve inside a combi decides whether the boiler's output goes to the radiators or to the taps.
The diverter valve inside a combi decides whether the boiler's output goes to the radiators or to the taps. When it sticks or the seals wear, you can get lukewarm taps, no heating, or both trying to run at once.
What it does
A motorised or pressure-operated head shifts a rubber-sealed disc between two ports, sending water to the hot water plate or the heating circuit but never both fully open together.
How it fails
Limescale and debris score the seat, the head motor's gears strip, or the seal perishes. The usual symptom is heating that stays on when a tap runs, or hot water that never gets properly hot.
Repair or replace
The head and cartridge can sometimes be replaced separately from the valve body. On older or badly scaled units we usually replace the whole assembly rather than chase intermittent faults.
Get a diverter valve checked
Describe what the taps and radiators are doing when the fault happens and we can usually tell you before we arrive whether this is the likely part.
Questions we get asked
Is diverter valve work Gas Safe-only?
Yes. It sits inside the boiler casing on the sealed system side, so any work on it is carried out by a Gas Safe registered engineer.
Can hard water cause this?
Yes, scale is one of the most common causes of a sticking diverter, alongside general age and worn seals.
Will a filter help?
A magnetic filter helps with circuit debris but does nothing for limescale, which is the more common cause with a diverter valve.