Combi boiler not switching from heating to hot water? Hot water cooler than it used to be? Radiators warming up when only a tap is open? All three are classic diverter valve symptoms. We replace the assembly across every major combi brand in London.
Inside every combi boiler sits a diverter valve, a small motorised assembly that routes the hot primary water either through the radiator circuit or through the DHW plate heat exchanger when a tap is opened. The valve relies on a tiny actuator motor that strokes a spindle a few millimetres, and on a rubber or PTFE diverter cartridge that seals the unused port. Both wear out, particularly on systems running without proper inhibitor or with a high level of sludge in the primary water. When the diverter sticks in the heating position you get heating but no hot water at all. When it sticks part-open you get lukewarm DHW with radiators warming up on tap demand, which costs you money and confuses every diagnostic that does not check the valve directly.
Our engineers carry OEM diverter valve assemblies for Worcester Greenstar 25Si, 30CDi, and 8000 series, Vaillant ecoTEC Plus 415 and 428, Ideal Logic 24/30/35, Baxi DuoTec, Viessmann Vitodens 050, 100, and 200 series, and Glow-worm Energy combis. We swap the valve, refill with fresh Sentinel X100 inhibitor in line with BS 7593, vent, balance, and run a flue-gas analysis before sign-off. Typical on-site time is 90 to 150 minutes including refill and commissioning.
Where this service applies
- Combi boiler heating works perfectly but tap-side hot water has stopped
- Hot water runs lukewarm even with a fresh DHW plate (diverter passing primary back through CH)
- Radiators noticeably warmer when a tap is open (stuck part-open diverter)
- Audible diverter motor cycling but no temperature swap (mechanical cartridge failure)
- Worcester Greenstar Sensor F6, Vaillant ecoTEC F.62, Ideal Logic L9, Baxi E168 in some firmware revisions
- Boiler making a clicking or knocking noise on every DHW demand (diverter motor stalling)
- Diverter previously replaced with non-OEM aftermarket part that has failed early
- System sludged and contributing to repeated diverter failure (often paired with a power flush)
How we deliver
No surprises, no upselling. The exact path every job follows.
- 1
On-site diagnostic confirmation
We verify the diverter is the genuine fault, not a flow switch, NTC sensor, or scaled plate exchanger mimicking the same symptom. - 2
Written quote
Fixed-price quote for the diverter assembly, labour, system refill, inhibitor dose, and FGA. Customer signs off before we start. - 3
Safe isolation and drain
Gas isolated. Boiler 230V isolated and proved dead. CH and DHW circuits drained at the lowest point on the boiler chassis. - 4
Old assembly removed
Motor unclipped, cartridge body unscrewed or unbolted depending on the model. Inspected for sludge and the cause of failure noted. - 5
OEM diverter fitted
New cartridge fitted with fresh O-rings and gaskets. Motor reattached and clipped. Manufacturer torque settings respected on bolted assemblies. - 6
Refill with fresh inhibitor
Sentinel X100 dosed in line with BS 7593 to restore corrosion protection diluted by the drain. - 7
Live operation test
Heating and DHW demands run in turn. Diverter stroke confirmed audibly and through tap-side temperature within 30 seconds of opening a hot tap. - 8
FGA and sign-off
Flue-gas analysis recorded. 12-month workmanship guarantee on the repair. Written job sheet emailed.