Vailant Boiler - Heating Problem

[mickw]

I'm not new to DIY, but this one has me beat!

I have a Vailant VCW282E Combi boiler (sealed CH system), when the system switches on the central heating works fine, heats up, thermostat kicks in and out when required and the boiler responds accordingly. However, when I turn on the hot water and the diverter valve slides over to the DHW heat exchanger, it does not slide back to central heating when I turn the hot water off, the only way to get the diverter to slide back is to switch the boiler off and then back on.

The system has been working fine for a number of years, the problem has become more persistent over the last couple of months.

The diverter valve is hydraulic and I'm not sure how it is operated so I don't want to take the usual pull it apart and have a look approach.

 

[CH4]

When you turn on a hot tap, the water flows through a venturi at the bottom of the diverter valve. This creates a differential pressure, which is applied to either side of a diaphragm (located just above the venturi) to provide the force to move the diverter valve, using a push rod.

The push rod may be sticking - aided by the flow from the pump through the diverter valve. Switching the boiler off stops the pump, thereby allowing the diverter valve to limp back to the CH position.

The diaphragm, venturi and push rod may just need cleaning, but if you have to replace them, it is probably easier to obtain the complete diverter assembly rather than individual parts.