Vaillant boiler (VCW GB 221) not heating up water or heating although the pilot is lit. Pump is heard working okay but there is small drips of water underneath boiler. Any idea's?

The movement of the diverter valve is critical on this appliance, and is located on rear right of the boiler, when hot water is turned on, theres are two microswitches, which must break (they both close in Central heating mode), if pump is running, it is recognising the hot water demand, check microswitches are breaking, if not, it tends to be a defective servo valve (located above the water valve), or a blocked/ restricted control line (small copper tubes which connect into the servo valve)

Drips of water obviously indicate a leak - locate the leak - althought it could be condensation unlikely & that would be in more than one place around the boiler.


obviously as it may sound - is the gas meter is a pre-paid one - check there is plenty of credit & the meter is ON and not on emergency credit

boilers are like women - not always easy to figure out and why they suddenly go wrong when all has been well !

leaks mean bar pressure dropping - so check for leaks and check if water is leaking from the pipe on the outside wall - problem could be with the Pressure Relief Value (PRV) not working properly - hence not allowing the system to maintain pressure and releasing it - so check bar pressure

possible problem with expansion tank?

or Printed Circuit Board - so check by topping up the system - by turning on top up tap - see if the bar pressure will go up to maximum - if it does close top up tap before maximum bar 3.5 - see if any water leaks outside - if so - monitor the pressure gauage and turn on the hot taps full
the the gas burner lights up - some progress - atleast the gas value isnt at fault necessarily - if the burner stays on for a while at least 10-15mins - then atleast it's ok to assume the heat sensors, the heat exchanger and gas value are ok. -

now see if the bar pressures is falling - if it is - then probs with PRV - probably needs replacing.

these boilers are OLD however i have found them to be very reliable - there are newer more efficient ones on the markert but why replace an entire boiler if it;'s been very reliable ?

there is a temporarily solution around the PRV and that's to leave the top up tap partially on - the water may not get as hot but hot enough and your outside wall will get very wet until an engineer visits - good luck but get a professional in.


- if it's falling there's an indicator it's the PRV -