Vaillant 182 EH - Intermittent hot water and heating

[Grammy]

Got a Vaillant Thermocompact VC GB 182 EH.
Has 2 x 2 port valves, each on heating and water circuits, which seem to be working fine (can see valve moving when heating or water is called for) but boiler doesn't always fire up when programmer calls for heat or water. If it does fire up, it seems fine, but if you remove the call for heat or water and then try to select heat or water again, boiler doesn't fire up!!!

This basically gives very intermiiten heat and hot water (and is very annoying!!). Has worked ok in the past though.

Any help appreciated.

 

[Andygasman2010]

It's probably one of the end-switches in either zone valve not quite making. You need to catch it in it's fault state and check if you have a demand at the boiler. You can do this yourself if you have a multimeter....?