Combi boiler, 100% hot water or nothing

[Nitoman]

Hi all, just moved into a house with a Vaillant Turbomax VUW GB 242/1 E Combi.
Central heating is fine but the hot water will only work at max flow, any taps have to be fully open and hot water only. So if using a mixer tap or shower there is hot water until a cooler temperature is selected then the boiler cuts out, so either full hot or cold, nothing in between.
Any ideas, easy fix? Fairly handy with tools and can’t get a plumber for love nor money at the moment, so any help appreciated.
Thanks.

 

[Deleted member 267285]

What’s the flow rate like? Sounds like the microswitch isn’t connecting properly or there’s a restriction in the Venturi.

 

[Nitoman]

Flow rate seems ok, but not sure what ok means! Is there a litres per min figure? It will fill a sink quite quickly with boiling hot water, but reduce the flow or add cold and it goes off.

 

[Nitoman]

Further investigation shows that there is a lever that closes the micro switch. That lever is only fully closing the switch at max flow, so not sensing minor flow.
I’ll investigate the diaphragm and Venturi.

 

[wannocks]

Sounds like the diverter valve or diaphragm needs changing.

 

[ianmcd]

Very common fault, you get a sevice kit for the diverter valve but easier to replace the complete valve, in the short term use long nosed pliers and make a small bend in the micro switch arm and it will work

 

[gas112]

yep best to change whole valve but at around 200 quid now plus labour you have to decide wether that is cost effective on a 25+ year old boiler