Combi boiler, 100% hot water or nothing

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19 Dec 2021
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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.
 
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What’s the flow rate like? Sounds like the microswitch isn’t connecting properly or there’s a restriction in the Venturi.
 
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.
 
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.
 
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Sounds like the diverter valve or diaphragm needs changing.
 
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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
 
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
 
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