Combi and thermostatic shower valve

[cherrymetal]

Hi all, I have a vaillant vcw242e boiler and a meynell viscount thermostatic shower valve. For a while I've been struggling to get decent hot water, and the only way I seemed to be able to fill a hot bath was to run the cold tap in the sink. I'd determined from stuff I'd read around the net including this forum that I needed a new heat exchanger, the water was travelling to fast through the hex without warming up and running the cold water reduced the flow enough that it got warmer. So ive changed the hex, and while it' got better it' still not great and you still need to run the cold tap in the sink to get a decent hot bath. When I was changing the hex, I isolated all the connections except the hot water outlet from the boiler, figuring that if the cold inlet was isolated once the bleeds from the boiler were drained there would be no other way that water could escape. Wrong! When I disconnected the old hex, water sprayed everywhere and I had to turn the water off in the road. So my theory somehow, cold water flow has been mixing with the hot and making it cold. I initially thought it was something else in the boiler but thought I'd eliminate the only other two possibilities - the two mixers, kitchen sink mixer and meynell viscount mixer shower. Turned off one of the kitchen feeds no change, turned off one of the shower feeds, hey presto! Hot water again without the need to drain off cold water. So my question (sorry for the long story!) This is wrong for a thermostatic valve isn't It? With the valve off the hot and cold feeds shouldn't be linked should they?? I've just changed the outer part of the valve, I guess it could be the inner bit?

 

[Gasguru]

You are right in that the hot and cold should not mix....you come across the problem every so often and I've even had it on Grohe shower mixers.

 

[cherrymetal]

Thanks for that, that's what I figured. I guess I'll just have to change bits of the shower until it stops doing it! There are some flow control thingies in each of the inlets, I wonder if these are also non return valves? Might be a place to start I guess.