Worcester 42cdi Hot water problem

[Pianogeezer]

Hello,
I have a problem with my Worcester Greenstar 42cdi. It was working fine until yesterday. when I noticed that the hot water would not get hot. upon further investigation, I am even more baffled.
The boiler is in the upstairs bathroom.
1. Bathroom sink, (closest to boiler) gives warm water.
2. Bath, warm water.

If I run both taps at the same time, the bath tap goes cold, but the sink tap gets hot. I cannot get the bath tap to give out hot water. Also no hot water from the kitchen sink.

Any help greatly appreciated as wife is frustrated!

 

[Gasguru]

Got any mixer taps?
Try isolating the cold inlet to the boiler and see if you still get water out of any hot taps.

 

[Pianogeezer]

Ive found the problem!...
the garden hosepipe is linked to the hot water system, It was fitted a few years ago but never been used. It is connected under the kitchen sink, (hot and cold merge into the one pipe which goes outside to the hosepipe).

I switched the hot on to fill the swimming pool for the grand daughter and forgot to switch it off afterwards. Obviously, it is not a one way valve as when this valve is open, the cold water is running into the hot water pipe.
What type of valve should this be?

Thanks for your help

 

[Alec_t]

Deleted.

 

[Gasguru]

You need double check valves on the hot and cold feeds...

 

[jeff the gasman]

You need a pressure equalising valve or a blending valve

 

[AGAS]

oh bless him