Hot water not hot (cold water flow into hot water pipe)

I have got a comb boiler and Prism Thermostatic Recessed Shower.

Recently the hot water is not hot any more but the boiler is working fine.

It's been also noticed that the hot water flow faster than before. A friend checked my water system and he said there was some wrong with the shower control part. The cold water flow into hot water pipe, because the part of hot water pipe in ground floor is still warm even hot. He didn't say which part exactly was faulty.

I barely know anything about plumbing, and I checked the fitting instruction and I found a part called non return valve, I guess (only guess from the name), whether it is this so called non return valve is not working properly, and cause cold water flow into hot water pipe?

I found the store where sell spare parts, but I am not sure exactly which part is faulty, anyone can help me out of this please?

it is worth to mention that the tap next to my shower has no hot water as well, that's the reason I guess that cold water flows into hot water circulation.

Also I am trying to check it by myself first, but I am stuck by the circlip, who can tell me how to remove it please?

is there anyone can help?

You've solved it. You need to replace the non-return valve (or check valve) on the hot inlet to the shower valve. Some shower valves have 5 year guarantees so worth checking if it's still covered.

To remove the circlip you may need circlip pliers which have two fine points to engage in the small holes on the clip. Alternatively a pair of dividers (geometry kit?) or a couple of fine pointed instruments (darts?) might be used.

many thanks chrishutt,
one thing I don't understand is that why there is no hot water from taps even I shut the shower off?
I mean when shower is turned off, is the water still runing inside the control part, from cold outlet into hot inlet (or outlet)?
is it normal or there is some problem with the cartridge valve as well?

to change the non return valve (25),
do I need to disassemble the elbow (2) first?

http://89.145.100.46/WebRoot/webstore/Shops/bristan/Products/CS DRAWING-1002915-D1.JPG

Joe07 said:

I mean when shower is turned off, is the water still runing inside the control part, from cold outlet into hot inlet (or outlet)?

Click to expand...

Yes, that's what happens. Don't ask me why they're designed like that, but I presume there's a good reason for it. You may find that the amount of cross flow is influenced by the temperature setting knob. No reason to suspect a problem with the cartridge though.