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A thermostatic mixer shower has been fitted in my parents house that is not working and I would appreciate advice. The plumber has run the cw supply from the tank in the loft to the mixer valve. He took the hw supply by connecting a short flexible pipe from the top of the hw cistern to the mixer valve. The cistern is in a cupboard adjacent to the shower valve. The shower was fine at first but the hw system soon developed an air lock which restricted flow not only to the shower but to all other hot taps. The plumber returned and cured the air lock but the same problem has returned within a week. When the mixer valve is set at cold, the pressure to the shower is okay. The plumber is now going to raise the level of the tank in the loft but is not convinced this will cure the problem. Has anyone met this problem before? Would an air release valve help?

Raising the tank may cure the problem, but I cannot guarantee it because this fault can have several causes.

Moving the tank is expensive and wouldn't necessarily be my first move.

Is the HW take off at the top of the hot water cylinder connected to the existing pipe or has he fitted a "S" flange?

http://www.awin1.com/awclick.php?mid=12 ... lange-22mm

The picture shown is upside down and the "probe" is hidden inside the cylinder and is designed to draw off the hot water without air being drawn in.

thanks plumbob.
The plumber has fitted a valve at the top of the cylinder. I'm not sure what sort of valve. It looks like its brass. The new flexible pipe is connected to this valve just below the existing copper hw pipework. The plumber is going to raise the tank f.o.c. as he was confident that he could fit a working shower to the existing system without providing a pump. My parents didn't want a power shower but increasing the pressure by raising the tank can only be a good thing.

I would also check the hot water temperature, i've had issues in the past where the cylinder stat didn't work anymore and the hot water temperature was too hot for the thermostatic valve and kept shutting down.