Shower & bath mixer taps not mixing

10 Aug 2022
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United Kingdom
Plumbers please help!

Our shower mixer tap and bath mixer tap are either giving pure hot or pure cold, and never warm. No matter how gently I adjust the tap.

Background: new bathroom installation in a 1930s house with regular boiler, unvented cylinder & cold water tank. Mixer taps installed, then cold water tank began constantly overflowing. Also very poor water pressure to new mixer taps despite great pressure throughout house otherwise. Plan was to update heating system anyway, so had a new boiler and unvented cylinder installed today. Now shower & bath mixer taps have great pressure, but only delivers scorching hot or freezing cold water, never warm.

Having done some Googling, I’m assuming the plumber hasn’t installed check valves on any pipe work, and the reason for the cold water tank overflowing is that the cold water supply was connected to the hot supply in the mixer tap(s) and pressurising the vented cylinder.

My question is - does the lack of check valves also cause the mixer taps not to effectively mix? Does one supply just overpower the other? Will fitting check valves solve the problem?
The taps are all brand new and good quality/not cheap. Shower mixer is WRAS approved.

Thank you!
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Your info is quite confusing. Do you have an unvented ,or vented ,hot water cylinder, your post quotes both , along with a cold water tank ??
Mixers require both hot and cold supplies to be balanced ( same pressure), so either both under mains pressure ,or both gravity.
Thank you for your reply. We had the old vented system and cold water tank taken out and replaced with a modern unvented system. We now have great pressure for both hot and cold. But the mixer taps don’t seem to be able to mix the water!
You need to get the installers back . Did mixers work ok with previous set up, temperature wise ??
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The temperature was fine, but not enough pressure to have even a dribble of a shower.
If your system has unvented hot water and mains cold pressure then unless the cold mains pressure is much much higher pressure than the unvented hot water pressure then check valves wouldn't be required If the cold mains is much higher than the restricted unvented then the cold supplies must be taken from the balanced pressure supply on the unvented hot water's combination valve.

Really surprised the trade that installed the system didn't check all of this. Shoddy finishing up if that's the case and as suggested time to get them back, it's their problem to sort out.

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