Bath/Shower Mixer Tap cold - imbalanced supply?

[Fox57]

Hi I have a simple mixer tap on the bath very similar to this https://www.screwfix.com/p/bristan-cadet-deck-mounted-bath-shower-mixer-tap/965gf

If I run mixed hot and cold through the bath outlet, no problems I can adjust the temperature to warm.

If I pull up the valve and run the shower - hot on its own will come through fine (scalding), but try to mix in any cold at all, water turns freezing in a couple of seconds.

Shower is either scalding or freezing, no matter how tiny an amount of cold tap I try to turn on, the hot cuts out straight away and it goes full cold.

The hot is gravity fed from a cylinder and I think the cold is mains fed. I searched around on here and saw people mention imbalanced supply - does that make it impossible for me to even turn on the cold a very tiny amount to mix with the hot? Also confused why it works fine out of the bath tap?

Is there anything I can try to fix the issue so I can use the shower?

Thanks!

 

[terryplumb]

Has the problem recently arisen, and previously worked well , or is the tap itself newly installed ?
I would guess the hot and cold don't mix until reaching the spout when filling the bath, when going to the shower hose, cold is overpowering the hot due to higher pressure on the cold mains

 

[Fox57]

We have another shower so this one almost never gets used. I think the last time it got used was over a year ago, and can't remember if it had the problem then or not. It's not a new install no. It was here when we got the house and I'd guess it has been in over ten years.
Could it have been wrong from the start?

 

[terryplumb]

If the cold is from the mains it was certainly plumbed in incorrectly.

 

[Fox57]

If the cold is from the mains it was certainly plumbed in incorrectly.

This might be a stupid question but how do I work out if it's supplied directly from mains or from the cold tank?

 

[terryplumb]

Turn off your mains cold water stopcock, if cold water continues to flow from the tap it's not from the mains !

 

[Fox57]

Turned off the mains stopcock and nothing is coming out of the cold side. So yes bath mixer shower cold has been plumbed directly from the mains.

 

[Johntheo5]

If you can't or don't want to feed the cold from the CWST you could install a lever or gate valve on the cold supply to the bath/shower only if possible, then throttle it to give the same flowrate as the hot gravity fed one, you should probably be able ro get them to mix then.

 

[Fox57]

If you can't or don't want to feed the cold from the CWST you could install a lever or gate valve on the cold supply to the bath/shower only if possible, then throttle it to give the same flowrate as the hot gravity fed one, you should probably be able ro get them to mix then.

I don't know what is under the bath, haven't taken the side off yet because it will mess up the sealant. But, if there is an isolator valve on the cold supply to the tap could I just throttle that? Or don't they work the same way as a lever?

 

[Johntheo5]

Yes, if there is a cold water isolating valve you can throttle that one, first measure the hot flow only then throttle the isol valve until the cold flow matches the hot flow, mixing might then be possible with the two taps.

 

[JohnD]

Not a good approach. Save up for a pressure reducing or pressure balancing valve.

 

[Johntheo5]

Can you get a PRV that will reduce to say 0.4bar, I have gravity hot and mains cold in the bathroom but I have two separate "real" taps, I have the cold feed throttled as suggested above and perfect mixing, mind you I could get mixing without the throttling but it was far too sensitive resulting sometimes in a fluctuating mixed temperature.

 

[JohnD]

Pressure reducing valves

The right pressure makes the difference

www.caleffi.com

I also have an Aqualisa (round bodied) shower mixer with a pressure balancing cartridge.