Cold water in hot. HELP.

[hotspot007]

I've been having issues with my shower going hot and cold. I was told that maybe one of my mixer taps was passing cold to hot so I tested them and sure enough with the hot supply isolated at the boiler - water came out the hot side on every mixer tap (downstairs loo, bathroom and ensuite.)
I replaced the cartridge in the downstairs loo and was no change.
All the taps were directly fed so couldn't isolate to try and find leaking one.
Today I have fitted double check valves with isolators on each of the hot pipes, and isolators on the cold.
And they are still the same pressure is definitely in the hot side - as when I isolate each sinks hot side - the water stops. I've isolated kitchen sink (2 separate feeds), isolated all colds to the sinks and still pressure there. Oddly though with the sink mixer taps off-if I open the hot on the bath or the shower then there is nothing - no water at all. Head scratching time. Has anyone got any suggestions please
Currently bath is two separate taps. The shower is currently a manual mixer tap with two manual taps.

 

[CBW]

Have you missed anything? Did you isolate any valves on the bath or shower?

 

[hotspot007]

Never isolated the bath or shower, as they're separate feeds - so figured impossible to mix. The shower feeds are behind the tiles.

 

[oldbutnotdead]

Never isolated the bath or shower, as they're separate feeds - so figured impossible to mix. The shower feeds are behind the tiles.

WTF? If the bath taps are singles then fair enough but the shower? Of course it can mix, especially if you've got a big pressure differential between hot and cold (your combi hex is quite a resistive item so will be reducing pressure).
The oddity with the bath- without seeing the pipework its hard to tell, does sound as though someone has cocked up somewhere.

 

[hotspot007]

The shower is the same as the bath. I replaced the thermostatic shower bar with a manual mixer valve with 2 taps. I've took the hot side off the mixer tap whilst cold connected and turned off. No water comes out the hot shower pipework - nor the bath hot tap. I've proved the boiler isolation valve too - removed the key for the CH fill loop - so I know that's not passing.
I'm at a loss now - don't think there's anything left to do/try?

 

[terryplumb]

Do you have a blending valve anywhere,bath supply pipes possibly ?
As I understand your posts ,if you isolate the kitchen sinks hot and cold supplies ,you then don't get any water at all from baths hot and cold taps ,is that so ?? If it is, your isolation valves must also isolate the bath. What physical position are they located in ,relative to sink and bath ?

 

[hotspot007]

No sorry, with the hot isolated at the boiler. I isolated the kitchen sink cold feed, opened the hot and nothing comes out. I then isolated the hot and cold at the sink to test the other 3 sinks. Again, with hot isolated at the boiler - water still came out the hot supply on the 3 sinks, downstairs toilet (ground floor) , bathroom sink (1st floor) and ensuite sink (2nd floor).
With them 3 sink isolated (whether just the hot, or both hot and cold) or if all sinks and feed are left open on the 3 sinks- no water comes out the hot tap of the bath or the shower as it does in them 3 sinks. Cold water comes out of the shower and bath taps OK with the hot isolated at the boiler.
I've just measured the flow of the water passing and it's 0.8 liters a min - that's the hot water tap open whilst the water isolated at the boiler.

 

[hotspot007]

That 0.8litres a min was ground floor toilet sink, the 1st floor bathroom sink was 0.6litres a min, the 2nd floor ensuite sink measures 0.55litres a min.

 

[hotspot007]

Just took bath panel off and maybe the mystery is solved.
See pic.
There's something in between the hot and cold feed on the bath - and that then goes to a bank at bottom of Bath with 3 feeds.
What is it ‍

 

[CBW]

Thermostatic Mixing Valve (TMV), installed due to building regulations to prevent scalding.

 

[hotspot007]

Grrrrr... So I take it I can remove that - and just feed from baths hot supply?

 

[hotspot007]

But as it happens... This will not be affecting the shower will it? It feeds 3 outlets which will be the 3 sinks I have observed to be passing.
So the cold/hot shower can now only be the boiler.
I had B. Gas out for that. They changed the flow restrictor and housing - but didn't change anything. They then blamed poor pressure. I'm only getting 6 litres a min through the hot water tap - yet 9/10 in the cold. I contacted Worcester - who said it could be a mixer tap passing (hence me going down this path) or the plate heat exchanger blocking.

 

[terryplumb]

Replace that blending valve ( TMV),it's goosed.
And show us a pic of the shower valve

 

[hotspot007]

OK.
The one in my hand was the original thermostatic shower bar. I replaced the cartridge in that, but didn't change anything so fitted the manual mixer as seen attached.

 

[terryplumb]

Are you sure the hot to the shower isn't from the blending valve ?
Was the shower pipework part of the original plumbing installation ?