No cold when hot is on - mixertap shower

[dishman]

Trying to problem solve the following.

Mixer-tap shower fed via a combi boiler and mains pressure cold.

When using the mixer in shower mode, the cold seems to cut off and we are left with just hot. This happens even if the tap is fully open.

Some times in can be tricked into keeping the cold flowing by starting in tap mode and switching to shower.

My only theories are:

1) lower pressure from the mains cold (which is hard to believe) so hot water has dominance.

2) Failure in the mixer plunger so that the cold feed is prevented from mixing.

It is the most basic full turn rubber washer type mixer, similar to this:

http://www.screwfix.com/p/swirl-contract-deck-mounted-metal-head-bath-shower-mixer-tap/61344

On my version the mixer lever is separate from the hose and is pull up stopper type.

Thoughts?

 

[terryplumb]

when using just cold tap to fill bath ,what is the flow like ? and when you divert to shower head ,again cold only what is flow like ?

 

[dishman]

Cold tap 12l/m

Hot tap 8.5 l/min

Pressure may not be overly high. It was quite easy to stop both the hot and cold water flowing with my finger on the tap. This was done without too much effort.

When I compre this to another property (different area) it is nigh on impossible to stop the cold water coming out of the tap with my finger.

The boiler pressure gague reads at just under 3bar.

Another oddity is that when turning the cold tap on, there is a 3 /4 second pause before any water begins to flow.

 

[terryplumb]

and cold flow ,no hot at all, at the shower hose ,no shower head attatched ?

 

[dishman]

Without showerhead - cold is the same 12l/sec. The hot was slower at 5.45l/s.

One thought; if there is a 3/4 second delay before any water flows, this means air is in the tap. If the cold is turned on whilst the hot water is flowing could the pressure of the hot water prevent this air from escaping, hence forming an air lock? Just a thought.

 

[terryplumb]

thanks , so if you do as you just have ,cold on full ,hose only , let run for 15 seconds or so ,and then open hot tap fully and again let run 15 secs . observing the following .... 1.. any noticeable difference in flow when hot is turned on. 2...what is temperature like from hose. very hot ,very very hot or other. think I know what your problem is ,once above questions answered .

 

[dishman]

Using that method: Cold first for 15 seconds, then hot... The flow did not seem to decrease and the water temperature went from very cold to a bit less cold once the hot was added.

So that way round the cold does not seem to get prevented from flowing.

Another oddity, is that with the former issue i.e the cold not coming through, once the shower was ended and the hot water was turned off, the cold tap could be left fully open by accident but no cold would flow.

Does that again support some form of airlock issue.

Many thanks.

 

[terryplumb]

got nothing to do with air locks.the tap valve on the cold tap is not functioning properly ,replace it. you gave the following info :- 1. turn cold tap on ,water does not flow for 3 or 4 seconds. then you get 12 litres per minute to bath or same if diverted to shower. conclusion :- diverter works fine but tap does not when opening ,but eventual opens . 2. when cold to shower is running and then hot is introduced the shower does not run hot. conclusion hot is not preventing cold from running 3. when finish showering and hot tap is turned off. no cold runs from cold tap. conclusion cold tap valve has shut off flow without being manually closed. faulty valve. so you need to change the cold tap valve. or renew the whole thing .

 

[dishman]

Intersting, I thought a traditional rubber compression valve would give more obvious signs of failure.

Maybe an upgrade to a thermostatic mixer would be a good upgrade anyway.

Thanks for pproblem solving it with me.

 

[dishman]

Out of interest, what type of valve failure would cause this issue. The odd thing is that it seems intermittent.