Confused! Electric shower flowing water when isolation is off!

[5teves]

Hi

Soemthing really wired is happening and I'm not sure what.

I have 2 pipes going to the bath hot and cold. Both have isolation valve.

The cold has a T going taking a pipe up the wall for the electric shower.

Now the shower is not yet connected up and has a bit of pipe projecting out of the wall.

I turned off the cold isolator at the bath tap, and this should stop water going to the shower which is does.

I turned on the hot tap everything was normal. Until I then turned the lever to use the shower head on the taps.

When I did this water starts to come out the pipe at the top which is for the electric shower.
This is very strange and I cannot figure out why.

I checked the piping and it's absolutely correct.

Does anyone know what is happening here?

 

[flameport]

The shower head part of the taps is a crude mixer, which connects hot & cold together.
With no pressure on the cold side, hot water will flow through the mixer into the cold pipe and out of the open end.