Shower stops when HW tap is switched on

[thesmoothound]

Hi,
Had a new power shower fitted last week as the old one kept stopping and the pump sounded strained. Got the new put in, a Mira Vigour, no problem, job done...or so I thougt! Had shower yesterday and to my horror, the shower started doing the same thing the old shower did! My wife came upstairs as she could hear the strain of the pump and said that she just turned the downstairs HW tap on. So, I then ran the shower, switched on the upstairs HW tap and the shower started fine, then started spluttering, loosing power and motor whining! Can't believe it.
Have called the plumber and he isn't too sure either. The only thing he did say was that the T-Junction where the hot feed pipe is from is quote far up the (in loft). I had a new boiler fitted in March last year, like for like, and it seems that the problem started then thinking about it. I have a standard system boiler not a combi. The T-Junction was in exactly the same place, so how would this affect this one?
Please help.
Thank you
Surely I don'

 

[xr4x4]

How far away is the hot feed for the shower from the hot water tank?

It should be close to or lower than.

 

[thesmoothound]

Hi,
Thanks for quick reply.
Well the hot feed is upstairs in the loft and the cylinder is pretty much directly below it. I would say about 3ft.
Thank you

 

[xr4x4]

So close to the vent then, drawing in air?

Is it lower than the cold water tank outlet?

 

[thesmoothound]

It is slightly lower, only by about 70cm.
Could their be a blockage where the red gate valve is?
How can it draw in air?
Just seems strange that the old shower worked fine on the old boiler. Spoke to the original engineer and he said no domestic work was touched.
Thanks

 

[xr4x4]

In my experience any hot water feed connected higher than the top of the hot water cylinder is prone to problems.