Hi!
I've just had a new shower installed but I can either get only boiling hot or freezing cold water- there's no 'mixing' available.
This was exactly how my old bath/shower that was taken out used to be as well.
The plumber seems to think it's a boiler problem but if it was, surely my sink and my other en suite shower would have the same issue?
From googling it looks like it might be installed the wrong way round or something- does that sound right?
Any help much appreciated!
Thanks Victoria

What kind of tub and shower valve?
I believe all tub and shower valves today incorporate a pressure balancing valve. Perhaps that need to be adjusted.
Don't know how you could install a valve backwards.

The plumber seems to think it's a boiler problem
That certainly is not logical. Whatever the problem is, it is for the plumber to figure out.