Shower either boiling or freezing

[Victoriahl]

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

 

[stevetheplumber]

are the supplies balance?
is it a thermostatic shower?

 

[Victoriahl]

Hi!

Thanks for answering; not sure what you mean by 'are the supplies balance' but yes it's thermostatic...

 

[Unregistered]

I would suggest checking the supplies are the correct way round. If it's a thermostatic shower and you've got cold going in the hot end it will pull more and more hot water (going in the 'cold' end) to try and mix it down to the correct temp!

 

[Sparkgap]

If the cold and hot pressures are unbalanced (ie mains cold water, tank hot water) the mains cold will overwhelm the hot through giving you a cold shower until the thermostatic element moves far enough so that it starts letting the hot through, by which time there isn't enough cold to keep it at the right temp and it goes too hot. One sort-of-fix is to turn down the cold isolating valve to reduce the flow, but better to put in a pressure reducing valve if getting a higher pressure hot isn't an option.

 

[Victoriahl]

Thanks for the replies! I'll show someone who knows more about plumbing than I do (ie anyone) and hopefully I can get it sorted.
Do you think there's something wrong with the way it's installed inside, because my old bath (that's been taken out) had the same problem; you could turn the hot tap on and get super hot water but as soon as you tried to add a slight bit of cold, it just went freezing?

 

[JCplumb]

Did you bath used to have a mixer tap or seperate taps?
Is your boiler a combi, unvented or do you have stored hot water?

 

[Victoriahl]

Hi

The bath used to have mixer taps. My new one does too (the shower is over the bath) and it's the same problem as the shower- I can't get a mix of waters; just scalding hot or freezing cold...
And I've got a combi boiler- but surely if it was a boiler problem my other shower would be affected too?

 

[Sparkgap]

Hi

The bath used to have mixer taps. My new one does too (the shower is over the bath) and it's the same problem as the shower- I can't get a mix of waters; just scalding hot or freezing cold...
And I've got a combi boiler- but surely if it was a boiler problem my other shower would be affected too?

From this I take it the combi does both showers? Are they both the same type of shower valve? (what make?) Have you checked both have hot/cold connections the same? One thought: what is your mains pressure like? If you have very high mains pressure it could be the valve doesn't like high pressure differences between hot and cold as the combi will reduce the hot water pressure.

 

[Plumbstar Tom]

Can you feel the right side of it? the water feed to it i mean. if it is hot then it has been plumbed in wrong