shower pipework from existing bath pipes

[little_poppet]

hello again chaps, i'm looking to install some pipework for a thermostatic mixer shower in a bath.
i'm fairly inexperienced with plumbing and have a few questions
what is the ideal point to tap into the hot and cold pipework?
i have hot/cold pipes supplying the bath, am i ok to tap into these?
what is advantage/disadvantage of doing this? i don't really envisage using both the bath and the shower at the same time.
i've noticed the hot water pipe is bigger than the cold, 22mm for hot, 15mm for cold, is there a reason for this, and if i tap into these pipes, should i look to match the thickness?
i've created a digram of a side view of the existing pipework to the bath on the left, and on the right a diagram of proposed shower pipework. does my plan look feasable?

thanks

 

[lostinthelight]

A lot depends on whether your cold water feed comes from a tank in the loft or is mains fed.

 

[Hugh Jaleak]

Also, what setup do you have for providing your hot water? A Combi boiler or Unvented cylinder will provide fairly balanced pressures on the hot and cold supplies, however is cold is mains and hot is tank fed, you'll have issues, as the pressures are not balanced. 15mm cold/22mm hot suggests mains cold/tank hot fed supplies, although many leave the 22mm pipework in situ when swapping a tank system over to a combi.

15mm feeds for the shower will be fine, we just need to determine where you will need to take those supplies from.

 

[little_poppet]

A lot depends on whether your cold water feed comes from a tank in the loft or is mains fed.

it's mains fed

 

[little_poppet]

Also, what setup do you have for providing your hot water? A Combi boiler or Unvented cylinder will provide fairly balanced pressures on the hot and cold supplies, however is cold is mains and hot is tank fed, you'll have issues, as the pressures are not balanced. 15mm cold/22mm hot suggests mains cold/tank hot fed supplies, although many leave the 22mm pipework in situ when swapping a tank system over to a combi.

15mm feeds for the shower will be fine, we just need to determine where you will need to take those supplies from.

hey, its a combi, and yes it's likely the 22mm pipework is leftover from tank system as previously there was a tank/cylinder where the boiler is now located.

 

[Hugh Jaleak]

Should be ok to just couple in a suitable shower valve then, 15mm pipework for both hot and cold, (assuming that's the size of the valve connections), will be fine. Put some isolating valves in the supplies, (think you've shown these in your sketch), but always useful to isolate the shower for maintenance in the future.

The 22mm pipework was used for Tank fed hot supply to give a better flow rate, as the head/pressure would usually be lower than mains cold supply.

 

[little_poppet]

thanks for the advice Hugh, that will be my course of action.
and good to know abou the 22mm pipework, otherwise i would always have nagging thought in back of my head about why this was the case!

 

[oldbutnotdead]

You may want to chuck an adjustable pressure reducing valve in the coldfeedto the shower- not essential but (for thermo mixer) you'll get much steadier temperature, manual mix its much easier to get just the right temperature with hot and cold the same pressure (the pressure drop caused by boiler and convoluted pipework can be significant)

 

[little_poppet]

ok thanks will read a little into it