38kw combi boiler supplying a shower. 22mm or 15mm pipework?

Discussion in 'Getting Started FAQ' started by ahearsum, Sep 1, 2020.

  1. ahearsum

    ahearsum New Member

    38kw combi boiler supplying a shower. 22mm or 15mm pipework?


    I am refurbishing a 1st floor bathroom and installing a shower to replace a bath and have a 22mm/15mm pipe quandary. There is currently a newish Worcester Greenstar 38CDi Classic combination boiler and my question is should I redo historic mixed diameter pipework. The boiler has 15mm mains-in and DHW out connections. At some point in the past there was a cold water tank in the attic and presumably a hot water tank somewhere. There is a half meter of 15mm pipe coming from the DHW boiler output which becomes 22mm supplying the bathroom bath and sink 3m away. The 15mm mains water pipework comes up from the ground floor into the boiler cupboard and also is connected to an historic 22mm pipe to supply the bathroom. This looks an easy route that the previous installer took.

    I was considering replacing all the 22mm pipework with 15mm as that seems to be the general internet consensus from what I can gather but I want to make sure I am not going to throttle the supply to my shower as the aim is to have a GREAT shower. Any advice gratefully received.
     
  2. terrymac

    terrymac Screwfix Select

    As the boilers domestic hot water outlet is 15 mm ,the intervening 22 mm is pointless. Having said that there is no real benefit to strip out the 22 mm section.
     
  3. ahearsum

    ahearsum New Member

    Thanks for that. I had a hunch that was the case but wanted some confirmation as I will be submerging all the plumbing under a whole tanked wetroom and underfloor heating scenario and I didn't want to make a huge mistake.
     
  4. rogerk101

    rogerk101 Screwfix Select

    There is a benefit in that you won't be having to run a lot of cold water through the 22mm pipe before the hot water arrives, and then leaving that same amount of water to cool down between uses. The cross sectional area of a 22mm is roughly twice that of a 15mm pipe, so you waste twice as much water and twice as much heat in the water that's going to cool down before it's used.

    Every time I take a shower in my house, I regret not having changed the 22mm hot water pipes to 15mm, as it is a really long run, and takes a loooooong time before the hot water arrives. A future project when all the others are done!
     
  5. terrymac

    terrymac Screwfix Select

    Replacing 3 metres of 22 mm pipe with 3 metres of 15 mm pipe will make no appreciable difference. Your talking about ,roughly ,half a litre of water !! That would take several seconds to displace.
     
    Last edited: Sep 1, 2020
    ahearsum likes this.
  6. ahearsum

    ahearsum New Member

    Ah! Sounds logical. The only thing I'm reticent about is the possibility that I may be throttling the flow to my shower . In my previous house I was used to gravity fed CW and HW storage tank with. 3bar pump. The flow rate to the bath taps currently is 22l/m CW 14l/m HW. To the sink (15mm spur off the bath supply I presume) is 12 and 10. The mixer tap may be restricting the flow of course .
     
  7. rogerk101

    rogerk101 Screwfix Select

    A good shower experience is far less determined by the volume of water flowing mer minute and far more determined by the design of the shower head.
    These days you should not have to use a lot of water to have a good shower experience.
    I usually stick with Grohe or Hansgrohe shower heads and they are so well designed that there is no point in having the shower on full blast - half way is plenty.
     
    ahearsum likes this.

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