Pipe diameter to bath taps

Discussion in 'Plumbers' Talk' started by Possom, Apr 28, 2017.

  1. Possom

    Possom Member

    I know the copper pipes which bring hot/cold water to a bathroom basin are normally 15mm diameter.

    Are the copper pipes which bring hot/cold water to a bath also normally 15mm diameter, or wider?

    Thanks.
     
  2. Phil the Paver

    Phil the Paver Screwfix Select

    Bath is normally 22mm, its to do with flow rates, a bath requiring a lot of water quickly.
     
    Possom likes this.
  3. Possom

    Possom Member

    Cheers.
     
  4. Depends if it's being delivered from a combi boiler or unvented cylinder, in which case 15mm should be ok - and is usual.

    If it's vented/gravity, then 22mm will be required.
     
    CGN and Possom like this.
  5. Possom

    Possom Member

    It's a combi boiler.
     
  6. Phil the Paver

    Phil the Paver Screwfix Select


    Most Magaflows are 22mm outlets.

    i48-2978-011.gif
     
    Deleted member 33931 likes this.
  7. Phil the Paver

    Phil the Paver Screwfix Select


    It will depend on the boilers KW rating and water flow rate, less than 14 litres a minute and around 40 KW will require smaller pipe.
     
  8. Possom

    Possom Member

    Here is the data for the combi.

    It has a Central heating output (kW) of 24, and a hot water flow rate 35°c ∆T (litres per minute) of 10.2 l per min.

    Does this mean 15mm or 22mm pipe?

    [​IMG]
     
  9. Phil the Paver

    Phil the Paver Screwfix Select


    That's quite low so 15mm would be best for it.
     
    Possom likes this.
  10. Possom

    Possom Member

    And the bath is only 1.4 metres long, not the standard 1.7m, it won't require as much water as standard to fill in any case.
     
  11. Phil the Paver

    Phil the Paver Screwfix Select


    I hope your short then, otherwise your knees will be around your ears. :p:p:p:p
     
    CGN likes this.
  12. Possom

    Possom Member

    Wall is 1.4 metres long, so unfortunately was only option.
     
  13. If you look at your boiler's DHW outlet pipe, you'll find it is almost certainly 15mm.

    There will be no advantage in using 22mm pipe, in fact this will only cause a slight further delay in getting the hot water to your taps as there will be more standing water to shift.

    As it is being delivered by a combi - which is at mains pressure - 15mm will handle that full flow with no issues.
     
    Possom and KIAB like this.
  14. KIAB

    KIAB Super Member

    Correct. A*:)
     
  15. Correct. A* :D
     
  16. Possom

    Possom Member

    I'll be using the type of bath filling tap pictured below instead of a more traditional tap.

    It has a supply hose water connection left and right of 3/4". If I want to connect up 15 mm pipes to this, is there some sort of reducer available to do this?

    [​IMG]
     
  17. ramseyman

    ramseyman Screwfix Select

    Looks like you need two chrome 3/4 female to 15mm compression reducers, probably elbows depending on your pipe runs. You'll need pref tape for the tap end and try to Use copper olives on your chrome connecting pipe runs as they will
    Bite better
     
    Possom likes this.
  18. G&W Plumbing & Heating

    G&W Plumbing & Heating Active Member

    Combi's have a varied range of maximum hot water deliverance called "flow rate", the variance being the flow rates determined by either differing KW ratings or manufacturers.

    This of coarse is provided the specified minimum pressure and supply factors are available

    In any case it's impractical to maximise the KW on a combi that's available with domestic gas meter as an unvented cylinder will provide an unchallenged superior alternative.

    But to answer your question, you can only run 1 thermostatically mixed shower valve off a combi, any further need to be an electric mains fed type due to fluctuations that would be created from exceeding the boilers maximum hot water flow rate ability, conclusively calculated by adding together the flow rate demand total of hot water fitments on that system.

    In your case, you need to check the incoming cold main flow rate is ok for your boilers capability, then take 22mm cold up to the boiler supply to limit flow loss, then add your hot water demand, then restrict your hot taps etc to their minimum working flow rate (e.g basin 3-5ltrs/min). This will leave you with you min-max range considering your occupancy and max combined usage.

    You need to check your hot water flow rate at the showers proposed location, then source a shower that's compatible with the ranges of your calculations.

    Running 22 from a combi will not increase flow at all, it will simply increase the volume of dead leg cold water to be drawn off, and deliver a cooler supply of hot water for a longer period until the hot water has sufficiently blended into the greater volume of cold water in the 22 pipe
     
    Possom likes this.
  19. Possom

    Possom Member

    What is the pref tape? I couldn't see any on the ScrewfIx website.


    Thanks for this. Yes there would just be one shower in the house, which would be the over-bath shower.
     
  20. G&W Plumbing & Heating

    G&W Plumbing & Heating Active Member

    So you have 1 shower off the combi?

    That's fine, that pic is a combined bath overflow and filler, the 1/2" thread looks to be parallel so if you should be able to put a tap connector straight on it, do this before you fit it to the bath
     
    Possom likes this.

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