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Reduce pressure in 25mm blueline to WC

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  1. #1
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    Default Reduce pressure in 25mm blueline to WC

    Hi all,

    Just a quick question. I'm connecting two (in/out) 25mm mains pressure blueline poly pipes together to supply a toilet cistern. It's mains pressure and being 25mm I'm a bit concerned it might blow the cistern off the wall. Should i simply put in a 15mm threaded nipple ($5) in between the 90 degree poly pipe fittings to help reduce the pressure or do I have to go for a pressure limiting valve ($70). I can't reduce pipe diameter as its under concrete. Another option would be to have a poly ball valve ($28) that remains half closed. Whatever I do will be very difficult to get to once project is complete. Am I concerned about nothing?. Any suggestions will be appreciated.

  2. #2
    Seasoned DIY droog's Avatar
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    Pressure has nothing to do with the size of the pipe, even a half closed valve will still have the same pressure at the end of the pipe. All that will be effected is the maximum flow.

    Toilet systems are designed to be used at mains pressure so unless you have raised the pressure to run through the poly it will be exactly the same as connecting anywhere else on your property.

  3. #3
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    If you have mains pressure coming in, it will be fine.
    Whats the distance the 25mm runs out of interest?

    As an example, Ive mains pressure delivered from a pressure pump through 25mm blueline to toilets/showers 30m and 90m away with no pressure issue's.

  4. #4
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    I was just a bit worried as I assumed that all the pipework in the house is the 1/2 inch stuff. The 25mm mains 'in' pipe is about 15m from the meter and the 'out' pipe to the toilet is about 4m. What bart and droog suggest sounds logical thank you very much. It all goes through the usual valve and flex hose at the cistern end.

  5. #5
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    One thing I haven't mentioned is that the toilet is currently gravity fed from a retention tank and was taking 20minutes to fill between flushes with filthy water and thats why I am trying to hook it up to the mains.

  6. #6
    Deactivated User Marc's Avatar
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    What Droog said is true in a static situation. When the water is flowing, a small pipe will produce a drop in pressure that will equalise when the cistern is full.
    This is academic and means little.
    You can use a 2" mains pipe and the cistern valve will not be affected providing your mains does not go over 1000 KPA. It may even take more than that.

  7. #7
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    Quote Originally Posted by peejayjay View Post
    One thing I haven't mentioned is that the toilet is currently gravity fed from a retention tank and was taking 20minutes to fill between flushes with filthy water and thats why I am trying to hook it up to the mains.
    If ??? the cistern is set up for gravity low pressure feed then you may have to fit a new valve inside the cistern
    to accept the mains pressure.
    However the reason it is taking 20 minutes to fill at the moment could be because it already has a standard valve on a low pressure system
    or, the dirty water may be blocking the valve in which case it may still take 20 minutes to fill even on mains pressure.

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