Connecting from old 3/4" pipe work to metric

Discussion in 'Plumbers' Talk' started by ajohn, Jul 10, 2018.

  1. ajohn

    ajohn Member

    Having replumbed a kitchen a few other parts of the house I came to the point where some old piping needed disconnecting and removing. :-( Then I realised that this will result in no hot water to a small downstairs bathroom / shower / toilet we use a lot.

    It uses 3/4" pipe and there is a straight 3/4" coupler in a convenient place to connect to. Ideally with 15mm as the house switched to high pressure hot water years ago. The new run will be a lot shorter anyway and 15mm is ok elsewhere. A lot of it already is even to this pipe. I'm wondering about the best way to connect to it?

    My thoughts
    3/4" female tap connector to15mm but I'm not sure that the threads are the same size or that this will clamp up the olive already on the 3/4 pipe. Or maybe the same but one with a rubber seal.

    Remove a section with the olive and nut on, clean it up, make sure the pipe is dry and use a solder ring converter. Looks like I would have to go 3/4 - 22mm - 15mm- not so tidy.

    Maybe the 3/4" coupler nut can be used with 22mm pipe and a 22mm olive. I could open up the hole in the nut a bit if needed.

    Help - any ideas / suggestions.

  2. Dr Bodgit

    Dr Bodgit Well-Known Member

    I'd do it in end feed (or solder ring/Yorkshire): use a 3/4" to 22" converter coupling, then a 22mm to 15mm converter which goes inside the 22" coupling so really quite neat.
  3. KIAB

    KIAB Well-Known Member


    And BSP threads on fitting,taps,etc have never change,so no problems
    teabreak likes this.
  4. rogerk101

    rogerk101 Well-Known Member

    If I were to redo my current house, I would eliminate the long 22mm runs, as they need twice the amount of 'hot' water to run through before the real hot water arrives from my combi boiler ... and that's a really bad waste of water.

    By the same token the closer to source you can run your 15mm pipes, the less water you'll waste as you wait for the real hot water to arrive.
    Heat and teabreak like this.
  5. Pollowick

    Pollowick Well-Known Member

    As DrB suggests - solder joints. 3/4" x 22mm adapter and 22 x 15 mm reducer.

    If you go with compression, use a 22mm and 3/4" copper olive.
    Heat and teabreak like this.
  6. ajohn

    ajohn Member

    I can do it that way but I'll need compression on the 15mm side. Pipe will need to go through and up a wall and will probably need some artistic bending to get round stuff that was done years and years ago some probably with sand in the pipes. Also central heating pipework. The current pipe has been bent rather than using a union on the other side of the wall and plastered / cemented in. ;-) At least the mess getting that out is well out of sight so wont need a very tidy fix. Anything in the bathroom will. No heat needed if I use a tap connector also not much room to work with heat. It needs some building up before fitting how ever I do it.

    Hot water - it gets through too quickly now. I'm going to have to fit a thermostatic mixer valve to prevent people scalding themselves. 1/4 turn taps great but trying to mix with them is "difficult". We use a heat store so turning the temperature down wont work out. It's small and aimed at heating back up very quickly.

  7. Bob Rathbone

    Bob Rathbone Well-Known Member

    Could you push 15mm JG Speedfit plastic through the buried 3/4 pipe?
    retiredsparks and Pollowick like this.
  8. Pollowick

    Pollowick Well-Known Member

    Have seen that done before and can be an easy option - provided there are no compression fittings and the end is protected from scratches.
  9. ajohn

    ajohn Member

    I'll bear the plastic pipe in mind.

    It looks like I can cut and extend the old pipe with solder ring connectors that will be in the wall. That way I can work on the bends I need in 15mm and effectively have a compression fitting on each end. One the existing 3/4 and the other a 15mm for the pipe going up the wall. I can make it up then and pull it clear for soldering etc.

    Pipes are really close to the wall so cutting them will be real fun.

  10. ajohn

    ajohn Member

    Solved the pipe cutting problem. There was no where near enough room to get a pipe cutter on. I used a multi-purpose oscillating tool with a bi metal cutting blade fitted. The Titan mains powered one that screwfix have stopped selling. Not something I will use very often so bought cheap. I bought it to help cut door architraves while tiling a floor.

    The teeth on the blade are rather fine for copper so slow going but I speeded it up later which seemed to help.

    Risky tasks as I couldn't drain the pipe completely. So angled the blade to ensure that water could not run back onto the tool. Cut low down and stopped as soon as water appeared and then left it to drain.

    The Titan they sell now is battery powered. Might be a safer option.


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