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I want to connect 15mm internal pipework to the delivery side of a pump, inside my cellar... Can anyone enlighten me as to what connectors to use to achieve this? I'm trying to pump rainwater from an external tank, up to a cold water tank in the loft. I'm sure this is elementary, but... any help would be much appreciated! Thanks.

1 inch is 25 mm and they’re a few ways of doing this.

But the simple ways are:

You can just get mi coupler size 22mm x 1inch, and then put a 22mm to 15 mm reducer in which will cost about £13, £3 for the fitting and about £10 for the copper pipe.

Or use MDPE Blue pipe size 25mm and do the pipe run then put a coupler on it and reducer. (MDPE Female adaptor size 25mm x ¾ inch, mi coupler 22mm x ¾, reducing set 22mm-15mm.) This would cost you about £25, fitting £4, 25 meters of mdpe blue pipe £19.

I am sorry but I can’t remember is it a MDPE male or female adaptor, so check when you buy it first.

Hi, I am a Mechanical Engineer not a plumber who may have a special adaptor. 15mm compression to 1/2 male BSP fitting. What you do next depends on the fitting on the delivery side of your pump. If BSP ,get the correct 1/2 to ???? adaptor. Or you can get an 1/2 BSP hose tail.

I would not want to pump water into my loft unless I used propper pipework and fittings. What stops the pump when the tank is full?

Depends on the thread size on the pump. If it is a normal central heating pump the thread is 1 1/2" bsp.
Use a pump valve and a 22 x 15 mm reducer. If you are not surer take the pump to a plumbers merchants and say you want to come off it in 15mm. They will give you the right fittings