Whether it is 3/4" or 22mm to 15mm the fitting is the same. Its called a reducing coupler (end feed if you are going to solder or compression if you are using spanners).
The 'olives' are very slightly different sizes and its important to get the right one, the fitting will come with a 22mm olive and a 15mm olive, I suggest you but a 3/4" olive just in case.
Chris, February 2010
