Discuss 20mm to 22mm reducer in the Plumbing Jobs | The Job-board area at PlumbersForums.net
A philmac 21-27mm x 25mm transition coupling with a 22mm copper insert.
Btw it won't do any good it is the 20mm pipe that is restricting the supply.
20mm pipe has a wall thickness of 2.4mm so the ID is 15.2mm. Stick an inset in and you are down to around 12.5mm.
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