danny rawlings
Hi chris - the only answer to your question is yes. The main reason is the flow of water from the boilers main heat exchanger to the cylinder \ airing cupboard. At 15mm it is far too small and the heat exchanger will more than likley block up. Plus you will have a lot of overheating problems. And all boiler manufactures state 22mm as a minimum . If you let us know the size of the boiler kw wise + that would help even more. Regards Danny
