My plumber recently installed an Ariston presurised indirect cylinder for me. When he came to do it he replaced a 3 way valve (think he called this a y plan) with two 2 way vavles, one that came with the system and one that I bought. After he had finished I sat down and tried to understand why the need for 2 valves. I do not have any zones split up...Am I missing something? If so could someone point me to any diagrams that might clear this up.