Hi - This one has done my head in - Conventional system - 1 tank in attic - 2 x 22mm pipes - 1 feeds DHW cylinder 1.5m below other is connected to Cold water feed to bathroom tank also doubles up as expansion tank for hot water cylinder - water in kitchen ok - no hot or cold in bathroom - why ???? - tank is full of water - no blockages in pipework - worked ok until turned off for 2 months all is on top floor of 3 storey flat