central heating valve?

can any one help ,updated heating controls for seperate control of heat/water there is a isolating valve on a pipe coming from the bottom of the hot water cyclinder going to the 22mm pipe just above the water pump but below the mid position valve not sure if this valve should be open or shut .
when open all works well except the last 2 radiators ie, dining/frontroom, dont get very warm but if i close the valve all the rads get hot after about 25 minutes and then the boiler shuts down and it will only allow me control of hotwater until the next day . all the controls are new and the old programer use to have hotwater only/ heating/water together am i right in thinking this is a y plan system and can anybody tell me if the valve should be open/shut and what the hell its used for cheers
ps. can send photos but need your email address because cant work out how to post pics on forum . dave

dave
April 2007