tube diameter for heat leak for rayburn?
 

I am changing my solid fuel rayburn from a direct to an indirect system. I have to use 28mm tube from rayburn to cylinder as standard, off this I am installing a radiator as a heat leak. The advice I am looking for is do I use 22mm tube to feed and return, or should I use 15mm tube. I am conserned that maybe by using 22mm it will draw off to much hot water to heat the cyclinder. I have to at some point reduce the tube to 15mm to connect to the radiator anyway. I would like to use 22mm to ensure good flow. We have no attic so water tanks are sited on the upstairs floor with only a 6ft drop to rayburn. The radiator is to be sited 6ft away from rayburn with a 3 inch drop from rad to cold return in rayburn. The feed to the heat leak will have to run up 2 inches over 4ft but I reckon the gravity pressure will cope with that. Any advice is appreicated.