My son has a 3 bed terraced house and a damp patch on a ground floor timber floor/wall that I think is caused by a pin hole/failed joint in the copper pipe. Rather than just do a localised repair I intend to replace all the ground floor pipe. The exisiting setup has 22mm pipe dropping down from the first floor to under the ground floor it then tees off to the front and rear of the house (3m max in each direction) also in 22mm to the two pairs of rads. Is it okay to change to 15mm at the T for the flow/return to each pair of rads - the total heat output for the kitchen and dining room rad pair is 3.4KW and for the lounge and hall pair 3.6KW. I understand from this forum that 15mm is okay for a 6KW load so I think 15mm is suitable but wanted to make sure for the specific situation I have detailed before starting the job. The system is supplied by a Vaillant ecoTec Plus 831. Many thanks for a great knowledge base/advice source. Tks