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Discuss Central heating flow/return 3 bed terrace - 15mm or 22mm in the Central Heating Forum area at PlumbersForums.net

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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
 
15mm will support a 6KW load on a Delta T of 11°C with a water velocity of 0.9m/s. 15mm should be ok yes but as above is the system losing pressure?
 
Thanks guys. Its strange, the problem seems to be intermittent and the system loses some but not all its pressure, and it doesn't necessarily lose it when its filled. I spent a day at the house with the heating on full and the damp didn't get worse, a couple of days later there was damp on the walls. I don't believe there are any other water pipes in the area. The attached images are from under the floor in the vicinity of the damp patches which can be seen on the internal walls, the images seem to indicate there is localised damp under the floor. I haven't opened the floor up yet as its complicated by a 20mm solid T&G wood floor which is on top of the floorboards, the virus permitting I'll travel to London early next week to deal with the floor. Given the effort in taking the floor up I thought it sensible to replace all the pipe rather than deal with any localised repairs - assuming it is the heating system that’s causing the problem. Any other checks or things to look for?
 

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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
Yes 22mm for the circuit then 15mm for the rads and you can get away with a couple of decent rads off a 15 mm feed ..but not more than 3kw and a bit. I know there is loads of theory etc. but this is my honest experience as busy Heating Outfit and trying things out on my own houses. I did try 2x 3 kw rads back to back once off a 15mm drop only because I was lazy and it was a new extension added on. Result ...no chance
Centralheatking.
 

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