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jaydebruyne

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Gas Engineer
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Taken on a job - woman's house is part flow/return 2 pipe and part 1 pipe system..

I'm ripping out the remaining 1 pipe and joining into the existing flow/return.

Other than new valves if they're still the ones for 1 pipe systems, it no different to piping up a normal flow/return system is it?

It's on a combi boiler system.

Is there anything I need to be aware of as I've never worked on a 1 pipe system before. I understand how it works in theory but just wanted to check if I needed to be aware of anything when changing it over to 2 pipes
 
The recently added section of flow/return was put in in 15mm - is there any point in going to 22 after this? I was thinking of running the rest in 15 also..?? It's not a massive place, maybe 8-10 rads in total, half house in 1 pipe the other in two
 
Well work out the heat loading of the rads and calculate if 15mm can carry enough heat to make them perform as expected.
15mm can only convey so many Kw - just make sure its enough
 
Old school way for 15mm pipes (on an open vented system) is 20,000 btus - close to 6kw. I still see that as correct.
That's really two fairly large double radiators maximum, or 4 or 5 fairly small rads, to think in simple terms.
Obviously a sealed system, like on a combi, is more capable but personally I wouldn't do too much 15mm in main flow and returns on any type of system. Got to also consider length of any long runs.
I prefer 22mm doing most of a main run right to the last couple of rads. It keeps the system easy to balance, as the end part of a circuit is always the hardest to heat properly and reducing the pipes diameter there would make it worse. I have always thought it sensible to maintain the larger diameter throughout the circuits main flow & return.

Also, - Don't forget that many modern designer rads require good flow to them, due to thin restrictive waterways.
 
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