# When to reduce from 22mm to 15mm on Central Heating Circuit?

Discussion in 'Plumbing and Central Heating' started by PASUK, 14 Feb 2014.

1. ### PASUK

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Hello:

I've been on the forum earlier figuring out a problem on my CH
(http://www.diynot.com/forums/plumbing/central-heating-flowing-into-expansion-tank.393414/) and you guys are fantastic!

I now have a separate question...

What sort of calculations are made when deciding how far to use 22mm before reducing to 15mm in your central heating circuit?

On my system 22mm is used from the boiler and hot water tank for a about 1 - 2 m before it reduces to 15mm. I think three small radiators t off the 22mm before it reduces. Most radiators (seven other bigger ones) tee off the 15mm circuit. it is two rings (one each floor).
They seem to heat ok so not currently worried but I am planning on adding a couple more rads in the future.

Are there any methods to calculate the optimal points in a circuit to reduce from larger pipe to smaller pipe? Or do you pro's use a common sense approach?!

Hope this makes sense!

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3. ### Dan Robinson

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Rule of thumb is no more than 3 rads off 15mm flow and returns.

Our 6kw.

Things will work, but not optimally.

4. ### GavThePlumber

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If you are interested in this sort of thing and you have the time to learn all about another trade and hate your current career, do a 5 year course in Plumbing & Heating, Gas, Oil and MCS.

Join the club

5. ### chantelle

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Better let British Gas know then, apparently they have no problem with all rads being fed in 10mm pipework from a manifold. Tell that to any installer and he will fall about laughing. Nice theory on paper, complete Bull in real world.

6. ### chantelle

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Common sense approach,if your boiler is big enough Kilowatt size to take more radiators then pipe them up in 15mm to your existing circuit, you can drop to 10mm if it`s easier to manage/fit but stick with 15mm it looks nicer and clips better if it is on show anyway.

7. ### swbjackson

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That's a completely different scenario. With a manifold you'll have 22 up to the manifold.

8. ### gas112

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Chantelle do you know anything about heating absolute stupid comment and assumption shows total lack of knowledge also BG do not install 10mm

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10. ### D_Hailsham

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These documents tell you how it should be done; but most installers can't be bothered to work it out (or don't know how). So they stick in 22/28mm for the main run (depending on boiler size) and tee of that in 15mm to each rad.

Small Bore Heating Systems

Copper Tubes in Domestic Heating Systems

11. ### chantelle

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You know nothing about installation in the real world ,neither does any buffoon who believes any company will give you the time to sit down,calculate x,y values for each home, boiler size, pipe sizing and all the rest of your theoretical bull,great in the classroom, you won`t work for a week if you were to try that for any company including BG, I know I have worked for them and many other big name companies, so give me a break mister holier than thou,any idiot preaching theory on here has never had any experience other than theoretically.How many service engineers,gas rate gas fires boilers etc when they service them? None.Own a home where British Gas installed 10mm, Muppet.

12. ### Richardthe3rd

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You make some valid points dear, it would be nice to have the time to work out pipe sizes & ensure the circulation pump was sized correctly etc etc. but in the real world it's normally done on a tight price & the competition's prices are just mental. This is all a grim reflection on our trade, too many course cowboys & chancers that don't do a proper apprenticeship & are completely clueless when it comes to pipe sizing etc.

One rsole that I work with thinks he's God's gift to Plumbing, during a discussion on this self same subject, I asked him about the Index Radiator on a system, he looked at me with a vacant look & says 'what's that'?

Dan's rule of thumb is good and used by most. But when it comes to other tube sizes, seek advice or you'll make a James Hunt of the job.

13. ### srhawksy

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Nothing wrong with 10mm, will carry around 3kw so supplying each rad individually is fine, trunking down the wall and if your good at working with copper it'll look fine!

14. ### Richardthe3rd

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Not 10mm push fit mate. It has a pipe bore equal to 8mm copper tube.

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15. ### srhawksy

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I never even thought about push fit, I suppose it's ultimately the diameter of the insert, but still 8mm supplying a single radiator (not k1 I mean 1 rad) would be sufficient surely? I don't use 8mm copper myself and very rarely use push fit, but I have used 10mm push fit in one or two houses, something to bare in mind..

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