22mm to 15mm when

fran1 · 21 Jun 2019

hi, all ,
i'm getting ready to change my central heating pipes from the buried leaking ones to ones that go to the loft and drop down (bungalow)

question is when do i change from 22mm to 15 mm
it comes out of the combi in 22mm then disappears behind built in cupboards then reemerges from the floor to rads in 15mm.
bearing in mind this will be new pipe run going up into the loft..
any help welcome
not really looking forward to this lol

Madrab · 21 Jun 2019

The main run should be done in 22mm out to the furthest away rad and a 22mm return from the furthest rad back to the boiler, it then drops to 15mm when running the pipes from the main flow and return to the radiators. Keep the lengths of the 15mm pipes out to the rads as equal as possible.

fran1 · 22 Jun 2019

thank you for the reply,
it's lost on me slightly . i thought it was like a big single loop i.e it fills one rad then another and so on .. 22mm to first rad change to 15mm then fill each rad in turn ... then after last rad convert back to 22 before returning to boiler .

think i better have a better understanding before i start .. are there any diagrams i can be pointed to ?
many thanks once again

dilalio · 22 Jun 2019

You need to do it as a 2 pipe system, what you describe is a 1 pipe system and is not ideal.

Think of the 22mm pipes as a motorway and the radiators as bridges across it. The footpaths to each bridge are your 15mm pipes. The last bridge connects the two lanes at their ends, they are not connected otherwise.

Plenty of diagrams on Google images.

fran1 · 22 Jun 2019

think ive almost got it .. the existing system is a single loop set up ..
so i'm changing it to a flow and return ..
having a quick look do i need to put an automatic bypass valve in ?
and where .
many thanks

fran1 · 22 Jun 2019

thank you Dilalio , makes it more understandable for someone like me.

dilalio · 22 Jun 2019

Depending on your boiler, you may have an internal bypass but you should put one in as near to the boiler as you can.

fran1 · 22 Jun 2019

its a worcester combi greenstar junior , feeding 7 rads.
is there a preference for the make of bypass valve

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22mm to 15mm when

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