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so far the rads have been on for 5-10 mins and still not heating up, so does this mean we will have to have the new 2mm fitted in the next room?
 
I recon you have either a blockage or air lock, 3 rads with an output of 4500 btu's each 15mm should cope with it. things you need to try first to see if water is going through when you turn on one of the new rads does any of the pipes start to get hot ?? did the old rads work B4 and what was the output on those were they single or double panel rads
 
so just to clarify the 3 rads are currently on 22mm, previously were standard rads on 15mm. not sure of the previous rads but but were standard size etc.

one of the rads started to get hot when the water flowed in but then seems to cool down again.

the old rads worked fine which were single column

thanks again for your helps
 
ok so once I have turned half the working rads off, then what do I do? Should I turn off half the rads in the house off or just downstairs?
 
how do I check for a blockage or air lock??

Are you using a plumber or DIY ? you will need to disconnect one side off one of the new rads both ends closed, open up a valve let water into a bucket if that side is getting hot then repeat with other side , any plumber would know how to do these basic checks, total waste of time changing pipe to 22mm if its connecting back to 15mm, it should 22mm going onto 15mm, it seems your going to finish up digging up floors until you find the original 22 mm pipe.
 

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