cold rads

[godfrey1971]

hi all
I was wondering if any of you people can help, Ive been working on an heating system ,the people before me have put in a new combi in an extension ,but left job unfinished I drained down system to fit three remaining rads , the new pipework is 15mm but the original pipes are 10mm for the old rads ,my problem is the three old radiators on the 10mm system downstairs remain cold , I drained the system again today and when i refilled I opened bleed nipple and let run I get warm water when heating is on but it stops when bleed is closed, there are 12 rads in the system 9 are doubles ,the boiler is a main 24he with a built in grundflos up 15-60 AO pump , I was wondering if the pump has different flow settings or another pump would help, or is the boiler big enough? the rest of radiators all get hot , could it be a blockage somewere? does not make any difference when I turn all the other rads off. sorry about length of thread but Im getting frustrated with it
thanks john

 

[newbie1]

firstly has system been thouroughly flushed as 10mm will allways be more susceptible to blockages then once you have eliminated the prospect of blockages you will need to balance the system isolate the rads that are getting heat and once you have introduced heat to the cold ones then reintroduce heat to remaining rads
balancing the system can be time consuming
not sure that boiler is up to the job poss i believe correct ratio is 1l expansion for every rad so would need a 12l expansion vessel but dont quote me

 

[REDSAW]

believe, 4% roughly of system volume boiler,rads,pipes. etc

actually reading the thread, we fit main/baxi but i mainly do the pipe works only. i think the 24he is not enough for 21 rads (inc doubles) a 30 he would be pushing it depending on pipe runs.
i also think there is a prob with the 10mm or valves or blocked rad vanes.

 

[wnjswozo]

boiler not up to job,youll,need another exp vessel on it.try balancing system

 

[migoplumber]

just to test that your rads are plumbed in correctly and not blocked or mispiped, turn off all the others except for the three not working, and see what happens.
on top of that, yes the boiler is probably undersized depending on the actual size of rads, or to be more specific the volume of water in the installation, and yes again the expansion vessel could be too small for the installation.
that said your boiler should still actually heat everything on the system its connected to, providing its balanced out. if your boiler is undersized just means it will work harder. if expansion vessel not adequate just means your have over/under pressure issues relating to prv, if the pump is not powerfull enough then you wont have enough circulation and some rads will remain cold.