Radiators not getting hot enough

[Bob Croft]

I am having a problem with a section of a central heating system since it was modified. Prior to the mods there were two
radiators a 1000 mm * 600 type 11 in a bedroom and a 1000 mm * 600 type 22 in the dining room these were fed by 15 mm
twin pipes. The modifications added another radiator in the dinning room 1000 * 600 type 11 and a towel radiator 1200 mm *
420 mm in the ensuite adjoining the bedroom. All 4 radiators are branched off a pair of 15 mm feed pipes. All except the
towel rail have bi-directional TRV's on the return end of the radiator.
The problem is that the radiators do not get hot. I turned them all off except the largest in the dining room and the most I
could get on the flow pipe was 54° and about 40°C on the return. The boiler is a Rayburn 28kW and it was set to maximum.
Every other radiator in the house gets hot as expected. The only temperature control is on the boiler. The curious thing is
that when the boiler shuts off the flow temperature (measured on the cool radiators) continues to rise to about 54°C then
drops again presumably due to the pump circulating water.
The system was not drained to add the extra radiators, the feed pipes were frozen to allow the joins to be made. I have bled
all the radiators and run about 5 litres out of both the flow and return pipes at the lower level. The water in the system is as
black as tar, another issue!
Can anyone please help as to why the radiators on this section should be cold and why the flow temperature is so low. I
would have expected more like 70°C.
Sorry for the long post but I wanted to give as much information as possible. Any suggestions how to fix this problem
gratefully received.
Thank you.

 

[REDSAW]

surley this is a ballancing problem?. shut of all known working reds then see if cold ones get hot.
also depending on the routing of pipewok there could be sumping ie pipe goes down then back up, this collects all dislodged sludge, i expect this if the new lines were
t-eed in to run.

had this prob a while ago after 4 other plumbers and new pump later failed to diagnose it,.
on the other-hand i could be still wrong.

 

[Winston]

It would also be worth to get that sludge out, if its as bad as you say sludge remover isnt go to shift it. Take rads out and flush through with hose pipe. Reconnect close all rads bar one flush through with mains, close rad and open another and so on until flushing until all pipes run clear. Time consuming but worthwhile.

 

[david williams]

sludge,incorrect pipe size,balancing,

 

[Winston]

Pipe sizing yes but if cleared correctly it will help the performance and a dose of inhibitor. So many ch systems no inhibitor at all all in 15mm awful, I dont understand why so many dont use 22mm then tee off with 15mm. And for that micro bore thats awful. I went to one house where is was microbore then teed off with 15mm and it was just reeled in like a coil!

 

[richie*is200]

yeah theres a good chance it could be balancing.