need help !!! poor CH flow

[panda2006]

hi i have replaced some old radiators from singles to slighty smaller doubles, but have a problem that the new rads i fitted the flow pipe runs hot but the return pipe cold and i can only get heat to a few rads at once,
my older rads the other end of the house are cold except for one which has the same problem flow pipe hot return cold.

i know there is no sludge because i used x400 sludge remover and have flushed the system through three times i also have replaced the pump that feeds the heating because old one had ceased working!

could it be that because the rads are doubles i would need a bigger pump or are there other underlying problems.

p.s the pump air vent and rads have all been bled no air in system!!

this is really getting to me now!

it is a feed and return system through 22mm pipe teed to 15mm run through old potterton oil boiler.

 

[JohnD]

Try turning off all the rads except one. make sure the pump is turned to speed 3 (high) not speed 1 (low). If you have any air in the pipes this will tend to blow it out.

Go round turning on rad at a time (and turning off all the others).

See if this way, each of the rads will get hot. If so, open each rad a quarter or so and see if some get hotter than others (if so, re-balance them).

Are you sure the new pump is the same power as the old one? Check that the pump valves are fully open and there are no other valves you might have closed while doing your work.

What sort of sludge did you get out when you flushed (there is a possibility that you have disturbed some and is has formed a new partial blockage).

Have you got TRVs? Is there one rad without TRVs?

 

[panda2006]

the pump is the same i took the old one with me and they matched it up have tried this method of turning off all but one! i also teed off one new double rad into a room which had no radiator in and even with all the other radiators off the new one does not get hot. the extra one i installed is furthest from the boiler, if you open the bleed valve it pulls hot water through but then it will cool back down after about 5 mins!? i have got a few rads furthest away to get hot at the top but for some reason the water is not getting out the return pipe? as this stays cold. and where would a blockege be because when i drained the sytem down the third time the whole system drained down no problem whatsoever.

 

[panda2006]

i have just descovered a radiator where the feed and return pipes are hot but the radiator isn't and when i open the bleed valve the water only trickles out not a jet like the other rads and the radiator bubbles.

 

[JohnD]

Is this newly discovered radiator in the bathroom?

If you feel the flow and the return pipes at the pump, is the return cold? and the flow fully hot?

Is the hot water flow mostly going to the cylinder?

Is there a motorised valve that controls flow to Rads or cylinder, and which pipes on it are hottest?

When did your rads last get hot correctly?

 

[panda2006]

the discovered radiator is in a bedroom it has been replaced with a new rad because the old one had been leaking for about a year when i do get water out it seems like there is air in the water, the pipes that feed the hot water are on a separate pump and none of the pump valves are motorised just manual taps.the last time the heating worked was last winter but because we have no room thermostats i have to manually go round turning each rad off in the summer.

 

[JohnD]

Have you got TRVs?

 

[panda2006]

i do now all the new rads have trv's

 

[JohnD]

Sometimes TRVs jam. If you take the heads off, see if the little pin pops up and can be pushed down against its spring with a teaspoon.

 

[panda2006]

tried this already i failed to mention our system is a one pipe feed system