rads playing up!

[morlomanc]

hi guys
got a 4 year old persimmons new build with a baxi 50ff and megaflo cylinder piped on a s plan. custard foned me saying that 3 rads were getting hot but now are not. standard pipework to the rads ie plastic 10mm behind dry lining to a patress behind rads then elbows across to valves. now one of the rads has had a rad in the conservatory added to its drop, so one pair of 10mms serving 2 biggish doubles, so im not empecting them both to get red hot obviusly, but the other 1 is on its own drop from above. custard said its had a 15/60 pump change before i arrived on the scene. ive taken all 3 rads off and flushed them thru with hosepipe, run 2 litres of x800 in system for 3 days not much gunge in em at all. i,ve taken the pushfit valves off, all clear as a bell. i,ve put some compressed air up the f&r,s all seems clear. reinstated system everything gets hot for a couple of days, then all 3 drop off to luke warm at best! do u think there is a blockage where the 10 mm,s tee off on the carcass- or is there anything ive missed. if youre still awake id appreciate your thoughts!

 

[bengasman]

Balanced correctly?

 

[morlomanc]

hi ben
yes ive cranked the upstairs lockshields down to a quarter turn open and some of the downstairs rads shes got turned off on the trvs anyway, so its not a balancing problem. any more ideas?. its strange cos i can pull decent hot water thru each f&r valves on the 3 rads when they are the only 3 "on" and its seems to circulate fine for a couple of days then dies off

 

[bengasman]

How many rads are turned on these days whilst the 3 don't get hot?

 

[morlomanc]

there are 11 rads on the system, 3 are always off on the trv,s 3 ive got problems with and the other 5 are fine. one thing i have noticed is that when ive got the problem 3 on only, is that the boiler cycles often as though its a circulation problem which leads me to think there is a blockage on the tees somewhere as the pipe gets hot on the flow rad valves.

 

[bengasman]

Problem rads are either plumbed in wrong or have a blockage in the supply.
If the 3 are not fed from a central point on the main flow/return pipes, they are almost surely plumbed in wrong.
I suspect they are done as a 1 pipe system.

 

[morlomanc]

yes i think ur right ben. seeing as they worked fine for 2/3 years, im leaning towards blockage somewhere on the return drops to each rad. the inside bore of the plastic 10 mms are tiny- it wouldnt take much to block 1 of those, i doubt if it was cleansed properly on the first fix and if it something like wood splinters from the joist drilling then x800 probably aint gonna shift that! i,ll give the custard the good news that its gonna be a re-pipe, trouble is there house is like a freeking showhome- plenty of dust sheets in the van me thinks!