Return valve and pipe are always cold even after rad flush

[chezabha]

Hi. CH radiator in the living room is causing some concern and will not heat up correctly. It a 1200mm double radiator with a TRV. It heats at the top and probably halfway down at most. I removed the rad this morning and flushed it with a hose pipe, now there was an element of blackish water but nothing that looked like a blockage. Clear water was flowing almost straight away. Before re-fitting I renewed the TRV as this always had a problem with sticking. What I did find strange was I was able to change the complete valve without draining the system or turning the water off. The pressure seemed minimal and I was able to unscrew and refit a new TRV without soaking the place!
Refitted, bled and balanced the rad. When the heating comes on the rad gets hot at the top still and cold at the bottom. The valve on the return is cold as is the pipe. It appears that once the rad is filled with hot water it stops circulating and cools down. When the rad was off I opened the return valve and water was flowing out so this side appears to work but even the round ball at the bottom of the rad is cold (dont know the tech term!).
It appears to me that the rad is blocked inside and hot water is not reaching the return but why would the cold water from the hose flow through without any issue?
Does anyone have any advice before I go out and buy a new rad.

Cheers

 

[45yearsagasman]

bled and balanced the rad.
Have you balanced the rads on the whole system? You don't just balance one rad.Is your pump OK? Air locks cleared? Lots of things to check.Last resort is to replace the rad if all previous checks have been done.Try by turning all other rads OFF and see how lounge rad performs.

 

[Burnerman]

If you can change a rad valve without draining down, then you've either a combi boiler or a blockage!
Check the loft C/H header tank has about 4" of water in it, too.
John

 

[chezabha]

Thanks for the post. I've balanced all rads using radiator thermometers and all seem ok. The pump as far as I know is operating ok, but not sure how to check this. I have previously closed off all other rads in the house and it didn't make a single bit of difference

 

[chezabha]

John

My boiler is not combi, so guess there is a blockage or poor flow then. Its microbore pipework. Just checked the CH header tank and it has about 6 inches of water, sitting about an inch below the overflow pipe.

 

[Agile]

Do the bucket test as described in the FAQ on this site.

Tony

 

[Whitespirit66]

Its microbore pipework.

You kept this nugget of important info to the end. You have an open vented system. Leaving the pipes that join to the rads open should result in a steady stream of water flowing out of each pipe, and the header tank refilling to replace the lost water.

Microbore is a bugger for blocking. Looks like the pipe is blocked.