One cold dual radiator

Hi, I have one cold dual radiator in my kitchen. Also one cold-ish in my lounge. However there are 3 other radiators on the ground floor and 5 rads upstairs which are OK. I have bled the rads and they are ok. I have taken the kitchen rad off tonight and drained and flushed it. Here is a thing ,the front of the kitchen dual rad gets aired and the back seems to stay cold much longer. The input to the rad is hot. Any ideas?

Sounds like you know what you are doing but check the bleedin obvious first Does it have a trv on it? If so is it turned up/on? This could be faulty (or pin sticking). If not... are both valves at either end of the rad open (turn anti clockwise)?

Have you tried turning the hots ones off? Do the cold ones then get hot?

Hi, thanks for your interest. No TRV,just those screw valves. I havent tried turning off all the other rads yet. Help me understand,what will that prove? Thanks.

Firstly I would ask is there a by pass valve? If there is and it's open too much, then that may be the problem. An open by pass valve will provide an easy route for the water back to the boiler without going along the full length of the flow and return pipes. This means radiators furthest away either stay cold or just warm. by pass valve should only be open a little.
Likewise lockshield valves on radiators control the flow of water through a radiators, the more open the more flow and hotter the room. Problem is how to ensure each room is as desired? So it's a matter of adjusting the lockshields so all radiators get an equal share of the water being circulated. What has been suggested is to turn of the hot radiators to see if hot water is forced through the cold ones, if they now get hot it does indicate there is no problem with stuck valves or blockages. Also it may have moved any air lock that may have been the problem.
To achieve ideal conditions you may have to increase the boiler stat to give hotter water, you may need to adjust the pump speed and you may have to consider the effectiveness of the pump. The pump body can collect crud and reduce the flow through the pump.
I suppose a lot depends on the age of the system and how well its been maintained regarding cleaner and inhibitor.

Hi, I turned off all the rads and the temp on the cold one went upto 43C.
Still the back rad was 7 deg cooler than the front part of the dual rad!

I did hear a gurgle... I have now turned on all the rads and I am watching to seehow much the problem rad cools. So far in 15 mins 0.4 deg.
Thanks for your help.

As far as I'm aware some double panel radiators have a bleed valve for each panel, so the cooler back panel may still have a litte air in it yet.

when you bleed them, how much air comes out? And what colour is the water?

When you fliushed the kitchen rad, what sort of sediment did you get?

I have bled both sides of the rad. There was no air last night in either side.

When I flushed the rad,the water was black initially, but only for a few seconds. However when I first drained the rad,all the water was black.

I still struggle to understand why the temp difference on the rad,7 deg. 44c on one side 37 on other

Thank you for your interest.

the big temperature drop is caused by poor flow, due to blockage, or closed valves, or bad pipework.

if there is black sediment to be found, it may be causing a partial blockage in the pipes and reducing circulation. This is very common on older open systems.

It is fairly easy to clean out, if you add some X400 you will be amazed at how much is loosened.

this is quite an easy and cheap thing to do yourself, if you can afford an extra £100 for a Magnaclean you will be delighted to see how much it catches.