Gravity fed radiator problem

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I have an older CH system whereby the radiators are pumped and hot what is gravity fed - including the small bathroom radiator (which is on the hot water circuit & and sited next to airing cupboard).

Recently this gravity fed radiator has been getting cold at the bottom and part-way up the middle. The odd thing is that the inlet / flow pipe feels hot to touch but the section of the radiator immediately around where the inlet (hot) water enters the radiator is cold, which begs the question how is the hot water getting up to the top? The outlet pipe is barely warm.

although this is an old system, the radiator in question was replaced about 3 years ago, I flush the whole system every year and pour in fresh Fernox inhibitor every year, so would be concerned if it was a heavy sludge.

any help appreciated
 
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Once the system is treated with a suitable Inhibitor, provided the water in the system isn't drastically changed, e.g. by constant bleeding of rads, or pumping over, then the system shouldn't need any further treatment. Annual draining and replenishment of Inhibitor to be honest, is a complete waste of time.

Take Rad off, and flush it through with a hose. If you do get a lot of black filth out then there may lie your issue. I'd also open both valves, one at a time into a bucket whilst Rad is off and check you've free flow from both of those. In all honesty, it can only be a physical restriction stopping the bottom section from heating up. It may only take a slight trickle of flow to heat the top of the rad up.
 
Flushing every year is a bit foolish- all you're doing is introducing freshly aerated water of unknown pH. How well is the hot water performing (ie how long is it taking for the cylinder to heat up from cold). As above from @Hugh Jaleak for your best way forward, hopefully it is just the rad full of crud rather than (for instance) you having dragged all the slime in the F & E tank down into the system last time you flushed it and causing a blockage/restriction in the main DHW gravity loop
 
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Oldbutnotdead: I also clean and flush the header. The reason I do this each year is actually because five years ago we had a very bad instance of biological slime in the system, which took ages to get rid of and a small fortune in specialist antibacterial cleaner.

the hot water is ok, still seems to heat up in about the same time, there are times when I think it’s not as hot as it used to get, but that could just be because it’s getting colder!

so from you you guys all conclude, it sounds very much like a blockage.

one thing to note, none of the house radiators, on the pumped side, exhibit any problems and water did flow clean from a radiator that I needed to remove to decorate behind.

thanks to all
 
Your slime problem would probably be alleviated with a proper lid on the F & E tank...yes a blockage is the most likely suspect, gravity feed isn't particularly efficient but it is a very simple mechanism and it does work (still surprises me how much horizontal you can get away with if there's a decent vertical leg to start with). Easiest first job is drop the bathroom rad off and clean it out (and test-run valves as @Hugh Jaleak post to make sure they're running properly). If that doesn't fix it then you're looking for a blockage between boiler and that radiator- either run a magnet along the pipes & see if it sticks anywhere or try a thermometer (one of the IR jobs) and see if there's any significant dropoff in pipe temperature while the system is warming.
 
Now this is odd...... Today I was about to tackle the flushing of this radiator, went into the bathroom and found the radiator was hot - all over, all evenly!

obviously the threat of posting a notice on DIYnot was sufficient to jolt the radiator to stop messing me around, and cleared itself! I’ll just have to keep a watch over it.
 

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