Radiators cold at the bottom.

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A couple of years ago we had a combi installed to replace our ancient Vulcan. The rads had a high velocity flush out and then some radiator fluid put in to help prevent sediment. Just of late I have noticed that the bottoms of the rads are not warming up but the tops are. There is no air inside as I have checked. How on earth can the bottoms be cold when water at the top is red hot ? I don't understand how when gravity would surely bring the water down to the bottom.

Anyway, I think it's a balancing issue (so I was told by the last plumber we had who sorted the problem for a while) and Ihave been tweaking the taps here and there getting full heat back in some but not in others. There are 5 rads in all, 2 downstairs and 3 upstairs. All have 2 taps except the bathroom and front bedroom which only have one. Non have thermostats.

If anyone can instruct me on how to get to the bottom of this I would be very grateful as it's proving a headache. The front room rad is the one that always eventually runs cold after a few weeks without it being touched. Why once it has been set it should do this I don't know. The plumber didn't understand how it could happen either.
 
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Simple, build up of magnetite/sludge.... Guarantee it wasn't flushed properly so that they could get on with lashing in the boiler... Sorry to say. Best thing you can do is pull the rads off and flush each one through with a hose...
 
My wife saw them flushing and they did it for a long time with a lot of sludge out. Have flushed with a hose myself previously. The play,ber did check that last time and the water coming out was clean. It def isn't that for sure.
 
That sounds promising, the only other thing I can think of is that the water is going through those rads far to fast... Need to squeeze the valves right down to slow the flow... It's unusual for this to be the cause but I've had it happen a few times... If they are budget rads might be worth looking at some stelrads or similar.
 
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Ok, thanks. I will do that.

Sorry about multiple posts but it seemed that my reply wasn't uploading so I tried a few items and then they all appeared at once. Strange.
 
Nope, I am finding it nigh on impossible to balance them. They just keep shifting when I get them right. The bar pressure always stays between 1 and 2. No air getting in and no water getting out so how on earth can it keep going out ?
 
Boiler make and model would be useful. Since so many rads are affected it may not be sludge but diverter issues etc.

Once the system is up to temperature feel the temperature of the pipes on each rad. If there's no more than 10 to 20 c difference between them and the bottom of the rad is cooler then it's sludge. More than this differential and there's insufficient flow through the rad.
 
That sounds promising, the only other thing I can think of is that the water is going through those rads far to fast... .
or too slow if the pump is tired/spindle broke - built into the combi no doubt
 
I very much doubt it's sludge as the whole system was flushed for a long time with high velocity. The water that came out the rad in the front room earlier this year when the plumber took it off briefly was clean. As for the pump, it's only 2 years old. When we had the boiler serviced last year (one year on) the plumber said it was still mint.
 
Last night I managed to set it so the downstairs room rad was hot at the top and bottom. I haven't touched them since but today the heating is turned on and it's all changed again.

It's just impossible to set them :cry:
 
Had a small rad fitted in kitchen. Now the front room rad is cold. Have to turn the living (middle) room rad right down to almost off before a little heat comes out of it. Having Thermostats fitted next week so hopefully it will be addressed.

I manage to get the cold rad working by turning off all other rads and giving it a blast at full heat. This sets it working again. Then I turn all other rads back on and it still works. Great, I think but sure enough in time it goes back to cold. If it were sludge it wouldn't come on at all.
 
Again, tweaked rads to get the front room working and a few days later it has gone cold. Why do the goalposts keep shifting ?
 
How many radiators do you have in the house ? Whats your pipework like? 22mm with 15mm tails ? 10mm, 8mm ? Sounds like a big balancing issue.
Plus you say some rads only have 1 tap, If you have twin entry valves on some rads these cannot be powerflushed. so you may still have sludge flowing around your system.
 

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