why are my radiators cold at the bottom

[althesaint]

the rads are cold at the bottom but hot at the top, they are only 4 years old, i have tried to bleed them but just get fluid from the bleed valve.

[HowardKennitby]

possibility is they have sludge deposit in the bottom.

[welshlibranr]

or maybe air in the system..........try bleeding them!...........

[jomifl]

There wouldn't be that much sludge after 4 years.Most Likely the water flow is too low, for which there could be many reasons. Has this suddenly happened or has it been developing slowly? Is it a combi boiler or one that heats a hot water tank? are there any zone valves? Can you vary the pump speed? If it heats a hot water tank it is possible that too much water is being bypassed and not going to the rads.

[Coldicote]

Are they turned on to full/high heat? If so, I would expect them to be hot all over, but if on low or part heat then it could be a case of heat rising to the top.

[bartyarmy]

hi ive just had the same problem . I had to remove the rad and flush it out i couldnt believe how much black sludge came out . this about 5 years since they were flushed out before .

[jomifl]

barty are you using a corrosion inhibitor?

[boxtops]

If you search "cold at the bottom", there are lots of threads on AB about this subject.

[buildersmate]

Points to note:
A radiator that is hot at the top CANNOT ever be caused because it needs bleeding. Its a simple case of physics - gas sits at the top preventing the hot fluid reaching the top.
Black sludge can form very quickly in a system with no inhibitor installed - certainly inside one year. If the system is short of inhibitor (below the recommended strength), the rate of sludge forming will be less, but will still occur over a longer period. You cannot overdose the inhibitor - only underdose.

I agree with Jomifl answer, apart from his assertion that "there wouldn't be that much sludge after 4 years" - it isn't time dependent, it is time and underdose dependent.

[jenty]

heat rises...

[jomifl]

I agree with buildersmate, my statement re. sludge failed to take into account that inhibitor might not have been used. I don't think sludge will block the flow as circulating pumps are quite powerful and capable of pushing sludge along pipes if it hasn't consolidated. Sludge will accumulate to form a 'pile' between the inlet and outlet of the rad. so if you have a triangular shape cold patch (with the point at top middle) then sludge is very likely your problem.