Why Radiators Are Cold at the Bottom
A radiator that's warm at the top but cold at the bottom has the opposite problem to trapped air, and bleeding it won't fix it.
A radiator that's warm at the top but cold at the bottom has the opposite problem to trapped air, and bleeding it won't fix it. This pattern points towards sludge settling at the base of the radiator.
Why it happens this way round
Black iron oxide sludge is denser than water and settles at the lowest point of the radiator, blocking flow through the bottom section while the top stays clear.
Why bleeding doesn't help
Bleeding releases trapped air from the top, which is the opposite problem; a cold bottom needs the sludge removed, not air let out.
What actually fixes it
Depending on severity, this might mean flushing that individual radiator, or if several radiators show the same pattern, a full system power flush.
Get a sludged radiator sorted
Tell us which radiators show a cold bottom and we'll assess whether a local flush or a full system flush is the right scale of fix.
Questions we get asked
Can I flush a single radiator myself?
It involves isolating and removing the radiator, which is straightforward for a competent DIYer but easy to get wrong on a sealed system, so many people prefer an engineer.
Does this mean my whole system has sludge?
Not necessarily, one radiator with a cold bottom could be isolated, but it's worth checking others for the same pattern.
Will this damage my boiler eventually?
Sludge that's reached a radiator has likely passed through the boiler too, which is exactly why a magnetic filter is worth having fitted.