Cold at the top versus cold at the bottom
- Cold at the top, hot at the bottom — air trapped in the radiator. Bleeding it usually sorts it in a couple of minutes.
- Cold at the bottom, hot at the top — sludge. Bleeding will do nothing at all, because the problem is solid material sitting in the bottom of the radiator, not air at the top.
What the sludge actually is
It is mostly magnetite — black iron oxide produced as the steel inside your radiators slowly corrodes. It is heavy, so it settles at the lowest points: the bottom of radiators, the bottom of pipe runs, and inside the boiler's heat exchanger.
Corrosion needs oxygen, and the main way oxygen gets into a sealed system is fresh water being added. Which is why a system that has been topped up repeatedly to chase a pressure leak often ends up with a sludge problem as well.
Signs it has gone beyond one radiator
- Several radiators cold at the bottom, or cold at the far end of the run
- The boiler making banging or kettling noises as it heats
- Radiators that take much longer to warm up than they used to
- Dirty black water when you bleed or drain a radiator
- The boiler cutting out intermittently on overheat
Is a power flush worth the money?
Sometimes, and sometimes not — which is the honest answer rather than the sales one.
A power flush pushes water and cleaning chemicals through the system at high velocity to lift the deposits out. On a system that is dirty but sound it works well and can transform how the heating performs. On a system where the pipework itself has silted up badly — particularly microbore, which is common in housing built between the 1960s and 1980s — flushing may only buy you a season, and replacing the affected pipework is the real answer.
A good engineer will tell you which of those two you have before taking the job, not after.
Stopping it coming back
- A corrosion inhibitor dosed into the system, and topped up whenever the system is drained
- A magnetic system filter fitted on the return to the boiler, which catches magnetite before it reaches the heat exchanger — and which needs emptying at each annual service
- Fixing whatever is causing you to keep topping the system up, since that is where the oxygen is coming from
Common questions
Can I just bleed a radiator that is cold at the bottom?
No — bleeding releases trapped air from the top. If the cold part is the bottom, there is no air to release and bleeding only loses you pressure. It needs flushing, not bleeding.
How often should a system be power flushed?
It is not a routine maintenance item. A well-inhibited system with a magnetic filter may never need one. It is a repair for a specific problem, so treat anyone offering it as an annual service with caution.
Why is this so common in Washington and similar estates?
Housing built in a narrow window, much of it using microbore heating pipe, all reaching the same age at the same time. Microbore silts up far more readily than standard pipe because the bore is so much smaller.
Related guides
Why does my boiler keep losing pressure? What is actually included in a boiler service? What actually decides the price of a new boiler
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