Home › Guides › Radiator cold at the bottom
Radiator cold at the bottom but warm at the top?
If the top of your radiator gets toasty but the bottom stays stone cold, you've almost certainly got sludge. It's one of the most misunderstood heating problems — because the usual "just bleed it" advice won't touch it. Here's what's really happening.
Cold at the bottom vs cold at the top
These two are caused by completely different things, so it's worth knowing which you've got:
- Cold at the top, warm at the bottom → trapped air. Bleeding the radiator fixes it.
- Warm at the top, cold at the bottom → sludge. Bleeding won't help at all.
So what is the sludge?
Over the years, the inside of your heating system slowly produces a black, gritty iron oxide called magnetite — central heating sludge. Because it's heavy, it settles at the bottom of your radiators and in low points of the system. That layer blocks hot water from reaching the lower part of the radiator, so the bottom stays cold while the top heats up fine.
Why it matters: sludge doesn't just leave cold patches — it makes your boiler work harder, pushes up your bills, and can damage the pump and other parts over time.
What you can do
For a single sludgy radiator, it can be removed, taken outside and flushed through with a hose until the water runs clear. It's a messy job — dirty water everywhere — so plenty of people prefer to leave it to an engineer.
If several radiators are cold at the bottom, that's a sign the whole system is full of sludge, and a one-radiator flush won't cut it.
When it's a powerflush job
When sludge is system-wide, the proper fix is a powerflush: a machine pushes a cleaning solution through the whole system at high flow, lifting the debris and flushing it out. Done right, it restores even heat across every radiator, gets your boiler running efficiently again, and we'll usually add an inhibitor (and recommend a magnetic filter) to keep it clean for the future.
When to call an engineer
Call us if multiple radiators are cold at the bottom, if a single radiator is still cold after a flush, or if you're seeing cold spots alongside other niggles like a noisy boiler or rising bills. We'll diagnose whether it's a quick flush or a full powerflush — and tell you straight either way.