Radiator cold at the bottom? What it means and how to fix it

If a radiator is hot along the top but cold along the bottom, the cause is almost always sludge. It's one of the most common heating faults I'm called out to, and the good news is that it can be fixed.

Why the bottom of a radiator goes cold

Over time, the water in a central heating system reacts with the steel inside radiators and pipes. That produces a black sludge, mostly iron oxide, which is heavy and settles in the bottom of the radiator. Hot water can't flow through the sludge, so the bottom stays cold while the top still heats up.

Sludge doesn't just make one radiator less effective. It works its way round the system and can block the boiler's heat exchanger, wear out the pump and cut the efficiency of the whole system, so it's worth dealing with sooner rather than later.

Which part is cold tells you a lot

• Cold at the bottom, warm at the top: sludge. Bleeding won't fix it.
• Cold at the top, warm at the bottom: trapped air. Bleeding the radiator should fix it.
• Cold in the middle: usually sludge as well.
• Cold all over while others are hot: often a stuck valve or a lockshield valve that's closed or set too low.
• Radiators furthest from the boiler are cooler: the system probably needs balancing.

What you can check yourself

1. Make sure the thermostatic valve (TRV) is turned up. If the radiator stays cold, the pin under the TRV head may be stuck. With the head removed, it should move freely when pressed.
2. If the top is cold, bleed the radiator with the heating off and the radiator cool, using a radiator key. Close the bleed screw as soon as water appears.
3. After bleeding, check the pressure gauge on your boiler. Many sealed systems sit at around 1 to 1.5 bar when cold, but check your boiler's manual for the right figure and top up only as it describes.

How sludge is cleared

If one radiator is affected, it can sometimes be taken off the wall and flushed through outside. Where several radiators are cold at the bottom, the whole system needs cleaning. A power flush pumps water and cleaning chemicals round the system at high flow to lift the sludge out, then the system is refilled with clean water and inhibitor.

To stop it coming back, I recommend a magnetic system filter on the return pipe to the boiler, which catches the iron particles before they settle, and the right dose of inhibitor in the water. Both are worth having whenever a new boiler goes in.

Need a hand?

I clean and rebalance heating systems across Tadworth, Reigate, Epsom and the surrounding Surrey towns. If your radiators are cold at the bottom, get in touch and I'll tell you honestly whether it needs a power flush or a simpler fix.