You've turned the heating on for the first time in months, and one radiator is warm at the bottom but stone cold across the top. It's a frustrating thing to find on a cold Derby morning — and with the Ofgem energy price cap rising 4% to around £1,723 a year for a typical household from October, you really don't want to be paying to heat a radiator that isn't warming the room. The good news: this is one of the few heating jobs you can often fix yourself in five minutes. Here's how to tell whether it's a quick bleed or something that needs a plumber.
Why radiators go cold at the top
When a radiator is warm at the bottom and cold at the top, the usual culprit is trapped air. Over the summer, while the system sat idle, air can collect inside the radiator and rise to the top, stopping hot water filling that section. The fix is simply to let the air out — that's what 'bleeding' a radiator means. It's a normal bit of seasonal maintenance, not a fault, and most households can do it safely.
How to bleed a radiator safely
You'll need a radiator bleed key (a few pounds from any DIY shop) and an old cloth. Take your time:
- Turn the heating off and let the radiators cool completely — bleeding a hot system risks a scald from escaping water
- Hold the cloth under the bleed valve at the top corner of the radiator to catch any drips
- Turn the key slowly anti-clockwise, no more than a quarter to half a turn — you'll hear a hiss as the trapped air escapes
- As soon as water dribbles out steadily instead of hissing, close the valve firmly (don't overtighten)
- Once done, check your boiler's pressure gauge — bleeding can drop the pressure, and it may need topping back up to the level in your manual
When it isn't just air — and you should call us
Bleeding fixes trapped air. It does not fix everything, and it's worth knowing the difference so you're not endlessly bleeding a radiator that has a different problem.
Call a plumber rather than reaching for the bleed key if:
- The radiator is cold at the bottom but warm at the top — that points to sludge or a blockage, not air, and bleeding won't help
- Several radiators are cold and bleeding makes no lasting difference — the system may need balancing or a power flush
- You bleed it, but it's cold again within days — air shouldn't keep returning, so there may be a leak drawing air in
- You can't hold boiler pressure after topping up, or you see damp patches, drips or rusty water
"A radiator cold at the top is usually air you can release yourself. Cold at the bottom, or cold again a day later, is a plumber's job."
Staying warm without wasting money
With energy costing more this winter, an efficient heating system genuinely saves money — cold spots mean your boiler works harder to heat rooms that aren't warming up. If a quick bleed sorts it, brilliant. If it doesn't, don't keep paying to run a struggling system. We cover Derby and Derbyshire around the clock, so whether it's a persistent cold radiator or a boiler that won't hold pressure, you can get it looked at quickly and properly.
Cold radiators you can't fix? We're here 24/7
Call Out Repair covers Derby and Derbyshire around the clock for plumbing and heating. If a bleed doesn't do it, let our engineers take a look.
Published by Kirk Group Editorial
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