How to Bleed a Radiator (Under 10 Minutes)
If your radiator is cold at the top and warm at the bottom, it needs bleeding. Here's how to do it in under 10 minutes — all you need is a radiator key.
By Askento Editorial Team · 3 min read · Apr 1, 2026
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A radiator that's cold at the top but warm at the bottom has trapped air preventing hot water from circulating properly. Bleeding it releases that air and restores full heat. It takes about 5–10 minutes per radiator.
What You'll Need
- A radiator bleed key (£2–£5 — a flat-head screwdriver works on some radiators)
- A cloth or small bowl
- Optional: a towel for the floor
Step 1: Turn the Heating On
Turn your central heating on and let it warm up fully — around 15–20 minutes. This allows you to identify which radiators need bleeding.
Feel across the radiator from top to bottom. A radiator that's warm at the bottom but cold or cooler at the top definitely has trapped air.
Step 2: Turn the Heating OFF
Before bleeding, turn the heating system off and let the radiators cool down slightly. You don't want to work with scalding hot water. Give it 20–30 minutes.
The system pressure should still be high enough for the air to escape without water spurting.
Step 3: Find the Bleed Valve
The bleed valve is a small square-headed bolt, usually located at one of the top corners of the radiator. It might have a cap over it that you twist off first.
Step 4: Bleed the Radiator
- Place your cloth under the bleed valve to catch any water drips
- Insert the bleed key into the valve square and turn it counter-clockwise very slowly — just a quarter to half turn
- You'll hear air hissing out — this is exactly what you want
- Hold the cloth against the valve and wait
- When water starts to trickle or spit out steadily with no more air hissing, close the valve immediately by turning it clockwise — don't overtighten
The air has been released. The radiator should now heat evenly.
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Step 5: Check the Boiler Pressure
Releasing water reduces the system pressure slightly. Check your boiler's pressure gauge — it should read around 1–1.5 bar when cold. If it's dropped below 1 bar, you'll need to repressurise the system using the filling loop (usually a silver braided hose connecting two pipes under the boiler). Consult your boiler manual for this step, or search for your model number.
Step 6: Repeat for Other Radiators
Work through any other cold-at-the-top radiators. Do upstairs radiators first (air rises), then downstairs.
After Bleeding
Turn the heating back on and feel each radiator again after 15 minutes. They should now heat evenly from top to bottom.
When Bleeding Doesn't Help
If the radiator is cold all over (not just at the top), the problem isn't air — it may be:
- A stuck thermostatic valve (the head on the radiator valve may need replacing)
- Sludge buildup inside the radiator (requires a powerflush by a professional)
- The radiator being at the end of a long circuit losing heat pressure
If radiators repeatedly need bleeding every few weeks, there's likely a persistent air leak somewhere in the system — worth getting a heating engineer to check.
Related guides
Frequently Asked Questions
- How do I know if my radiator needs bleeding?
- The classic sign is a radiator that's warm at the bottom but cold at the top — trapped air has risen and is blocking hot water from filling the upper section. Other clues are gurgling sounds and rooms that are slow to heat. If the whole radiator is cold, that's a different problem, not air.
- Should the heating be on or off when bleeding a radiator?
- Turn the heating on first (15–20 minutes) to find which radiators have cold tops, then turn it off and let them cool before bleeding. Working on a hot, pressurised system risks scalding water spurting out. Bleed when the radiators are warm but not hot.
- Why does my radiator keep needing to be bled?
- Repeated air buildup usually means air is getting into the system — often through a small leak, a faulty automatic air vent, or corrosion producing hydrogen gas inside the radiators. If you're bleeding the same radiator every few weeks, have a heating engineer check the system.
- Do I need to check boiler pressure after bleeding?
- Yes. Releasing air also lets out a little water, which drops system pressure. Check the boiler gauge — it should read about 1–1.5 bar when cold. If it's below 1 bar, top it up using the filling loop before running the heating again.
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