If your radiators are warm at the bottom but cold at the top, there's trapped air in the system. Bleeding the radiator — releasing that air — is one of the few heating maintenance jobs you can safely do yourself, and it takes about five minutes per radiator.
Here's exactly how to do it, plus the warning signs that your cold radiators are telling you something more serious.
What You'll Need
- A radiator bleed key (flat-head screwdriver works on some modern radiators)
- A cloth or small towel
- A bowl or cup to catch drips
Bleed keys cost around £1–2 from any hardware store or DIY shop.
Step-by-Step: How to Bleed a Radiator
1. Turn your heating on Run your heating for 10–15 minutes until all radiators have had a chance to warm up. This lets you identify which ones have cold spots and ensures the system is pressurised.
2. Identify which radiators need bleeding Run your hand carefully along the top of each radiator. If it's noticeably cooler at the top than the bottom, there's trapped air. If the whole radiator is cold, that's a different problem — see below.
3. Turn your heating off and let it cool for 30 minutes You don't want to work on a hot, pressurised system. Turn the heating off and wait until the radiators have cooled before bleeding.
4. Locate the bleed valve The bleed valve is a small square or slotted nut, usually located at the top of the radiator on one end. Place your cloth underneath it and have your bowl ready.
5. Turn the valve anticlockwise — slowly Insert the bleed key and turn it anticlockwise by about a quarter turn. You'll hear a hissing sound as air escapes. Hold your cloth beneath the valve to catch any drips.
6. Wait for water to appear, then close the valve When the hissing stops and water begins to trickle out, the air has been fully released. Turn the valve clockwise to close it. Don't overtighten — it only needs to be snug.
7. Check your boiler pressure Bleeding radiators reduces the pressure in your central heating system. After bleeding, check the pressure gauge on your boiler. Most boilers should sit between 1 and 1.5 bar when cold. If it's dropped below 1 bar, you'll need to repressurise the system via the filling loop.
8. Turn the heating back on and check Run the heating again and check the previously cold radiators. They should now heat evenly from top to bottom.
How Often Should You Bleed Your Radiators?
Once a year is a good habit — typically in autumn before the heating season begins. If you find yourself bleeding the same radiators repeatedly throughout the year, it suggests air is entering the system somewhere, which warrants a closer look.
When Cold Radiators Mean Something More Serious
Bleeding won't fix everything. If you've bled your radiators and they're still cold — or if the bottom of the radiator is cold rather than the top — you're likely dealing with a different problem:
Cold spots at the bottom of the radiator This is almost always sludge — the black magnetite that builds up inside radiators over time. Bleeding won't shift it. A power flush of the system is usually the solution.
One radiator completely cold while others are fine The thermostatic radiator valve (TRV) may be stuck. This is common on older valves, especially after a summer when the heating hasn't been used. Try gently removing the TRV head and moving the pin underneath with a pair of pliers.
All radiators cold but the boiler is running Could be a failed pump, a blocked filter, or a pressure issue. This needs a Gas Safe registered engineer.
Radiators warm but one room never heats up properly May be a balancing issue — the valves across the system aren't calibrated correctly. An engineer can balance the system in an hour.
You're frequently losing pressure on the boiler If your boiler pressure keeps dropping and you're bleeding and repressurising regularly, there may be a leak in the system — in a radiator, a joint, or the boiler itself. Get it checked.
Need Help With Your Heating in Hertfordshire?
If bleeding doesn't solve the problem, or you'd rather have an engineer check the system properly, we're based in Watford and cover all of Hertfordshire. Call us on 0208 092 1359 or send a message — we'll get back to you quickly.