Bleeding a radiator is the single most useful bit of heating maintenance a homeowner can do, and it costs nothing but a few minutes. Trapped air collects at the top of radiators, leaving them cold up there and forcing your boiler to burn more gas to heat the rooms. Releasing that air gets your radiators working properly again - and most people can do every radiator in the house in under half an hour.
A radiator bleed key (a couple of pounds from any DIY shop, or a flat-head screwdriver for newer valves) and an old cloth or small container to catch drips.
Signs Your Radiators Need Bleeding
You don't need to bleed radiators on a schedule - only when they show the signs. The classic one is a radiator that's warm at the bottom but cold across the top, where the air has gathered.
- Cold patches at the top of one or more radiators
- Radiators slow to warm up, or never getting properly hot
- Gurgling or trickling sounds from the radiators
- The whole house feels like it's working harder to heat
Before You Start
Air rises, so to bleed effectively you want the system off and the air settled. Doing it with the pump running just stirs everything up.
- Turn the heating off and let the radiators go cold
- Lay down a cloth or towel - bleed water is often dirty and can stain
- Have something to top up boiler pressure afterwards (you'll likely need to)
Bleed a Radiator in Five Steps
It's the small square nipple at the top corner of the radiator, set slightly back. Modern valves take a flat screwdriver instead of a key.
Hold the cloth or container right under the valve to catch the water that follows the air.
A quarter to half a turn is plenty. You'll hear a hiss as the trapped air escapes - don't open it fully.
Once the hissing stops and a steady stream of water appears, the air is out. Close the valve snugly, but don't force it.
Dry the valve and the radiator, then move to the next one that needs doing.
Which Order to Bleed Them In
Order matters more than people realise. Start with the radiator on the lowest floor that's furthest from the boiler, then work your way towards the boiler and upwards. Working from the far, low point back means you push the air out efficiently rather than chasing it around the system. In a two-storey house, that usually means finishing on the upstairs radiator nearest the boiler.
Top Up the Boiler Pressure Afterwards
Every bit of air and water you let out drops the pressure in a sealed (combi or system-boiler) setup. If you don't top it back up, the boiler may lock out and refuse to fire.
Look at the pressure dial on the boiler. It should sit between 1 and 1.5 bar when cold.
Open the filling loop (a small silver braided hose under the boiler) until the needle reaches about 1.5 bar, then close it.
Turn the heating back on and feel each radiator - they should now heat evenly from top to bottom.
How Often Should You Bleed Radiators?
There's no fixed schedule - bleed them whenever you notice cold tops or gurgling, and it's worth a quick check at the start of each heating season in autumn. If you find yourself bleeding the same radiator every few weeks, that's not normal: air is getting back in somewhere, which can point to a leak, a failing pump seal, or corrosion creating gas inside the system. That's worth an engineer's eyes.
Still Cold After Bleeding?
If a radiator stays cold even after a proper bleed, the problem isn't air. Cold at the bottom means sludge; cold all over means a valve - our guide on why a radiator won't heat up walks through both. And if no radiators are heating at all, it's the boiler - start with the boiler not working checklist. No heat at all on a freezing night? You can get an emergency heating engineer on the way on the way fast.