Bleeding a radiator is the one heating job we actively encourage homeowners to do themselves. It takes five minutes, costs nothing, and fixes a genuinely common problem. But cold radiators can also be a symptom of something more serious — here's how to tell the difference before you reach for the bleed key.
What you'll need
- A radiator bleed key — £1–2 from any hardware shop, or a flathead screwdriver works on some modern valves
- A cloth or old towel to catch drips
- That's it
When to bleed: what you're looking for
Put your hand along the top of each radiator when the heating has been on for 15 minutes. If it's warm at the bottom but cold or lukewarm at the top, there's trapped air — that's exactly what bleeding fixes.
Important distinction: if the radiator is cold at the bottom rather than the top, that's sludge — bleeding won't help. See below.
Step-by-step
Step 1 — Turn the heating on. Run it for 15 minutes to identify which radiators have cold tops and confirm the system is pressurised.
Step 2 — Turn the heating off and wait. Give the radiators 20–30 minutes to cool. You don't want to bleed a hot pressurised system.
Step 3 — Find the bleed valve. It's the small square or slotted nut at the top of the radiator, usually on one end. Hold your cloth beneath it.
Step 4 — Turn it anticlockwise — just a quarter turn. You'll hear a hiss as air escapes. Keep the cloth underneath; some water will follow the air.
Step 5 — Close it when water appears. As soon as a steady trickle of water comes out, close the valve clockwise. Don't overtighten — snug is enough.
Step 6 — Check your boiler pressure. Bleeding releases water from the system and drops pressure. Check the gauge on your boiler — it should sit between 1.0 and 1.5 bar when cold. If it's dropped below 0.8 bar, repressurise via the filling loop (the small lever or valve under the boiler — open slowly until the gauge reads 1.2 bar, then close it).
Step 7 — Test. Turn the heating back on and check those radiators. They should now heat evenly top to bottom.
How often should you bleed?
Once a year before winter is a sensible habit. In Hertfordshire's hard water areas — 300–400ppm — slightly more frequent air ingress is common due to the higher rate of internal corrosion causing micro-bubbles. If you're bleeding the same radiators every couple of months, that's a different problem.
When bleeding won't fix it
Cold spots at the bottom of the radiator. This is sludge — magnetite that's settled and blocked flow. Bleeding won't touch it. The fix is a power flush, especially common in properties with older systems or those that have never had inhibitor added.
The whole radiator is cold but others on the same system are warm. The thermostatic radiator valve (TRV) is probably stuck — a common issue after summer when the heating hasn't been used. Try removing the TRV head and moving the pin underneath with pliers. If it's seized solid, the valve needs replacing (a straightforward job, usually £80–£120 + VAT).
Multiple radiators are cold after bleeding. Could be a pump issue, a balance problem, or a blockage in the pipework. This needs an engineer.
You're bleeding frequently and the boiler pressure keeps dropping. There's almost certainly a leak somewhere in the system — in a joint, a radiator valve, or the boiler itself. Don't ignore a continuously dropping pressure reading. Call us — finding a leak early is far cheaper than dealing with water damage.
When to call us
We're happy to bleed radiators as part of a wider heating check, but if your issue is more than trapped air — persistent cold spots, pressure loss, or a system that just isn't heating the way it used to — call us on 0208 092 1359. We cover all of Hertfordshire and can usually get to you same day.