How to Bleed a Radiator (5-Minute Guide for UK Homes)
You can bleed a radiator in about 5 minutes with a tea towel, a small bowl, and a £2 radiator key. Turn the heating off and let the radiators cool. Find the bleed valve at the top corner of the radiator, hold the bowl underneath, slot the key in, and turn it slowly anticlockwise — about a quarter turn. You’ll hear a hiss as trapped air escapes. The moment water starts trickling out instead of air, close the valve again. That’s it.
You should bleed any radiator that’s cold at the top while the bottom is hot, or that’s not warming up properly. Cold-at-top usually means trapped air, which bleeding fixes in minutes. Cold-all-over usually means something else — sludge, a stuck valve, or low system pressure — and we’ll cover what to do about those further down.
What you need
- A £2 radiator key (B&Q, Wilko, Amazon — universal square fitting)
- An old tea towel
- A small bowl or mug
- 5 minutes per radiator
If you don’t have a key, a flat-blade screwdriver fits some modern bleed valves — but a key is cheaper than the time you’ll spend hunting for the right screwdriver.
Step-by-step
- Turn the heating off and let the radiators cool for 15–30 minutes. Bleeding hot, pressurised radiators sprays scalding water.
- Find the bleed valve. It’s the small square nut at the top corner of the radiator, usually on the opposite side to the thermostatic valve.
- Hold the tea towel and bowl directly under the valve.
- Slot the key on and turn slowly anticlockwise — about a quarter turn. You’ll hear a hiss.
- Wait for water. The moment air stops and a steady trickle of water appears, close the valve immediately by turning clockwise.
- Wipe up any drips and move to the next radiator.
Which radiator first?
Start with the radiator furthest from the boiler, then work back towards it. In a two-storey house, do all the downstairs radiators first, then go upstairs. Air rises through the system, so working low-to-high pulls trapped air out as you go instead of pushing it around.
Check the boiler pressure afterwards
Bleeding lets water out of the system, which drops the pressure. Once you’ve finished, look at the pressure gauge on the front of the boiler:
- 1.0 to 1.5 bar (cold) — normal. Do nothing.
- Below 1.0 bar — top up via the filling loop (the silver braided hose under your boiler). Open both taps until the gauge reads about 1.2 bar, then close them.
- Below 0.5 bar — the boiler may have shut down on low pressure. Topping up should bring it back; if it doesn’t, the boiler needs resetting per the manual.
Your boiler’s manual will show exactly where the filling loop is and how to use it. If it’s not obvious or you’re not sure, call us on 0116 488 6858 — we can talk you through it on the phone before anyone needs to come out.
Common problems and what they mean
| What you see | What it usually means |
|---|---|
| No air, no water comes out | Valve is seized or the bleed pin is closed inside — don’t force it; call a plumber |
| Water comes out black, not clear | Sludge build-up. The system needs a chemical or power flush, not just a bleed |
| You bled it last month and it’s air-locked again | A small leak somewhere in the system, or a failing automatic air vent on the boiler |
| Hissing won’t stop, water won’t come | Pressure is too low. Top up via the filling loop first, then bleed |
| Radiator cold all over even after bleeding | Stuck valve, sludge, or a circulation problem — needs a heating engineer to diagnose |
When to call a heating engineer
Bleeding is one of the few heating jobs you can safely DIY. But if any of these happen, stop and call a professional:
- The radiator is still cold all the way across after you’ve bled it
- The water that came out was black, not clear
- You’re losing pressure repeatedly even after topping up — that suggests a leak somewhere
- The boiler keeps cutting out even with pressure restored
- You can smell gas — leave the property and call National Grid on 0800 111 999 before doing anything else
For Leicester homes, our central heating service in Leicester covers radiator diagnostics, power flushes, and pump replacements. We’ll diagnose the actual cause before quoting a fix — no upselling a power flush when a £20 valve will do.
FAQs
Do I need to turn the heating off to bleed a radiator?
Yes. A hot, pressurised radiator will spray scalding water out of the bleed valve. Turn the heating off and let the radiators cool for at least 15–30 minutes before you start.
How often should I bleed my radiators?
Most UK homes need it once a year, usually at the start of the heating season in October. If you’re bleeding the same radiator every few weeks, there’s a deeper problem — typically a small leak or a failing air vent — and it’s worth getting it looked at.
What if water doesn’t come out when I bleed it?
The valve might be seized, or the bleed pin inside is fully closed. Try a tiny bit more turn, but don’t force it — you can snap the pin off, which means draining the system to fix. If nothing’s happening, a plumber can free it without breaking the radiator.
Do I bleed upstairs or downstairs radiators first?
Downstairs first, then upstairs. Air rises through the heating system, so working from low to high pulls the trapped air out as you go.
Why is my radiator cold at the top but hot at the bottom?
Trapped air at the top of the radiator. That’s exactly what bleeding fixes — it’s the most common reason to bleed.
Why is my radiator cold at the bottom but hot at the top?
That’s sludge, not air. Black, oxidised water has settled at the bottom of the radiator and is blocking circulation. Bleeding won’t fix it — the system needs a chemical flush or, if it’s bad, a power flush.
Need a heating engineer in Leicester?
If you’ve bled the radiators and something still isn’t right — pressure won’t hold, the boiler keeps cutting out, or the heating’s still patchy — we can help. We answer the phone 7 days a week and we’ll talk through what’s happening before sending anyone out.
📞 0116 488 6858 — answered 7 days a week
See our central heating service in Leicester →