Top of the radiator stone cold but the bottom's warm? That's trapped air. It takes about 60 seconds per radiator to fix, you don't need a plumber, and once you know the order to do them in you'll never have to think about it again.
Here's the no-nonsense engineer's method we teach customers on the phone every winter.
First — do you actually need to bleed them?
The 60-second test:
- Set your heating to come on for 10 minutes
- Wait until every radiator is calling for heat
- Run your hand from the top of each rad down to the bottom
Cold top, warm bottom = trapped air. Bleed it.
Cold bottom, warm top = sludge. Bleeding won't help — you probably need a powerflush. See our 7 signs you need a powerflush for a proper check.
Cold everywhere = thermostatic valve closed, or zone valve fault. Turn the TRV head fully open and wait. If still cold, call us.
What you'll need
- A radiator bleed key (£1.50 from any DIY shop — most modern rads also take a flat-blade screwdriver)
- An old towel or jug to catch the water — it will be dirty, do not let it touch carpet or pale flooring
- Access to your boiler filling loop for the pressure top-up afterwards
The right order — downstairs first, then upstairs
Air naturally rises, so working from the lowest rads in the house up to the highest pushes the air ahead of you and avoids re-trapping it. Order:
- The rad furthest from the boiler on the ground floor (usually the lounge or dining room)
- Work your way back through the ground floor radiators
- Then upstairs, again furthest from the boiler first
- Towel rails (bathroom) almost always last — they're typically the highest point in the system
Step-by-step — one radiator at a time
- Turn the heating off. Pump pressure makes bleeding messier and less effective. Let the system sit for 10 minutes to cool slightly.
- Find the bleed valve — small square nipple at the top, on one end of the radiator.
- Hold the towel/jug under the valve.
- Turn the bleed key anti-clockwise — quarter to half a turn, no more. You'll hear a hiss of air.
- Wait for the hiss to become a steady drip of water. When water trickles out cleanly without spitting, the air is gone.
- Turn the key clockwise to close. Snug, not gorilla-tight — over-tightening damages the valve seat.
- Wipe the valve dry and move on to the next radiator.
Now top up the boiler pressure
Bleeding drops your system pressure — the boiler will lock out if it falls below 0.5 bar. Check the pressure gauge on the boiler:
- Should read: 1.0–1.5 bar when cold (the green band)
- If lower: top up via the filling loop — usually a silver flexible hose under the boiler with two black tap handles
To top up:
- Open both filling-loop taps slowly until the gauge needle moves
- Stop at 1.2 bar (mid-green)
- Close both taps firmly
- Reset the boiler if it locked out (most modern boilers auto-reset once pressure is back)
Step-by-step pressure top-up with diagrams is in our boiler pressure guide.
How often should you bleed radiators?
For a healthy Lancashire heating system: once a year, ideally in early autumn before the heating goes on full-time. If you're bleeding more than every 6 months, you have a hidden problem:
- Air keeps coming back = a leak somewhere is sucking air in. Often a failing automatic air vent, a perished radiator valve, or a microleak in pipework. Call us — it gets worse, not better.
- Pressure keeps dropping but no visible leak = heat exchanger pinhole or PRV discharge. See our losing pressure guide.
Common mistakes we see
- Bleeding with the pump running. Hot water sprays everywhere. Always turn the heating off first.
- Opening the valve too far. Half a turn is plenty. A full turn can pop the bleed nipple out — and it's impossible to re-fit on a pressurised system.
- Forgetting to top up pressure. System then locks out on a cold night and you've got no heat.
- Bleeding in the wrong order. You end up doing some rads twice.
When to stop and call an engineer
Call 01282 914 044 if:
- Brown/black water comes out instead of clear — that's sludge, you need a powerflush
- You bleed the rad, it's still cold at the top after 20 minutes of heating
- The bleed valve won't turn or won't seal again afterwards
- Boiler keeps cutting out on low pressure within hours of topping up
The 60-second post-job test
Once you've bled every rad and topped up pressure, fire the heating back on, wait 15 minutes, and run your hand down each radiator again. Hot top, hot bottom on every single one = job done.
If you're in Burnley, Blackburn, Preston, Nelson or anywhere across Lancashire and a radiator just won't play ball after bleeding, give us a ring — sometimes 5 minutes on the phone diagnoses what 30 minutes of YouTube can't. Call 01282 914 044.