Every October it's the same story: you turn the heating on for the first time and one radiator is warm at the bottom and stone cold at the top. Nine times out of ten it's air trapped in the system. The room takes twice as long to warm up, the boiler works harder than it needs to, and the fix takes literally five minutes. Here's the complete guide to how to bleed radiators yourself, safely, plus what to check if one of them still won't heat up afterwards.
How to tell if your radiator has air in it
Not every cold radiator is full of air. Look at the pattern:
- Warm at the bottom, cold at the top: that's air. Bleeding will sort it.
- Cold at the bottom, warm at the top: that isn't air, it's sludge and rust sitting in the bottom. That needs the system flushing, and it's a job for a plumber.
- The whole radiator cold and the inlet pipe cold too: the valve is closed, or the pin under the thermostatic head has seized.
- Sloshing, bubbling or gurgling: the classic sign of air moving around the system.
What you need
- A radiator bleed key (the usual square one) or a flat-head screwdriver, depending on the valve your radiator has.
- A small container or a glass to catch the water.
- An old cloth. The water in the system comes out murky and it stains.
- Gloves if your heating is already hot: the water can come out at over 60 °C.
How to bleed radiators step by step
- Switch off the boiler or the pump and wait 30 to 60 minutes. With the water still, the air rises to the high points and comes out in one go instead of circulating.
- Start with the lowest radiator closest to the boiler and finish with the highest and furthest away. In a building with several floors, work from the bottom up.
- Put the container under the bleed valve and hold the cloth just below it.
- Open the valve a quarter to half a turn anticlockwise. Don't unscrew it completely: it can come out altogether, and then you really do have a problem.
- You'll hear a hiss. Keep it open until a steady stream of water comes out with no bubbles.
- Close it without forcing it. Overtightening damages the washer and it ends up dripping.
- Repeat on the rest of the radiators in the same order.
- Check the pressure. Bleeding takes water out of the system. With the heating cold, 1 to 1.5 bar is normal. If it's dropped below 1 bar, top it up using the filling loop and bleed the highest radiator again.
I've bled it and it's still cold: what to check
Before you call anyone out, check these four things:
- A seized pin. Unscrew the thermostatic head and take it off. Underneath you'll see a small metal pin. Press it a few times: it should drop and spring back on its own. If it's stuck, work it gently with pliers; if it won't budge, the head needs replacing.
- The valves. Make sure the inlet valve is open and the lockshield on the return is too.
- Sludge. If it's cold at the bottom, bleeding won't achieve anything. It needs a chemical flush.
- A whole cold riser in a block of flats. That isn't down to your home: report it to the building management.
If you're having to bleed them every few weeks, air is getting into the system somewhere. It's worth having it looked at before the cold really sets in.
Getting more heat out of the same radiator
While you're there, make the most of it:
- Vacuum inside the fins. Built-up dust acts as insulation and, when the radiator kicks in, it spreads it around the whole room.
- Don't block it. A long curtain in front sends the heat straight to the glass, and a sofa pushed up against it absorbs the lot.
- Reflective panel behind it if the radiator is on an external wall.
- Leave the thermostatic head out in the open. Boxed into a cabinet, it misreads the temperature and shuts off too early.
- Set the temperature sensibly: 20-21 °C in the living room and 18 °C in the bedroom. Every extra degree shows up on the bill.
- Air the room with a through draught for 5-10 minutes rather than leaving a window ajar all morning, and close the thermostatic head while you do it.
If you're getting the house ready for autumn, the Velia Select shop has home and cleaning products to get everything sorted before the frosts arrive.
How often should you bleed radiators?
Once a year is enough for most homes, and the best time is right before you switch the heating on, between late September and October. Do it again every time you top up the water in the system, because fresh water always brings air in with it.