Bleed a radiator
A radiator that's warm at the bottom and cold at the top has air trapped inside. Letting it out takes a radiator key, which costs a few dollars, an old towel for drips, and about ten minutes per radiator. Start with the radiator farthest from the boiler and work your way back, and check the boiler pressure afterward, since letting air out can drop it a little.
Why air gets trapped
Your radiators are part of a closed loop: the boiler heats water and pumps it through each one. Over time, air works its way in and rises to the top of the radiator, where it sits in a pocket the hot water can't reach. The small valve at the top corner is there to let that air out. Once it's gone, water fills the whole radiator and it heats evenly again.
This is for hot-water radiators, the kind with a small square or slotted valve at a top corner. Steam radiators have a bullet-shaped vent on the side and bleed themselves, so leave those alone.
What else it could be
The classic sign: cold at the top, warm at the bottom. Bleeding fixes it.
Cold at the bottom but warm at the top means rust and sludge have settled inside. That needs a flush, not a bleed.
If the boiler pressure is too low, water can't push up to every radiator, especially upstairs.
Thermostatic valves have a small pin under the head that can stick. If one radiator stays cold top to bottom, that's a likely suspect.
| What you notice | Likely cause | Fix |
|---|---|---|
| Cold at the top, warm at the bottom | Trapped air | Bleed it. |
| Cold at the bottom, warm at the top | Sludge inside | Needs a system flush. |
| Completely cold, others are warm | Stuck valve or closed valve | Check both valves, tap the valve pin. |
| Upstairs radiators cold, downstairs fine | Low system pressure | Check and top up the boiler pressure. |
| Gurgling or banging | Air in the system | Bleed the radiators, starting downstairs. |
Let the air out
- Turn the heat offSwitch the heating off and give the radiators 20 to 30 minutes to cool. The water inside can be scalding, and air collects better when the pump isn't running.
- Find the bleed valveLook at the top corner, usually on the opposite end from the pipe that feeds it. It's a small round valve with a square pin or a screwdriver slot in the middle.
- Open it slowlyHold the towel and cup under the valve. Fit the key and turn it counterclockwise about a quarter turn. You'll hear air hiss out. That's what you want.
- Close it when water comesWhen the hiss stops and a steady trickle of water appears, turn the key clockwise to close it. Snug, not forced.
- Check the pressureTurn the heat back on. Look at the pressure gauge on the boiler. If it dropped below its usual level, top it up following the boiler's instructions, then check each radiator warms top to bottom.
Watch it done
- No more hiss when the valve is opened, only water
- Each radiator warms from top to bottom
- Boiler pressure is back at its usual level
One radiator still cold? Check the pin on its thermostatic valve.
If it stays cold after bleeding
If one radiator stays completely cold, take the head off its thermostatic valve and gently tap the pin underneath. It should spring up and down.
If you have to bleed the same radiator every week or two, air is getting in somewhere. Have the system checked for a small leak or a faulty part.
What a pro typically charges (a rough estimate; prices vary by area): A heating technician typically charges $100–$250 for a visit. A full system flush for sludge costs more, often $300–$700.
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