Heating
Bleeding a radiator, properly
Two minutes a radiator, in the right order, with the pressure checked afterwards. And the one symptom that bleeding will never fix.
Written by the ARA Services engineering team · Updated August 2026
The short answer
Turn the heating off and let the radiators cool. Start with the radiator nearest the boiler on the lowest floor and work outwards and upwards. Open each bleed valve a quarter turn until the hissing stops and water appears, then close it. Check the boiler pressure afterwards and top up if it has dropped. Cold at the top means air; cold at the bottom means sludge, and no amount of bleeding will shift that.
The order matters
Air collects at the highest points, but it moves as the water moves, so working at random pushes it around the system. Heating off and cool, then start downstairs at the radiator closest to the boiler and work away from it, finishing with the highest radiator in the house.
You need a bleed key, a cloth and a small container. Towel rails and any radiator that has been moved or added since the system was installed are the ones that hold air most stubbornly.
Not sure which radiator is nearest the boiler? Describe the layout and we will tell you where to start.
077 2715 4746Doing it
Turn the heating off and give it half an hour. Bleeding a hot system risks scalding and lets air straight back in.
Fit the key to the square pin at the top of the radiator and turn a quarter turn anticlockwise — no further.
Let it hiss. When the hiss turns into a steady dribble of water, close the valve. That radiator is done.
Do not remove the valve completely, and stop if water comes out under pressure with no air first — there is nothing to bleed.
Then check the gauge. Bleeding releases pressure as well as air. On a sealed system the gauge will drop, and if it falls below about 1 bar the boiler may lock the heating out. Top up through the filling loop as the manual describes.
Pressure dropped and will not hold? That is worth a look before winter.
Heating repairsWhat bleeding will not fix
Air is only one of four reasons a radiator stays cold, and the pattern of the cold tells you which one you have.
Cold at the bottom
Sludge and corrosion debris settled in the bottom of the radiator. Bleeding does nothing. The system needs chemical cleaning or a powerflush, and a magnetic filter fitting to stop it coming back.
Stone cold, top and bottom
Usually a thermostatic valve seized shut. Take the head off and check the pin moves freely; if not, the valve or the head needs replacing.
One floor hot, another cold
A balancing problem, or a tired circulation pump. Balancing means adjusting the lockshield valves so the far radiators get their share of the flow.
Air back within weeks
Air being drawn in through a small leak, or hydrogen produced by corrosion inside the system. Both need finding rather than bleeding again.
Cold at the bottom, or the same radiator every year? That is a cleaning job, not a bleeding job.
System cleaning and balancingOnce a year is enough
Bleed at the start of the heating season, ideally as part of the annual service, and then leave the system alone. Repeated bleeding through the winter usually means something is wrong: a leak somewhere, an expansion vessel that has lost its charge, or corrosion that needs inhibitor putting back into the system.
Cold radiators with the heating due to run again next week? Get an engineer out now, not in December.
Call 077 2715 4746Radiator questions
How often should I bleed my radiators?
Once a year, at the start of the heating season. More often than that is a symptom rather than maintenance.
Why does air keep coming back?
Either air is being pulled in through a small leak or a failing vessel, or the system is corroding and producing hydrogen. Both are fixable, but not by bleeding.
Should I bleed the towel rail too?
Yes, and it is often the worst offender because it sits high and holds air easily.
The bleed valve is dripping now. Is that serious?
It usually means the valve did not seat properly or the washer has gone. Close it gently; if it still weeps, it needs replacing, which is a quick job.
Get the heating checked
Bled the radiators and still cold? Tell us the postcode and what is happening and we will book a slot.