How to bleed a radiator (and what the cold spots mean)

First, tell air from sludge

Air is lighter than water, so it collects at the top of the panel. Feel the radiator with the heating off: cold across the top and warm at the bottom is trapped air.

Sludge, the black magnetite grit that wears off the inside of steel radiators, sinks. Cold at the bottom with a hot top, often with a grey or brown stain near the base, is sludge. It usually shows first on the radiator nearest the boiler.

How to bleed a radiator

Work through the house starting with the radiator furthest from the boiler and finish on the closest, so the air moves towards the valve you are opening.

• Turn the heating off and let the panels cool, a hot radiator sprays scalding water when you open the valve
• Find a bleed key or a flat screwdriver, most modern valves have a square or slotted head at the top corner
• Wrap a cloth round the valve and hold a cup under it to catch the dribble
• Turn the screw a quarter turn anticlockwise and wait for the hiss to stop
• Tighten it back up by hand and no further, over-tightening splits plastic threads
• Top the boiler pressure back to between one and one and a half bar, cold

Check the pressure afterwards

Every radiator you bleed drops the system pressure a little, because water takes up the space the air left behind. If the gauge was already low when you started it will be lower still when you finish, and a boiler short of pressure locks out before it heats anything.

Look at the gauge again the next morning. If it has dropped with no bleeding involved, water is escaping somewhere and the pressure post explains where it goes.

When bleeding is not the answer

A radiator that comes back cold a week later has a cause behind it, and bleeding is only treating the symptom.

• Cold at the bottom every time, sludge in the panel or the pipework below it
• One radiator cold while the rest are hot, a stuck or closed thermostatic valve pin
• Pressure falling after every bleed, a leak somewhere on the heating circuit
• Air returning within weeks across the whole house, the system is drawing air in

What we look at when the heating will not warm up

Not one radiator but the whole circuit: pump speed, inhibitor strength, whether a magnetic filter was ever fitted, and how the system was balanced when the radiators went in. An unbalanced system leaves the rooms furthest from the boiler lukewarm all winter, and no amount of bleeding changes that.