How to Bleed a Radiator and What to Do If It Is Still Cold

Radiators fail in two distinct ways and the pattern of cold spots diagnoses it instantly.

Cold at the top, warm at the bottom — trapped air. Bleeding fixes it in two minutes.

Cold at the bottom, warm at the top — sludge. Magnetite, which is corroded iron, settling in the bottom. Bleeding will not help at all.

Cold all over while others are hot — a closed valve or a stuck thermostatic head.

Bleeding, properly

1. Turn the heating off and let the system cool for an hour. Bleeding a hot system risks scalding, and the circulating water makes it harder to tell when the air is gone.
2. Find the bleed valve — a small square socket at the top of one end of the radiator.
3. Hold a cloth and a small container underneath. The water that comes out is black and stains carpet permanently.
4. Turn the key anticlockwise a quarter turn. No further. You will hear a hiss.
5. Wait for the hiss to become a steady dribble of water, then close it immediately.
6. Repeat around the house, working from the radiator nearest the boiler outward, and downstairs before upstairs.
7. Check the boiler pressure afterwards. Bleeding removes water from the system. If the gauge has dropped below about one bar, top it up using the filling loop until it reads between one and one and a half bar cold.

If it is cold at the bottom

That is sludge, and it will not improve on its own. Options in order of cost:

Remove and flush the single radiator. Close both valves, note the number of turns on the lockshield so you can restore the balance, disconnect it, carry it outside, and run a hose through it until the water runs clear. Expect the water to be alarmingly black. This is a real job — a radiator full of water is very heavy — but entirely doable.

Have the system power flushed. A professional job using a pump and chemicals. Worth it if several radiators are affected.

Fit a magnetic filter. A small canister on the return pipe that collects magnetite before it settles. Should be standard on every system and often is not.

If it is cold all over

1. Check both valves are open. The thermostatic valve at one end and the lockshield at the other.
2. Remove the thermostatic head and look at the pin underneath. If it is stuck down, the valve is closed. Free it by tapping gently and pulling with pliers, or by working it up and down. Stuck pins after summer are extremely common.
3. If the pin moves freely and the radiator is still cold, the system may need balancing — the radiators nearest the pump are taking all the flow. Closing the lockshield valves on the hot radiators slightly pushes flow to the cold ones.

The order that saves time

Bleed everything first. Then balance. Then consider flushing. Most heating complaints resolve at step one, and the ones that do not are diagnosed clearly by which part of the radiator stayed cold.