How to Bleed and Balance Radiators

1. Turn the heating off and let the radiators cool — hot water can spray out.
2. Hold a cloth under the bleed valve at the top corner of the radiator.
3. Turn the valve anticlockwise about a quarter turn with a radiator key. You'll hear air hiss out.
4. When water starts to come out steadily, close the valve.
5. Do the rest, starting with the radiator nearest the boiler, then check the boiler pressure.

On a sealed system — most combis and system boilers — bleeding lets pressure out. Check the gauge with the heating cold: most want around 1–1.5 bar, but follow the boiler's manual.

1. Find the filling loop, usually a braided hose or a lever under the boiler.
2. Open its valves slowly and watch the gauge rise to the right pressure.
3. Close both valves firmly. If the loop is a removable hose, take it off afterwards.

Cold at the top, warm at the bottom means air — bleed it. If it keeps coming back, air is getting in somewhere and an engineer should find out why.

Cold at the bottom, warm at the top means sludge. Bleeding won't fix it; the system needs cleaning, then inhibitor and a magnetic filter — see the filter and inhibitor guide.

Cold all over while others are hot is usually a stuck TRV pin, a closed lockshield or an unbalanced system.

Balancing sets the lockshield valves so water is shared fairly. Without it, the radiators nearest the boiler take most of the flow and the furthest ones stay lukewarm.

1. With the heating off and cold, open every TRV fully and every lockshield (under the cap) fully.
2. Turn the heating on and note the order the radiators warm up.
3. On the first to warm up, close the lockshield, then open it a quarter to half a turn. Open the next ones a little more, and the furthest fully.
4. Wait, then fine-tune until they all heat up at a similar rate. With a pipe thermometer, aim for the flow and return temperature drop the boiler maker recommends.
5. Set the TRVs back to the temperatures you want.

Each radiator has two valves. The TRV (thermostatic radiator valve) has a numbered head and opens and closes on its own to hold the room temperature. The lockshield has a plain cap and is set once, when balancing, then left alone.

A TRV that won't open after summer often has a stuck pin: take the head off and gently free the pin. Old valves that weep or won't turn are worth replacing while the system is drained.