How to Bleed a Radiator and Get the Heat Back Fast

The heating is on, the boiler is working, yet one radiator is warm at the bottom and stone cold at the top. That cold band is almost always trapped air, and learning how to bleed a radiator fixes it in about five minutes with a key that costs less than a coffee.

This guide covers the tools, the exact steps, how to top up the boiler pressure afterwards, and the few situations where bleeding will not help. It is written for the standard wet central heating found in most homes, with a gas or oil boiler and panel radiators.

Signs you need to bleed a radiator

Air rises, so it collects at the top of each radiator and stops hot water filling that space. Look for these clues:

• The top 5 to 15 cm of the radiator is cool while the bottom is hot.
• You hear gurgling, trickling or knocking when the heating starts.
• Rooms take noticeably longer to warm up than last winter.
• Radiators on the top floor are worse than those downstairs.

If a radiator is cold at the bottom but warm at the top, that is a different problem, usually sludge, and bleeding will not fix it. More on that below.

What you need before you start

• A radiator key. A small brass or steel key that fits the square bleed valve. They cost very little in any hardware shop, and it is worth buying two because they vanish. Some modern valves take a flat-head screwdriver instead.
• An old cloth or tea towel to catch drips.
• A small cup or jar in case water comes out faster than expected.
• Access to your boiler's pressure gauge, usually a small dial on the front or underneath.

Clear the area under the valve too. Move rugs, curtains and anything electrical away from the radiator, because the water inside is often dark grey or black and stains fabric easily. If the radiator sits behind a sofa, pull the sofa out by 30 cm rather than working blind. A torch or phone light helps you see the valve pin clearly, especially on radiators fitted low under a window.

How to bleed a radiator step by step

1. Turn the heating off and wait. Give it 30 to 60 minutes so the water cools. Hot water can spit out at the valve and scald you, and a pump running while you bleed can pull more air in.
2. Find the bleed valve. It sits at one of the top corners of the radiator, a small round fitting with a square pin in the middle.
3. Hold the cloth under the valve. Fold it a couple of times and keep it pressed below the valve with one hand.
4. Turn the key anticlockwise a quarter to half turn. You should hear a hiss. That is the air escaping. Do not unscrew it further; the pin can come out completely and you will be chasing it across the floor while water runs.
5. Wait for water. When the hiss stops and a steady dribble of water appears, the air is out. This usually takes 10 to 30 seconds.
6. Close the valve clockwise. Snug, not forced. Overtightening damages the seat and causes slow leaks.
7. Repeat on other radiators. Start downstairs at the radiator furthest from the boiler and work your way up and toward it.
8. Check the boiler pressure. See the next section, because this is the step people forget.
9. Turn the heating back on and feel each radiator after 20 minutes. It should be evenly hot across the top.

Topping up boiler pressure afterwards

Bleeding lets out a little water as well as air, and in a sealed system that lowers the pressure. If it drops too far, most combi boilers lock out and show a fault code, which can look as if you broke something.

Check the gauge with the heating cold. On most domestic boilers, the target is between 1 and 1.5 bar. If it reads below 1, top it up:

1. Find the filling loop, a braided hose or a pair of small taps under the boiler.
2. Open both taps slowly, or the single lever, and watch the needle rise.
3. Close them at around 1.2 to 1.3 bar. It climbs fast, so go slowly.
4. Reset the boiler if it shows a fault.

Your boiler manual will show the exact layout. If you have an older open-vented system with a small tank in the loft, it refills itself and there is no gauge to check.

Why bleeding every radiator once a year is not the answer

A lot of home advice says to bleed every radiator at the start of every heating season, as routine maintenance. We do not think that is good advice for most homes.

In a healthy sealed system, very little air gets in. If nothing is cold at the top, bleeding achieves nothing except letting out water and lowering pressure. Each top-up then adds fresh water, which brings in new dissolved oxygen, and oxygen is what corrodes the inside of steel radiators and creates the black sludge you are trying to avoid.

Instead, bleed only when a radiator shows symptoms. And if you find yourself bleeding the same radiator every couple of weeks, stop treating the symptom. Repeated air usually means one of three things: a small leak somewhere drawing air in, corrosion producing hydrogen gas inside the system, or a pump or air vent problem. A heating engineer can test for these, and an inhibitor added to the water often helps the corrosion case. If you do need to pick up other small fixes around the house this season, our guide to stopping condensation on windows pairs well with getting the heat working properly.

When bleeding does not fix it

• Cold at the bottom, hot at the top. Sludge has settled in the base. The fix is a power flush or removing and flushing the radiator, which is a bigger job.
• Cold everywhere. Check that both valves at the bottom are open. The thermostatic valve may be stuck; tap the pin under its head gently.
• Some radiators hot, others barely warm. The system needs balancing, which means adjusting the lockshield valves so water is shared evenly.
• No water comes out at all. Pressure may be too low already, or the valve is blocked with sediment.

For more step-by-step fixes like this one, browse the how-to guides.

Frequently asked questions

Should the heating be on or off when you bleed a radiator?

Off, and cooled down. With the pump running, air can be pulled back in, and hot water can spit from the valve. Turn the heating off for 30 to 60 minutes first, then bleed, then turn it back on.

Which radiator should I bleed first?

Start on the ground floor with the radiator furthest from the boiler, then work closer and move upstairs. Finishing upstairs catches the air that has risen there. In a bungalow, simply work from the furthest radiator toward the boiler.

How often should you bleed radiators?

Only when one shows a cold top or makes gurgling noises. For many homes that means once a year or less. If you need to bleed the same radiator every few weeks, ask a heating engineer to look for a leak or corrosion.