How to Bleed a Radiator — Step-by-Step Guide

If a radiator is warm at the bottom but cold at the top, it has trapped air inside — and bleeding it takes about five minutes per radiator with a simple bleed key. This guide walks through the safe procedure, what to do if water doesn't come out, and how often the job needs doing.

What you'll need

• A radiator bleed key (£1–£2 from any DIY shop) or a flat-blade screwdriver — modern radiators usually take one or the other
• A small bowl or dish to catch water
• An old towel or rag
• Access to the boiler pressure gauge afterwards

If you don't have a bleed key and need to bleed urgently, a small flat screwdriver fits most modern radiator bleed valves. Avoid using pliers — they can damage the brass valve.

Step 1: Turn off the central heating

Switch the heating off at the thermostat or programmer. Wait at least 30 minutes for the system to cool — bleeding a hot radiator under pressure causes scalding water to spurt out and is genuinely dangerous.

You'll also get a better result on a cool system. When the pump is running, air is partly forced into solution and bleeds out incompletely.

Step 2: Find the bleed valve

The bleed valve is a small square or slot-headed valve at the top corner of the radiator — usually one corner only, sometimes the opposite end from the inlet pipe. It's a 6mm square brass fitting in older radiators and a 5mm or 4mm square in newer ones, occasionally a slot for a screwdriver.

If you can't see the bleed valve, check both top corners. Designer and column radiators sometimes have it on the side or at the top centre.

Step 3: Position the bowl and cloth

Hold the bowl directly under the bleed valve, with the cloth between the valve and your hand. Water will trickle out when the air finishes escaping — it's stained brown or black from corrosion residue and will mark carpets and floors if it drips.

Step 4: Open the valve a quarter turn

Insert the key or screwdriver and turn the valve anti-clockwise by no more than a quarter turn. You'll hear a hissing sound as trapped air escapes. Don't open it further — a half turn or more risks the valve coming out completely.

If you hear no hissing and water comes straight out, the radiator wasn't air-locked — close the valve again. Cold spots in this case may indicate sludge or a closed lockshield rather than air.

Step 5: Wait until water flows steadily

Keep the valve open until the hissing stops and a steady, drip-free trickle of water appears. This confirms all the air is out. The whole process usually takes 10–30 seconds.

Step 6: Close the valve firmly but not over-tightened

Turn the key clockwise until the water flow stops, then quarter-turn more. Don't crank it hard — over-tightening can damage the valve seat and cause future weeping.

Step 7: Check the boiler pressure

Bleeding releases water along with air, which drops the system pressure. Check the boiler gauge — should read 1.0–1.5 bar when cold. If it's dropped below 1 bar, repressurise via the filling loop before restarting the heating. See our repressurising guide for the steps.

Step 8: Restart the heating and test

Switch the heating back on. After 15–20 minutes, feel the radiator — it should now be hot evenly from top to bottom. If a cold spot remains, you may need to bleed again or the cause may not have been air after all.

Which radiators to bleed first

If multiple radiators need bleeding (common at the start of the heating season), follow this order:

1. Start with the radiator closest to the boiler on the ground floor
2. Work outwards through the rest of the ground floor
3. Move upstairs and bleed in the same outward order from the boiler
4. Finish with the radiator furthest from the boiler

This pushes any remaining air systematically towards the bleed valves rather than chasing it around the system.

How often should radiators be bled?

Most systems need bleeding once a year, at the start of the heating season (September or October). Air builds up gradually as the system cycles between hot and cold over the summer.

If you're bleeding more often than annually, there's an underlying issue:

• Air being drawn in. Usually a failed pump shaft seal or a low pressure system pulling air through bleed valves
• System pressure too low. Causes water to vaporise at high points
• Failed expansion vessel. Allows pressure to fluctuate, drawing air on each cycle
• Hydrogen generation from internal corrosion. If bleed water smells like rotten eggs or you can ignite the gas from the valve briefly, internal corrosion is producing hydrogen — needs a system flush and inhibitor top-up

If no air or water comes out

• Check the radiator is on. A closed TRV or lockshield means no water is reaching the radiator at all
• Check the system pressure. If the boiler is locked out from low pressure, repressurise first
• Check the bleed valve isn't blocked. Insert a fine pin (carefully, with the bowl underneath) to clear any debris in the bleed port

When to call a plumber

• Bleed water is black or contains visible debris — system needs flushing
• Radiator stays cold at the bottom after bleeding (sludge, not air)
• Radiator stays cold at the top despite repeated bleeding (air is being drawn in)
• Multiple radiators consistently need bleeding more than once a year
• You smell gas at the bleed valve (hydrogen — corrosion issue)

If radiators continue to underperform after bleeding, our central heating team can diagnose and resolve the underlying cause across all PE postcodes. See also our guide on radiators not heating up for the wider diagnostic walkthrough.