How to Bleed a Radiator


Bleeding a radiator is an essential maintenance task that helps ensure your home’s heating system runs efficiently. Air can enter the system and accumulate in radiators, reducing their effectiveness. By bleeding the radiators, you release trapped air, allowing hot water to circulate fully and provide uniform heat throughout your home.

Bleeding radiators is a straightforward process that most homeowners can do themselves. All you need are a radiator key, cloth, and a container to catch the water. With a little time and patience, you can bleed all the radiators in your home. We will walk you through the complete process step-by-step in this guide.

When to Bleed Radiators

Here are some signs that indicate it’s time to bleed your radiators:

  • Cool spots at the top of a radiator, with the bottom being hot. This shows air is trapped in the radiator.
  • Gurgling sounds or hissing noises coming from a radiator. Air bubbles moving through the radiator system cause these noises.
  • Radiators take a long time to heat up or don’t get hot even after the heating has been on for a while.
  • Lower than normal heat output from the radiator system even when the heating is switched on.
  • Increased heating expenses as the system works harder to compensate for trapped air.

As a general maintenance practice, you should bleed your radiators at the start of the heating season or any time the above symptoms appear. Bleeding once a year should be sufficient in most cases.

How to Bleed a Radiator Step-By-Step

Follow this simple process to effectively bleed the radiators in your home:

Gather the Necessary Supplies

You will need:

  • Radiator key – This special bleed key fits into the radiator bleed valve and allows you to open it.
  • Cloth or rag – For catching any drips of water and wiping up spills. An old towel or piece of cloth works well.
  • Container – A bucket or bowl to catch the water drained from the radiator.
  • Gloves (optional) – To protect your hands from hot water.

Turn Off the Heating System

Start by turning off your home’s heating system. Locate the main switch or valve that shuts off the boiler and heating pipes. This prevents hot water from entering the radiators while you’re bleeding them.

Locate the Bleed Valve

Examine each radiator and find the bleed valve. This small square or hexagonal valve is located at the top of one end of the radiator. Modern radiator bleed valves use a square radiator key, while older valves may need a special hex key.

Place the Cloth and Container Underneath

Position your cloth or rag below the bleed valve to catch any drips. Place the bucket or bowl under the valve as well. Opening the valve will release a stream of water, so having these in position prevents a mess.

Open the Bleed Valve with the Radiator Key

Insert the radiator key and turn it counter-clockwise to open the valve. You will hear a hissing sound as air starts to escape.

Allow the Air to Escape

Keep the valve open until water starts dripping out in a steady stream. This shows all the trapped air has been released.

Initially, you may see spurts of water mixed with air bubbles. Let this drain out into the container until there is a constant trickle of water.

Close the Bleed Valve

When only water flows out, use the radiator key to turn the bleed valve clockwise. This closes the valve and stops the water from draining out.

Make sure to fully tighten the valve to prevent leaks. Check that water has stopped flowing before removing the cloth and container.

Turn the Heating Back On

Once all radiators have been bled, you can turn your heating system back on.

Set the thermostat to the desired temperature. The radiators should heat up faster and more evenly than before.

Troubleshooting Tips

Here are some troubleshooting tips in case you have difficulty bleeding a radiator:

  • If no water or air comes out when you open the bleed valve, tap gently around the valve with the radiator key. This helps dislodge any blockages.
  • Try closing the valve and opening it again. Repeating this a few times can help start the flow of water.
  • Older radiators may need extra force to open. Insert the radiator key and press down firmly while turning it counter-clockwise.
  • If the bleed valve is damaged or rounded out, you may need a replacement bleed valve. Contact a heating contractor.
  • For stubborn air pockets, bleed the radiator from the lowest point by attaching a hose. Consult a heating professional to do this properly.

How Often Should You Bleed Radiators?

  • Bleed radiators at the start of each heating season – This prevents air from accumulating over the summer.
  • Any time you notice signs of air trapped in the radiators – Don’t wait until your system is badly affected before bleeding.
  • After doing any work on the heating system – Servicing the boiler or replacing parts can introduce air into the system.
  • Once per year as routine maintenance – Annual bleeding keeps your radiators working at peak efficiency.
  • When you move into a new home – Bleed the radiators to ensure the heating system works properly in your new home. The previous owner may not have maintained them well.

Signs of Effective Radiator Bleeding

When you have successfully bled the radiators, you will notice these signs:

  • Radiators heat up quicker when the heating turns on
  • The top and bottom of the radiator are the same temperature
  • No more cool spots at the top of the radiator
  • Radiators emit consistent heat across the whole surface
  • No more gurgling noises or hissing sounds from the radiators
  • The system requires lower water pressure to function well
  • Rooms heat up faster and the home stays warmer
  • Lower heating bills as the system doesn’t need to work as hard

Radiator Bleed Key Types

There are two main types of radiator bleed keys to open the valve:

Square Bleed Keys

Most modern radiators use square-shaped bleed valves. The special square key fits over the valve and allows you to turn it counter-clockwise. This is the most common type you will find.

Hex (Hexagonal) Bleed Keys

Older cast iron radiators often have hexagonal bleed valves. You need a hex bleed key with a hex-shaped end that fits over the valves to open them.

Make sure you have the proper bleed key for your valve shape before starting. The keys are inexpensive and available at hardware stores.

How to Bleed a Radiator Without a Key

If you don’t have a radiator bleed key, try these methods:

  • Use pliers – Grip the valve firmly with pliers turned sideways. Protect the valve with a cloth and slowly turn counter-clockwise.
  • Use a small wrench – A 6mm wrench fits over some square radiator valves. Carefully turn it counter-clockwise.
  • Use a screwdriver – For slotted valve heads, insert a sturdy flathead screwdriver and turn gently. Protect the valve with a cloth first.
  • Use a coin – Insert a large coin into the slot and slowly turn counter-clockwise. Protect the valve first and be gentle.
  • Use vice grips – Lock vice grips onto the valve after protecting it with a cloth. Slowly turn the vice grips counter-clockwise.
  • Buy a replacement key – If you can’t find a DIY solution, buy a proper radiator bleed key. Don’t force the valve and damage it.

How to Bleed a Radiator That’s Stuck or Seized

If the radiator bleed valve is stuck shut and won’t turn, try these techniques:

  • Spray lubricant like WD-40 into the valve and let it sit for 15 minutes. The lubricant will loosen up stuck parts.
  • Tap the valve gently with a hammer. Be careful not to damage the valve body. The vibration can help free stuck components.
  • Push down firmly on the radiator key while turning it to break loose a seized valve. Apply penetrating oil and let it work before retrying.
  • Heat helps expand metal. Warm the valve with a hair dryer set on low for 30 seconds before attempting to open it.
  • For corroded square bleed valves, scrape out corrosion with a small flat screwdriver. Apply lubricant and open it with pliers.
  • If the bleed valve cannot be freed, replace it. A new bleed valve will ensure you can easily bleed the radiator in the future.

Why Does Air Get Trapped in Radiators?

Here are the main reasons air can build up inside radiators:

  • System leaks – Leaks in heating pipes and connections allow air to enter. Even small leaks introduce air over time.
  • Low system pressure – Insufficient water pressure doesn’t keep the system full of water. Air gets sucked in.
  • Servicing work – Work on radiators or the boiler releases trapped air into the system.
  • Faulty pump – A worn circulation pump can’t efficiently move water, allowing air to accumulate.
  • Damaged bleed valves – Cracked or leaking bleed valves let air enter but don’t let it back out.
  • Corrosion – Rust and mineral deposits inside radiators provide places for air bubbles to form.
  • Temperature changes – Heating cycles create expansion and contraction that releases trapped air.

Regular bleeding allows air to escape so it can’t reduce your heating efficiency. Fixing leaks and maintaining proper water pressure prevents air from building up continuously.

Safety Tips for Radiator Bleeding

Follow these important safety precautions when bleeding radiators:

  • Turn the heating off – Shut off the boiler and allow radiators to fully cool before starting.
  • Protect your hands – Wear gloves to prevent burns from hot radiators or water.
  • Use a container – Have a bucket or bowl ready to catch drained water to avoid a spill.
  • Carefully open valves – Use the proper key and open bleed valves slowly to prevent damage.
  • Check for leaks – Watch for drips after closing the valve and wipe up any spilled water.
  • Supervise children – Don’t leave kids unattended near open radiators due to burn risks.
  • Avoid electrical danger – Make sure hands and tools don’t contact any live electrical components inside the radiator.
  • Allow heat to dissipate – Radiators stay hot after bleeding. Don’t touch them until they’ve cooled down.

Following basic precautions makes radiator bleeding a safe DIY heating system maintenance task.

FAQs About Bleeding Radiators

How do I know when a radiator needs bleeding?

Signs a radiator needs bleeding include cool spots at the top, loud gurgling noises, very slow heating, and limited heat output compared to the bottom of the radiator. Trapped air reduces efficiency.

Does bleeding radiators reduce heating bills?

Yes, bleeding radiators can lower heating bills. Removing air allows them to heat up quicker and work more efficiently. The boiler doesn’t need to work as hard to compensate for trapped air.

Where is the radiator bleed valve located?

The bleed valve is located at the top of one end of the radiator, usually the opposite end from the supply pipe. Refer to your radiator manual to identify the bleed valve if you’re unsure.

How much water will come out when bleeding a radiator?

Just a small amount of water comes out when bleeding. Expect anywhere from a few tablespoons up to one cup from a single radiator. Have an old towel or container ready to catch any drips.

Should I bleed radiators when the heating is on or off?

Always turn off your heating system before bleeding radiators. The water coming out can be hot and could burn you. Bleeding with the heat off also releases air more effectively.

Can I damage a radiator by bleeding it too much?

No, there is no risk of damage from bleeding a radiator too frequently. It may be unnecessary if you don’t observe signs of trapped air, but bleeding once a month won’t cause any issues.

My radiators feel slightly warm but don’t heat fully. Will bleeding help?

Yes, this is a classic sign of trapped air preventing proper water circulation. Bleeding the radiators should allow hot water to reach all areas and provide full heat.


Bleeding radiators is an easy process that you can tackle yourself using just a radiator key and cloth. By opening the valves at the top and releasing trapped air, you restore your radiator heating performance and efficiency.

Carry out this basic maintenance any time you notice gurgling, cool spots forming, or limited heating from your radiators. Just be sure to turn off the boiler, have a container ready, and open valves slowly.

Bleeding radiators prevents air from accumulating and lets hot water circulate properly. Take a few minutes each year to bleed all the radiators in your home. You’ll enjoy better heating, greater comfort, and lower energy costs.