How to Bleed a Radiator in 7 Easy Steps

Bleeding a radiator valve

You’ve set the thermostat on full heating mode, but the radiator next to your favorite couch doesn’t feel as warm as it used to. You gently place your hand near the top of the radiator just to check on the temperature, and realize it’s not even warm. That’s a major sign of air build-up inside the radiator, which needs to undergo a bleeding process. Don’t worry, you won’t see any blood. Our step-by-step guide will show you how to bleed a radiator and enjoy full heating again. 

When switching on the thermostat, you may realize that one or more of your radiators is taking a long time to heat up, or making gurgling noises. Bleeding a radiator involves getting rid of all the air that is keeping hot water from flowing through the entire heating unit. It’s an easy process where you switch off the entire heating system, identify the radiators that need bleeding, release all that trapped air, and double-check on the boiler pressure. 

We’ll go through each and every step to make sure you get the bleeding process done right, without exacerbating the issue or harming yourself.

What Does Bleeding a Radiator Mean? 

Over time, pockets of trapped air will naturally accumulate in your hot water radiators. They can also result from tiny leaks in the valves, or when rusty sludge forms at the bottom of your radiator. Trapped air prevents hot water from effectively circulating in radiators, reducing heating efficiency. It’s also the reason why the radiator keeps making strange sounds, or why the top half of the unit feels cold when in full heating mode. 

Bleeding a radiator means getting rid of any trapped air in the radiator. It is quite easy to do, and will only take a few minutes of your time. The bleeding process allows hot water to circulate evenly and freely throughout the heating system, maximizing its performance. But how can you tell when a radiator needs bleeding? 

When to Bleed a Radiator

Fortunately, you don’t need to be an HVAC professional to determine that a radiator needs bleeding. Here are the signs to look out for:

  • Strange noises: When air is trapped inside the radiator, you’ll hear gurgling and rattling sounds. This is one of the most common signs that it needs bleeding. 
  • Cold patches: This suggests that air has risen to the top of the radiator and prevented hot water from flowing through. You can feel the cold by gently placing your hand on the top half of the unit, but more on this later. 
  • Slow heating rate: If one radiator takes longer than the others to heat up, then it could be the result of trapped air. 
  • High pressure: You check the boiler pressure gauge which indicates high pressure (more than 1.5)
  • Mold: If things weren’t bad enough, you may also notice mold growth or dampness surrounding the radiator

If you’ve encountered one or more of these signs, then the radiator definitely needs bleeding. Before getting into the actual process, let’s take a look at the supplies you need to get it done cleanly and efficiently. 

Supplies You Need to Bleed a Radiator

No matter the maintenance or home improvement task, it’s always good to be prepared. Having the right supplies helps you get the job done without hurting yourself or damaging the property. When it comes to bleeding radiators, the tools and supplies you’ll need are pretty simple. 

  • Radiator bleed key: It’s a specialized tool that fits into the radiator bleed valve and allows you to open it. Doing so will release the air trapped inside the radiator. If you don’t have a radiator key, you can also use a flat-head screwdriver (same result). If you’ve lost your key, you can always purchase a new one at your local hardware store. 
  • Flat-head screwdriver: This is not just an alternative to the radiator key. To complete the bleeding process, most modern radiators won’t even require a key. It’s important to note that you should not use a crosshead screwdriver, as it can damage the bleed valve. 
  • Cloth: You should keep one in your hand to capture any water that escapes during the bleeding process. You can also put extra towels below the radiator to keep moisture off the floor. 
  • Bucket: If you don’t want to use a lot of towels, you can insert a bucket underneath the bleed valve to capture any excess water. No one likes having a wet floor, especially if it leads to accidental slipping. 
  • Safety gloves: Though there are no sharp tools involved, the gloves are to protect your hands from boiling water. 

Now that you’ve got all the supplies you need, it’s time to get rid of all that excess air. The following section will go through each step to successfully complete the bleeding process. 

How to Bleed a Radiator

Woman bleeding a radiator
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As mentioned above, bleeding a radiator shouldn’t take longer than a couple of minutes. However, it’s all about the execution. If you rush or skip any of the following steps, you might do more harm than good. The entire process can be broken down into 7 stages:

Step 1: Identify Radiators That Need Bleeding

To determine which radiators in your home need bleeding, you must switch the heating on to full power. Wait until all of the units in your home have heated up. For the best results, you may have to wait up to 30 minutes. Following this, cautiously feel all the radiators in your home. Hot radiators are no joke, so we recommend putting on light gloves while feeling them. 

The bottom of the radiator, including the pipes, should feel quite warm. You should then run your hands along the top of the radiator. If it’s cold at the top, then you’ve identified a unit that needs bleeding. You may also notice it’s taken far too long to heat up, or strange gurgling noises. 

It’s highly important that you separate the radiators that need bleeding from the ones that don’t. If you bleed radiators that don’t have air trapped in, you risk having the boiler pressure drop too low and your entire heating system failing. 

Step 2: Switch Off the Heating System

When you’re done identifying the radiators that need bleeding, switch off the heating and wait for the radiators to cool down. You should wait at least 20 minutes, or up to 1 hour just to be extra safe. 

Leaving the heat on may introduce more air into the heating system, thus ruining the whole process. You can also burn yourself when your skin comes in contact with hot metal (radiator), or when hot water bursts out of the bleed valve when the trapped air comes out. 

While you’re waiting for the radiators to cool down, you can move on to the next stage. 

Step 3: Prep Work

Grab all the supplies you need to perform the bleeding and prep the area near the heating units. Lay down some towels or cleaning cloths underneath the radiator (or radiators) that need bleeding, especially on the side of the bleed valve. This is to keep water from damaging your floor or carpet. You should also keep a towel or cloth in your hand to hold it near the bleed valve when needed. 

Step 4: Locate the Bleed Valve

The radiator’s bleed valve is usually located at the top of your radiator, either on the left or right-hand side. The bleed valve basically looks like a round hole with a square screw or bolt inside it. This is where you’ll be releasing all that built up air inside your heating unit. Now that you’re ready, let the bleeding begin! 

Stage 5: Loosen the Bleed Screw

Slowly turn the radiator key (or screwdriver) anti-clockwise about half a turn to release the air. You should start to hear a hissing noise as trapped air leaves the radiator. You can use the cloth in your hand to help grip the radiator key if the bleed valve is tight. It should take about 20 to 30 seconds for the trapped air to escape the radiator. When the bleeding is complete, you’ll witness a steady stream of water out of the valve. Use a towel to catch the water drops. 

You should not open the radiator valve entirely, or keep it open for too long. This is to prevent too much water from pouring out and boiler pressure dropping dramatically. 

Stage 6: Close the Bleed Valve

With no more hissing sound and water escaping from the bleed valve, you’ve gotten rid of all the trapped air!

It is now time to gently close the radiator’s bleed valve. Position your key or screwdriver properly and turn it clockwise to tighten and secure the valve. Use the cloth again for a better grip. Be careful not to tighten it too much to keep the valve from breaking, especially if it’s an aging radiator. Wipe down excess water to prevent the metal from rusting in the future. Repeat the same process with all the radiators that need bleeding. 

Note: If the air stops and no water is coming out of the bleed valve, contact an HVAC pro as there may be other issues preventing the radiator from filling up with water. 

Stage 7: Check the Boiler Pressure

You may have released all the trapped air, but the bleeding process isn’t quite over yet. After you bleed all the radiators, make sure the boiler is functioning properly and maintaining normal levels of pressure. 

To check for normal pressure, look for the hydraulic gauge on the front of the boiler. The pressure indicator should be between 1 and 1.5. A digital gauge will show an alert indicating low water pressure. Anything below 1 means your boiler needs repressurising.  If the pressure seems normal, turn the heat back on and test the radiators. The top half of each unit should be as warm as the bottom. 

If that’s the case, you’ve done it! You can enjoy the fruits of your labor and relax in the warm, cozy living room. 

Why Bleeding Your Radiator is Important

The most obvious reason is letting it work smoothly and keeping your home warm. If air is trapped in your heating unit, you may need to set it on a higher temperature and let the entire system run for longer periods of time. This leads to one thing: higher energy costs. 

So when you release trapped air from one or more radiators, your heating system will require less time and effort to heat up your home. This will help you save money in the long run. Overall, bleeding maintains boiler efficiency and even heat distribution. 

If you find yourself repeating this process quite frequently throughout the year, you should contact a boiler technician. You could be dealing with a bigger issue than air pockets. 

How to Keep Air From Filling Your Radiator

Unfortunately, you can’t keep air entirely out of your heating units. The continual heating and cooling of water will inevitably lead to air build-up.

The best way to prevent any major issues, including cold radiators, is annual maintenance or tune-ups by trained technicians. This allows you to catch any issues before they get worse. Generally, it’s best to have your boilers serviced during the summer months before switching on the unit. This prepares your boiler for winter’s arrival. 

You should make it a habit to bleed your radiators before the fall and during the spring. By doing so, you’ll ensure there is no trapped air during the period of inactivity. 

Is Your Radiator Still Cold?

Hand touching a radiator
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This is a worst case scenario. You’ve followed every step correctly, released all the trapped air, but the radiator is still cold. Why is that? 

You could have an issue with the thermostatic radiator valve. It’s usually located on the bottom of the radiator, with temperature control numbers on. The pins located underneath the cap can often get stuck in the shut position. This keeps water from flowing to the radiator. You can loosen the pin by using a set of grips to gently move it up and down. If that doesn’t work, then it might need to be replaced by an HVAC technician. 

A cold radiator may also mean that your radiator needs flushing. This involves draining the entire unit, taking it off the wall, and letting water flush through the radiator valve inlet to get rid of any sludge. This is not as simple as bleeding a radiator, and should be done by professionals to prevent further damage. 

FAQ About Bleeding a Radiator

Which radiator should you bleed first?

HVAC pros recommend starting with the radiator that is furthest away from the boiler, and work your way back to the closest. Radiators closest to the boiler system tend to have less air trapped in them. If you own a 2-story house, it’s advisable to start with the radiators located downstairs. 

However, it’s not disastrous if you don’t follow this particular order, just a recommendation for a more efficient process!

Is bleeding your radiator the same as flushing? 

In short, no. Flushing is a far more complex process that affects the entire central heating system, not just a few radiators. It’s the removal of substances, such as sludge, that develop over time and affect the boiler’s performance. It is also not DIY-friendly and should be performed by trained HVAC professionals. 

Can you bleed multiple radiators at once? 

Even if you have magic skills that put you in two places at once, you shouldn’t bleed multiple radiators simultaneously. As mentioned above, start with the radiator that’s furthest away from the boiler. It is best to complete the bleeding process one radiator at a time, otherwise you risk causing big leaks that affect the entire system. 

Replace Trapped Air With More Heat

Bleeding a radiator isn’t so time-consuming, with simple steps that any homeowner can follow without being an HVAC expert. It’s an important process for efficient and consistent heating throughout your home. 

With detectable signs of trapped air within the radiator, such as cold spots and gurgling sounds, there is no reason to neglect the issue and let it affect the entire central heating system. 

If bleeding hasn’t solved the issue and some rooms remain colder than others, you should hire a local boiler professional to perform all the necessary repairs and tune-ups. They’ll get your entire home warm and comfortable in no time. 

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Jeffery Keusseyan

Jeffery Keusseyan is a writer with a knack for research and handiperson skills. He enjoys writing about home improvement projects and efficient ways to maintain your home exterior.