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How do I Bleed a Radiator?

Nicole Madison
Updated May 16, 2024
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To encourage your heating system to work at optimum efficiency, it is important to bleed your radiators on a regular basis. Though it may sound difficult to bleed a radiator, it is actually quite easy to do. There is no special training required, and all you need is a radiator key and a small bowl.

If the heat in your home isn’t circulating well or the bottom portion of your radiator is warm while the top portion is cold, there may be air trapped inside the radiator. When this happens, it reduces the radiator’s ability to heat effectively. This trapped air may even lead other parts of your heating system to function poorly. Sometimes, most of a radiator will fill with air, and in such a case, there will be no noticeable temperature difference — the whole radiator will be cool to the touch.

As you prepare to bleed a radiator, you will most likely be starting with a system that has been turned on. As such, there should be warm water in the radiator. Before you begin, turn the central heating system off.

You’ll need to have a bleed key to bleed a radiator. If you don't have one, you may be able to buy a new one from a hardware store or use a flat screwdriver, if the bleed valve allows for it. This valve is typically positioned at the top of a radiator, near the end. Insert the bleed key or screwdriver into the bleed valve and turn it counterclockwise. Don’t turn it too much; a half a turn is usually enough.

As you turn the radiator key, the valve will open and you will hear a hissing sound. This is completely normal and is simply caused by the air escaping. Once water begins to leak out of the radiator, it is time to close the valve. To do so, turn the bleed key clockwise for half a turn. You can then move on to the next radiator that needs to be bled before turning your central heating system back on.

When you bleed a radiator, keep a small bowl or rag handy to catch the water that drips down. If you have a sealed heating system, be sure to do a pressure check and add water if needed. Once you’ve done so, you are finished and may turn your heating system back on. Store your bleed key until the next time you need it.

HomeQuestionsAnswered is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Nicole Madison
By Nicole Madison , Writer
Nicole Madison's love for learning inspires her work as a HomeQuestionsAnswered writer, where she focuses on topics like homeschooling, parenting, health, science, and business. Her passion for knowledge is evident in the well-researched and informative articles she authors. As a mother of four, Nicole balances work with quality family time activities such as reading, camping, and beach trips.

Discussion Comments

By anon356841 — On Nov 28, 2013

When adding water to your radiators to fill them up, when do you need to stop? I live in a three story house and I don't know how much water I need or when to turn it off.

By anon326724 — On Mar 23, 2013

The bottom half of my radiator is cold, but the top half is hot. What do I do?

By anon230469 — On Nov 19, 2011

my radiators needed to be bled, but only air is coming out -- no water.

By anon225283 — On Oct 26, 2011

I try to bleed my valves and air just comes out and no water then the air stops all together.

By anon170597 — On Apr 26, 2011

3 story georgian, built 1919, 13 radiators,gas boiler in basement, city water with manual feed valve and overflow, we bleed first turn on of the season, I will add water till it overflows (i.e., pressure is released) then we start on the top floor and work toward the furnace.

I use a key and rag. We repeat after the system is warmed up. The water is hot but never been burned. On a quiet night, if there is still air, you can hear the radiator gurgle, just bleed the noisy one(s)! this is how my wife's grandad did it. he built it, now it is hers. The biggest hassle is the R&R of the covers.

By anon170489 — On Apr 26, 2011

I had a leak from my boiler, and it was fixed, but they drained the system. now I get noises from the tank when the system boots up. guests in that room thought it had been raining outside heavily - the noise is terrible. I bleed two radiators constantly, but still have noise from the tank and the pipes.

By anon154469 — On Feb 21, 2011

please help just moved into a house with my kids and its freezing! radiators are warm at the sides but just the middle is cold when i try bleed it water comes out? please someone help.

By ikaye — On Feb 12, 2011

If no water comes out and just a small hiss, is this enough?

By anon146729 — On Jan 27, 2011

when bleeding the system, start with the bleeder closest to the boiler in the direction of travel and work your way down the line till you get to the last heater.

By anon146161 — On Jan 25, 2011

I have never bled radiators before, yet my bedroom is absolutely freezing, but the rest of my house is comfortable. Someone told me bleeding is not necessary. I am not sure what to do.

By anon132861 — On Dec 08, 2010

When I had the furnace replaced, the plumber installed new bleed valves on all the radiators, these could be opened with either the radiator key or a screwdriver. If you have any work done on your furnace, it might be worth the extra bucks to have the bleed valves replaced when the plumber is at your house, especially if your valves are old an/or painted over.

By anon131349 — On Dec 02, 2010

If you can't find a bleed key in your area try the local music (instruments) store. A drum lug key works just as well.

By anon131108 — On Dec 01, 2010

I can tell the radiator needs bleeding as it's only hot at the bottom, so I've undone the valve all the way and taken it out (there wasn't even any hissing) and no water came out - I assume that's because there is so much air inside? With the valve out will the air inside the radiator still come out?

By anon115859 — On Oct 04, 2010

I was reading this radiator info as we have been having similar problems including not being able to turn the valve with the key. As I was reading this, I noticed that the editing had been done by one Bronwyn Harris. My name is Bronwen. Needless to say, I don't meet many other women with my name.

By anon108269 — On Sep 02, 2010

I have a new boiler fitted in my flat. However my heating doesn't work. When i turn the heating on the boiler starts to heat water and i can hear the radiator starting but then it stops and won't heat. I have bled the radiators so only water comes out now. Any ideas on why the boiler stops heating?

By anon71187 — On Mar 17, 2010

My boiler's pressure is too high. Am I getting the pressure down by letting some water escape from the radiators?

By anon68494 — On Mar 02, 2010

i need to keep adding water to system daily, no visible leaks.

By anon64673 — On Feb 08, 2010

If you do not have a radiator key or one valve has rounded, I have found a pair of needle nose pliers works nicely.

By anon59803 — On Jan 10, 2010

When I bleed my radiator, water flows from it like a faucet. Is this OK? I suspect not.

By anon59122 — On Jan 06, 2010

For anyone that needs a radiator key - try a local Ace hardware. I live in an area with many older houses and the local Ace hardware had a big dish of these keys near the cash register. They have been

selling a lot of them lately. $1.89, so not a big investment and my house heated up nicely after I bled the radiators.

By anon58552 — On Jan 02, 2010

my Uncle left me a old house with radiators. I want to bleed the radiators but have no key. What do I do?

By anon58514 — On Jan 02, 2010

I just bought a radiator key after my 25 year-old key broke last week. I went to bleed the radiators and got the familiar hissing of air, but no water came out.

I checked to see if water would come from the radiators that I bled last week. Nope. From this thread I take it that the boiler is not pressurized.

How do I repressurize the boiler? It is a one year-old furnace, but the water tank is probably 60 years old and there is no gauge to indicate pressure.

Should I fiddle with the various knobs and see what happens? Also, the high temperature today is 10 degrees F, so I am kind of concerned.

By anon55817 — On Dec 09, 2009

My radiators were cold on one side of my house after i had a boiler installed. It was summer at the time. I did not turn on the heat until a year later. I bled the radiators, buckets of water came out and the radiators heated up. A lot of air came out. Mrd

By anon55284 — On Dec 06, 2009

I bled my rads earlier and now the hot water or heating is not working. I have re-set it but it's still not working. I don't know what to do!

By honeyb — On Aug 25, 2009

My radiators downstairs are cold in the middle and bottom but piping hot at the top and the overflow pipe has sprouting water for the past eight months. Please, any suggestions on what i could do myself would be most apprdciated. thank you.

By anon26485 — On Feb 14, 2009

My central heating is not working after bleeding some radiators.

By Lwedenbine — On Jan 23, 2009

I have an old radiator heating system. I've bled the radiators and got water out immediately. Problem is, the bottom 3/4 of the radiators are cold - only the top is hot. Any suggestions on how to fix this problem?

By Mainedogg — On Jan 19, 2009

My downstairs radiators are always warm. On my second floor I have to let the air out of the radiators many times during the winter?? If the heat is turned down and then back up that is when I have to let the air out of the ones on the 2nd floor. I have an American Radiator Company boiler system--can't find anything about them. I don't understand the gauges. I do have a closed water system and rarely have to add water. I would like to know proper maintenance on my system.

By armond — On Nov 30, 2008

I live in an old row house with cast iron radiators. When I bleed the radiators, I get air from two of them (top 3rd floor and water from the others (the heating system is on). What could be the problem and should I add water to the system? If so how? How do I check the pressure and what should it be.

By liljen10 — On Nov 16, 2008

hi, we are bleeding the radiators in our home and the one's on the 2nd and 3rd floors are remaining cold. any suggestions on how to fix this problem and make them warm?

By Loydaf — On Nov 03, 2008

I have an old house with iron cast radiators. How do you bleed these? I took out the pressure valve, no water came out. Is there another place on them to bleed? Some of them turn on and other don't. Can you give me some suggestions?

By bussbass — On Oct 28, 2008

OK, I went to bleed my upstairs radiators and all that came out was air,...no water. Is that because the water only rises as it is heated ? or what? I read that when I turn on my boiler that the gauge should read 10-12 psi (which it did when I turned it on) but I don't want to turn it on and let it run to build pressure and then "bleed" it to see if water comes out. I'm afraid it'll blast out way too fast and way too hot.

I checked the valves coming out of the floor and they are all the way open. I guess I need some help here,..any answers or recommendations ??

By anon20127 — On Oct 26, 2008

If nothing comes out when you turn the key that means that you need to re-pressurize the system by adding water at the boiler. There should be a valve that you can turn on and a pressure gauge at the boiler to tell when it is re-pressurized.

By thisisbam — On Oct 23, 2008

Hello wiseGeek. I have managed to thread one of the bleed valves, so I can't bleed the front half of the radiator - any suggestions? Thank you

By anon18888 — On Oct 01, 2008

I don't know what a bleeding key is????

By anon12273 — On May 03, 2008

After bleeding all the radiators in the flat, I switched on the boiler but non of the radiators seems to be heating up and I find the boiler makes some terrible noise on and off. What do I do next?


By anon7253 — On Jan 22, 2008

i was wondering when bleeding radiators how much water should you take out ?

By cpslot — On Jan 12, 2008

thanks. I'll give it a try.

By Shelzbells — On Jan 12, 2008

Look where the main water supply comes into your house. Follow the pipe that leads to your boiler. Somewhere along there should be a ball valve where you can shut the water feed on & off. On ours, just after where that valve is there is a regulator..(not sure what you call it) It's about the size of a large grapefruit. That is what I tapped with a wrench. Once I did that,the water ran in. If you placed your hand around the pipe you could feel the cold water feeding in. Hope this helps you.

By cpslot — On Jan 12, 2008

Shelzbells: Where do I find the auto water feed? And congrats!

By slewis5272 — On Jan 12, 2008

where do i buy old radiator keys?

By Shelzbells — On Dec 18, 2007

I finally fixed mine!!! The auto water feed was turned on but must have been stuck, I took a wrench and banged the valve once and it immediately then filled up. I then went back upstairs and opened the valve on the cold radiator. A LOT of Air gushed out, and after 10 seconds or so, water also came out. Now all of them are working normally.

By Erush — On Dec 18, 2007

The second floor radiators are cold and when I bleed them I get some air but water never comes out...what should I do?

By Shelzbells — On Dec 06, 2007

I am having the same exact problem as cpslot, I get water from almost all of them. 2 are cold and only a small amount of air comes out, but no water. another is hot but no water comes out. Is water supposed to fill them automatically?

By philmurphree — On Dec 05, 2007

I have an old 3 story house with hot water radiator heating. the ones on the 1st and 2nd floor are hot and when I bleed them, water comes out. The 3rd floor radiators are cold and when I bleed, I get some air but water never comes out...what should I do?

Phil Murphree

By jomama92371 — On Nov 07, 2007

i don't have central air-so does my thermostat heat need to be on or off?

By anon4369 — On Oct 15, 2007

must water flow before I turn off the key? What if there is no hissing sound?

By cpslot — On Oct 13, 2007

I live in an old row house with cast iron radiators. When I bleed the radiators, I get air from two of them, and water from the others (the heating system is off). Also, I get nothing at all from another radiator. Any thoughts? thank you

Nicole Madison

Nicole Madison


Nicole Madison's love for learning inspires her work as a HomeQuestionsAnswered writer, where she focuses on topics like...
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