How to Bleed a Radiator

24 Hours 7 Days a Week emergency repair call out service 0800 246 1375

For all those families having heating systems at home is good to know that a lack of maintenance or prompt fixing of the radiator, might cause big damages to all the central heating, and it can also raise the electricity bill. Radiators are devices located in different parts of the house, -usually attached to the walls-, with a series of pipes through which hot water passes, and this is what sends the heat out. Sometimes, while using your radiator you might notice your place is not warm enough, or you might hear weird sounds coming out of it, well, these are clear signs that something’s wrong and you may need to check and see if your radiator needs to be bled. You can call an expert to do this, or you can do it yourself. It’s not a hard thing to do but it requires you to take some precautions and follow a few steps to do it properly and avoid causing a bigger problem. If you’re ready to give it a try, then you will find the following information very useful. But first, let’s have a quick overview of how a radiator works: The whole heating system consists of a boiler and connecting pipes containing water. The boiler heats the water and a pump pushes it through the pipes, which finally heats the air around them. Then, once the water is cool again, it returns to the boiler which means the same recycled water is being used to operate the system. Radiant heat systems (or water-based) are more recommended to have due to they’re more environmentally friendly. But, there are some other air-based and electrical systems that usually can be more powerful but also can cause you health problems due to they dry out the air around the place where they’re installed. Now that you know a bit more about radiators functioning, let’s dive into the process of identifying if they need to be bled and the steps to do it.


We’ve covered lots of useful information in this article to help you get the best results. See our headings below!


One of the first things to do before bleeding a radiator is making sure it’s not working properly and there are clear signs it needs bleeding. A clear sign is if it feels hot in the bottom and cold in the top section. You will need to touch it to identify this, take your precautions first, and get a little towel or cloth to protect your hands before touching it. If it feels cooler on the top part, it means the hot water is not circulating effectively due to air trapped inside, and then it’s needed to get that air out. Also, if the whole radiator remains cold while the heating system is on, it’s a sign it needs to get repaired. Another good sign that your radiator is not working effectively is if you hear gurgling, banging or clanking noises. Pay attention to this and make sure there aren’t other exterior objects causing the noise if you hear the noise comes from the inside it might be a sign it needs to be fixed. One last thing that could happen is that your radiator feels cold at the bottom part and hot at the top, this usually means there’s some sludge on the inside, and instead of bleeding, it will need to be flushed with a hosepipe. When you’re completely sure you need to bleed your radiator, get ready for organizing all you need to get it done. Make sure to keep in mind that you should start with the ones located downstairs (if your house has more than one floor) and also make sure you start with the ones located further away from the boiler, this is to avoid letting air back into the system as you make your way along the water lines. Then, continue with the ones on the top floor. It’s great that you’re going to give it a try and bleed the radiator yourself, but also it’s always good to have a plan B. So, if you’re not an expert or it’s the first time you try this, maybe ask somebody to help you with this process, or you can also have the phone number of an experienced plumber, just in case something goes wrong. Use comfortable clothes, avoid your favourite t-shirt or pants as they might get wet or dirty, and make sure you have enough time, maybe plan this activity for the weekend or a day you don’t have to work.

What do You Need?

The first thing you will need is a radiator key, these are special keys for doing this specific job. Usually, when you buy a radiator you get one of those. If you don’t have one or can’t find where it is, you can always go and buy one at any hardware store, they’re very cheap and easy to find. There are different types of radiator keys, like the four-way key, three ledge key, double-ended key, Hex or Allen, Spanner, Universal key, and there are a few more types. Neither of those can be used for bleeding, so when you buy it make sure you ask for a “Radiator Bleeding Key”. If you definitely can’t find this specific key, you can also use pliers. It’s not the best option though, as the valve might get damaged, but consider it if it’s an emergency or you need to urgently bleed your radiator. Some modern radiators are easier to bleed and you will need just a standard flathead screwdriver, so first check what kind of radiator you have at home. You will also need a cloth or rag to catch any leaking water, and a container (preferably a plastic one), for bigger amounts of draining water you might get during the radiator bleeding.

The Process (easy step-by-step)

  • Turn on the central heating system: the first thing to do is to check if your radiators are working properly or not. The way to do it is to turn the central heating system on and wait until it’s fully operating. Make sure the radiator is turned up fully and wait about 20-30 minutes until they have reached their normal operating level.

Thermostatic valves are very helpful in modern radiators because they show you if they’re turned up to the maximum. Anyway, for older-style ones, it’s still simple, you will need to turn the valve fully in the anti-clockwise direction.

  • Diagnose if it needs to be bled: once you’ve turned the heating system on, make sure you use gloves, cloth, or something to protect your hands. You need to touch the radiator carefully and identify if it’s hot or cold.

Whether you find it’s totally cold, or maybe just cold on the top but warm or hot on the bottom, then it’ll probably need to be bled. If there is more than one radiator at your place, you should check them all and diagnose one by one, if all of them are presenting the same diagnosis it might be a general problem on the central heating system, and you might need professional assistance to fix it. There might be other problems like a leak on the radiator, or an accumulation of sludge on the inside. So for any of these other problems you might need to ask for the professional help of a plumber.

  • Turn off the central heating system: after identifying if your radiator needs to be bled, you need to turn the central heating system off and wait until it has cooled down completely. This is a safety measure, to avoid you scalding while touching it, and also to avoid water to spread all over the floor.

Do a couple of checkings before you start, touch the radiator surface in search of hot spots, or even warm ones. Remember even though it feels cold on the outside, the water inside might still be hot, so give it plenty of time for it to cool down.

  • Bleeding the radiator: place the cloth, rag or towel on the floor, below the radiator, to catch any water coming out of it. Find the bleeding valve, it’s usually located at one of the top sides and it looks like a round hole with a square inside. Now get the bleeding key and insert it into the bleed screw on the bleeding valve and turn it anti-clockwise to open it.

Keep the key in that position and you will notice the air will start to come outside, you’ll hear a hissing sound. Take precautions and if possible protect your hands with a cloth while turning the key, as air and water coming out might be hot. We recommend not opening the valve fully but just half of it, this way you’ll be able to control the air and water coming out. Anyway, keep a container nearby in case lots of water starts rushing out. You should let all the air come out and once the water starts dripping out, then it’s the time to close the valve. Now turn the key clockwise to seal the radiator. Tighten the valve, strong enough to stop the water coming out but noo too much so don’t damage the valve and so you’re able to open it again next time. Make sure to wipe down all the water around the radiator, to avoid future rust on it, and now repeat all this process with the other radiators you have at home. Remember, first floor’s first and then the ones on higher floors.

  • Repeat the process (on other radiators): if you have more than one radiator at home, then you should consider repeating this process in each one of them. You don’t need to bleed all of them straight ahead, but at least do the diagnose and check which ones need it.

Also, as we said before, you should start bleeding the radiators located on the ground floor and then going up to the top levels, this is because the air can rise through the whole heating system so doing it this way avoids you having to repeat the process more than once.

  • Turn on the central heating system (again): yes, you need to do this again. Remember you’re making sure the radiator was bled properly. So you need to turn it on and give it some time again for it to fully operate. Wait about 30 minutes and do all the radiators checking again, check one by one and see if now all the surface feels warm or hot, look for cold spots, if there isn’t any it means they’re good now.
  • Check boiler pressure: after bleeding all radiators in your place, you need to check your boiler pressure and make sure it’s operating on the optimal level. Next to your boiler’s water supply, you will usually find a tap or a lever, you need to use the filling loop to boost the pressure, in case it’s under the optimal level.

It is normal that after bleeding the radiators the boiler’s pressure might decrease, the normal pressure with the boiler switched off should be around 1.0 to 1.5 bars, and 2 bars when it’s switched on. Also, you can check if the water pressure in your heating system is ok by checking the needle gauge on the boiler, if the needle is facing green it means it’s ok, if it’s facing yellow you might need to re-pressurize the system, and if it’s red it means it’s over pressured and you might use the bleed tap to depressure it. Keep in mind not all houses require the same boiler pressure, bigger houses might require more than the small ones. And finally, if your boiler has an automatic filling system, it might usually maintain the optimal level pressure without you doing any work, if not, you might have to add water manually, using the boiler’s water feed valve.

  • One final check: the process might look long but it’s very simple and easy, it might take you an hour or so. After you’ve levelled the boiler’s pressure and check all your radiators are working properly and your place is getting enough heating, it’s time to just relax and enjoy.

Other Problems

  • Some radiators are hot and some others are cold: if the ones remaining hot are downstairs, it might be that the expansion tank and feed are dry and this is usually a problem with the valve, that can easily be fixed by just replacing the old valve for a new one. And if the hot radiators are the ones upstairs, it might indicate a bigger problem in the central heating pump, in this case, you should call a professional plumber to check and fix it.
  • Radiators close to the boiler are warmer: generally, this means the heating system needs to be balanced. What you should do is turning the valves down on the radiators closer to the boiler and turning them up for the ones far away from it. That way all radiators will share the hot water equally.
  • Leaking problem: leaks are one of the more common problems on radiators. The first thing to do is to identify where the leak is, if it’s coming from one of the pipe joints, it could mean the nut has loosened and in this case, you just need to turn the monkey wrench. If the leak comes from the front or back surface you might be able to fix it with a leak sealer. But if you notice the leak is going out of control, it’s better to turn the water system off and call a professional plumber to fix it and avoid bigger damages at your place.
  • Sludge problem: if you feel there are cold spots on your radiator, it might be a sign they have sludge on the inside, this means it has built some corrosion deposits on the inside and it’s needed to get rid of it by flushing the radiator with a cleanser. Usually, radiators need to be removed from the wall to make this flushing process.
  • Can I move my radiator?: if you need to move your radiator from to a different place in the same room, it can be done with no further complications as long as the pipework is being properly extended. The only thing it may cause is it might take a bit longer to heat, and in some cases might cause the boiler to turn on and off. We recommend asking a professional plumber for this type of work as it’s a bit more complex and delicate.
  • Radiator fell from the wall: this may happen if the radiator has been there for a long time and some screws got fitted. It’s not safe to put it back yourself if you don’t have experience with this type of work, or if you’re not a professional plumber. While you figure out prompt fixing you’ll need to turn the central heating system off, to avoid any bigger problems.
  • Radiator getting too hot: when this happens you need to turn down the thermostatic valve on the radiator. Usually, this means the thermostat might be damaged. By turning the central heating down on the boiler you may solve the issue, if not you will need to change the thermostatic valve.
  • Radiator remains cold: usually, when this happens it’s a problem with the valve set, if you turn all the other radiators off, switch the heating on and the radiator heats up, then it’d probably be that. Or it could be dirt on the valve set’s spindle, you can solve that by replacing the thermostat cover.
  • Boiler making loud noises: one of the reasons could be that there isn’t enough water in the boiler, in that case, you just need to fill it until required for proper operation. Also, check if the water supply pipe has frozen, if so, you need to turn the water supply off and, open the tap and thaw the frozen parts, for this you can use a hairdryer or a hot water bottle.

The thermostat being set too high can be another reason for noises, this could cause water overheating, so the way to fix it would be turning it down, but that may cause the radiators not heating enough.

  • Balancing your radiators (detailed): when some of your radiators are hot and others cold, it might mean a lack of balancing, so you need to do the following simple steps to balance it:
  • Bleed all radiators, then turn off the central heating system and let them cool down.
  • Remove the lockshield, whether it has a wheel-head valve or a thermostatic valve, use it to turn the radiator off.
  • Open up the valves on all radiators, turning it anti-clockwise.
  • Turn central heating on again and make a list of the radiators heat up order (which ones get hotter first?). Then, turn the heat off and wait until other radiators cool down again, this process can take a little bit but it’s necessary.
  • Go to the first radiator on your list and close the lockshield valve, then open it again by a quarter and take the temperature of one of the pipes, using a digital thermometer or a multimeter with thermometer function.
  • Now (on the same radiator), take the temperature of the other pipe and open the lockshield valve gradually until there’s a difference of 12°C between this now and the temperature you took on the step above.
  • Then repeat this same process in all radiators in your place. You will need to open the lockshield valve more for those radiators further away from the boiler.
  • After this, all your radiators should be balanced and you shouldn’t have differences in the heating between them.


Radiators require maintenance to keep operating properly, so a professional check should be done at minimum, once a year. Some other checks should be done more often, for example, steam radiators require the boiler’s low-water cut off to be flushed every week and the safety valve should be checked at least once a month. If you’re not a big fan of manually changing or adding water to through the radiator’s valves every month, then you should think about getting an automatic valve that will do the job for you. Does your radiator look too old? Maybe it’s operating perfectly but you just don’t like the way it looks, well, you could solve that simple issue by getting a radiator heat cover. You can find lots of designs, styles, and materials, like wood and metal. There are other types of radiators (not steam or hot water ones), Electric oil-filled radiators. These don’t require much maintenance but they can cause other electrical problems, so it’s also recommended to have the yearly professional checking. Have you heard about the radiator’s boosters? They are devices designed for increasing the efficiency of the central heating. They’re a great option, especially for being automatic and so they only run when the radiator is hot, so it will benefit you without increasing your electricity bill. It’s not fully recommended to place radiators under curtains due to drapes being able to block the heat, so make sure you place it where the whole room can get the heat without being affected by other obstacles. Some old heating systems and radiators might operate well but doesn’t have controls or valves to adjust them and to turn them up and down, they just have the option of turning them on and off, which can be a bit of a problem if you want to customize the heating on different parts of the house. So, consider updating or upgrading your radiators after a few years of usage, then you’ll have access to all the benefits and functions that bring new technologies.

What’s the Best Radiator For You?

Whether you want to replace the old one you have at home, or just want to install the whole heating system for the first time, you’ll have to consider a few things before choosing the right one. There are two types of radiator: central heating and electric ones. The firsts ones (water-based) operate with a boiler and they are the more common ones to find in most places. While the electrical radiators contain a thermo-fluid that gets heated once the device is connected to the power source. You will find a very wide variety of shapes, so you can choose what fits better depending on the size and type of place where you need it to be installed. So you’ll find single, doubles, horizontal, vertical, and all of them in different sizes. Another thing to consider is the type of material. Cast iron radiators are the most traditional and probably the more expensive ones because of their classic look, they might take longer to heat and also to cool down, but if you’re looking for a fancy shape then this should be your choice. Steel radiators are low-cost and very common too, and they can keep rooms warmer for longer periods, even after turning them off, prices will depend on the type of steel and design. If you don’t like waiting much and you prefer a radiator that heats quick, then aluminium radiators are the best option for you. As soon as you turn them on they start heating, they’re not as heavy as other models and they’re also easy to install. You will also need to calculate the BTU (British Thermal Unit) to figure out how much heating your place needs. The higher the BTU level, the hotter the radiator can get, so the bigger the radiator the more expensive it will be.


Hopefully, all this information has been useful to you, we’d like to make a little recap about radiators and the way to identify if they need to be bled, and other related subjects. So, a radiator is part of a heating system, a device usually attached to the wall that contains water and works with the help of a boiler, once the hot water is pumped from the boiler it goes through the radiator pipes and heats the air, making a place warm and cosy. If a radiator is making weird noises, and if it’s warm or hot at the bottom part but cold at the top part, it will probably need to be bled. Once identified the problem, you will need a radiator key (or a screwdriver), a cloth and a container, with these simple tools you’ll be able to bleed the radiator and get rid of the air inside of the pipes that aren’t allowing it to heat properly. The bleeding process is pretty easy, first, you will need to turn the central heating system on and once all radiators are fully operating, check one by one’s surface by touching it –being careful of not burning your hands-, you´ll need to check if they´re hot or cold at the bottom. Turn the heating system off and wait until the radiators have cooled down. Then get the bleeding key and using a cloth for protecting your hands and a container to collect the missing water, open the bleeding valve just a quarter and let the air come out, once the water starts coming out it means there´s no more air inside and you can close the valve again. Repeat the process in all radiators and then check the boiler’s pressure, if it needs to be re-pressurized you will need to use the filling loop to boost the pressure. There are other types of problems related to radiators that you can easily solve, like replacing valves or balancing the heating system. If you have radiator leaking problems, you could solve it by adjusting the valves or using leak sealers. If the whole radiator remains cold or hot, it could be a problem with the thermostatic valve, you will only need to turn it up or down, or if it’s definitely damaged you will have to replace it for new a new valve. The process for balancing all radiators in your place is not complicated and it can benefit you a lot, as you will maintain equilibrium on each room’s BTU and it will help you to avoid future damages to these devices. After bleeding all radiators, letting them cool down and turning them off, you’ll have to open the valves and turn them on again, to measure the temperature in each one of the pipes located at the bottom of each valve. Then, open the lockshield valve gradually until you reach a difference of 12°C between the two pipes, and repeat this process in each radiator. Taking in mind that the closer they are to the boiler, the less you’ll have to open the lockshield valve. We also talked about some recommendations for improving the radiator’s proper operation. They usually don’t require professional maintenance as long as you do a few checkings every month. For steam radiators, you should be changing the boiler’s low-water cut off every week, and also check frequently if the surface feels hot or cold to do bleedings when it’s needed. You can also improve the radiators functioning by adding a booster or improve its appearance by getting a fancy heat cover. There are lots of different types of radiators, you should study carefully what the best option for you is, taking in mind the material (cast iron, steel or aluminium), or also the shape (horizontal, vertical, single, double). But always make sure you’re getting a radiator with the proper BTU for your specific place’s needs. Remember radiators can consume lots of energy if you use them all day, it’s recommended to use it during short periods. If you want your rooms to remain warm while using the radiator, make sure you’ve locked all Windows and doors. Try to use it at those times when the temperature is lower –like at night or early mornings. Also, although you might feel confident to do some fixings on your radiator when you feel there’s a small damage, make sure you have enough information and the right tools to do it properly. Otherwise, it’s always good to call a professional plumber to do bigger jobs like fixing sludge problems, installing or uninstalling a radiator from the wall. There are lots of ways how you can make your heating system more efficient and avoid damages on the boiler or radiator, and also avoid larger electricity bills. Think of using your radiators and all other heating devices responsibly, and avoid over-usage when it’s not fully necessary. Last but not least, take care of all your heating devices. Implement maintenance routines frequently so you prevent the central heating system to fail in the middle of the Winter. And once everything is functioning perfectly, just get a cup of tea and enjoy it.