How To Bleed A Radiator
 General Information
Water based central heating systems only work well if there is minimal air in the system. Air in the system can cause radiators to not get hot at all or radiators are cold over the top part of their radiating area. These effects reduce the efficiency of the central heating system and can lead to increased costs. Getting all the air out of the central heating system needs to be done before trying to balance or set up the system. Bleeding radiators should only be necessary annually or after modifications and repairs to the heating system. The information on his page does not apply to oil filled radiators.. Bleeding the central heating system to remove trapped air is a job that can be attempted by most people if a little care is taken.
There is always some air dissolved in heating system water which in a properly designed system collects at the highest point and is vented to the atmosphere. Air naturally rises to the highest point of the system it can which means the top of individual radiators and the highest point in pipe runs.
What Does Bleeding The Radiator Mean?
Bleeding a radiator means opening a small valve known as the radiator bleed valve to allow any trapped air to be expelled. Trapped air can stop hot water flowing to all parts of the radiator.
 Does A Radiator Need Bleeding?
If a radiator is cold and used to work the problem could be due to air in the system. In this case it is best to bleed all the radiators and any other bleed valves you can find, sometimes there are two in the loft (usually in bungalows). If the radiator gets hot at the bottom and not at the top then the radiator probably needs bleeding. Air in the system can also cause a knocking noise in the pipes when the heating is running and bleeding all the radiators will often solve it. The knocking noise is caused by air trapped in the pipes.
How To Bleed A Radiator
Most radiators have a small bleed valve at one end at the top (double radiators may have two bleed valves), usually these require a square or hexagonal radiator bleed key or a screwdriver to open them.
 Air is expelled either through a small hole adjacent to the bleed valve or more usually around the thread of the bleed valve itself. Radiators are often painted which means that the bleed valve is clogged with paint and difficult to open. Sometimes the bleed valves are damaged and cannot be opened using the proper key, in this case an adjustable spanner or a small socket may do the job. When all the radiators are being bled it is sometimes necessary to go round them all more than once. Success is when each radiator has a uniform temperature over the whole of it's surface area.
If you find you have to bleed the one or all radiators regularly to keep them hot then there is an air leak somewhere in the system which needs finding and fixing. If bleeding the radiators doesn't solve the problem of cold radiators the chances are there is an airlock in the system or the system is low on water which needs to be investigated and fixed. There are often other bleed valves apart from those on radiators such as on the pump or even in the loft (usually associated with bungalows where heating pipes are often routed through the loft).
Before starting make sure you have the correct tool to open the valve, plenty of rag and a small container to catch any water expelled.
1. Run the heating system until it has reached normal temperature and then turn it off.
2. If thermostatic valves are fitted to radiators open them fully but record where they were set to enable them to be reset afterwards.
3. Wrap a rag loosely around the bleed valve.
 4. Open the bleed valve a small amount anticlockwise until air starts escaping (usually a hissing sound is heard) , leave the valve open until water starts coming out. This can take a while especially if there is not much pressure in the system. Be careful as water can spray out suddenly. Once all the air is out close the valve.
5. Reset the thermostatic valve if fitted.
6. On pressurised systems adjust the system pressure back to normal following the manufacturers instructions.
7. Check that the bleed valve is not leaking.
Radiators Continually Need Bleeding
While there is always some air in the water the heating system should be designed in such a way that it is vented into the atmosphere. If a radiator or radiators need bleeding regularly and no modifications or changes have been made to the system and if this is a new problem there is a problem. AIr is getting into the system somewhere and the system needs to be investigated. Possibly this is due to low water level, especially if it is the upstairs radiators.
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