Plumbing Repairs: How to Bleed a Radiator System

How to bleed a central heating radiator

Bleeding or venting a radiator is a simple DIY task that anyone can do with a little bit of understanding and some basic tools. A clear sign that a central heating radiator requires the air venting out of it is if the top of the radiator (where the air has risen too) is cold whilst the radiator is warm further down when the heating is on.

Once you know how to this job is easily accomplished and also saves the expense and inconvenience of involving a plumber.

What you will need

A the bare minimum you will need the following

  • Radiator vent key, these are available from any hardware or DIY store, alternatively they can be bought on line.
  • Old rag, this is to soak up any excess water, heating water is not clean water (it is often dirty in colour) it is also likely to contain chemicals.

I would advise getting a brass radiator vent key as they tend to be more robust, other fancy radiator venting tools exist such as ones with built in tanks to catch any stray water and it is a personal decision if this kind of gadget is for you.

Safety first

Like all elements of DIY there is a small risk of injury and steps should be taken to keep this risk to a minimum. It is best to vent radiators when the heating system if off and cold, if any problems do then occur you are dealing with a flow of cold water then a potential flow of boiling hot water.

Know where the water stop tap is, alternatively this could be the stop tap feeding the feed and expansion tank. Again it is best to know its location before hand as it is the last thing you want to be searching for in an emergency. Knowing where the water stop tap is located is also useful should you ever have any leaks from your plumbing systems.

Before starting

It is worth knowing what kind of central heating system you have before venting your radiators, there are two main types.

Tank fed

This type of system uses a small water tank often called the feed and expansion tank to fill the central heating system. As it uses gravity to push water into the system it will be found at the highest point of the central heating system usually in an attic or loft.

Sealed system

This more modern system is more often found on combi boilers, the system is sealed and water must be added manually via a filling loop. The installer most likely showed you this when the boiler was installed, it usually consists of a silver braided hose with valves attached.

Potential problems

It may be possible that you are unable to vent the air out of your radiator and this can call for further action. If you are an accomplished DIYer you may feel able to undertake these tasks yourself, if not it is recommend to involve a professional plumber or heating engineer both to diagnose the exact fault and rectify it.

Potential problems include things such as

  • Worn/rounded vent plug so unable to vent radiator
  • Radiator not filling with water
  • Radiator vented and full of water but still not getting warm
  • System requires venting frequently

Doing the job

All radiators are fitted with a small square bleed vent on, these are always at the top of the radiator located at one of the corners. Some double panel radiators will have a vent on both panels of the radiator. Using your bleed key turn this square vent anti clockwise until you hear the hiss of air movement as the water forces it out. Take care not to turn the vent too far, always leave some of the threads holding the vent in. This is to stop you losing the vent plug and causing an incident, it is better to vent the radiator more slowly. At this time hold the old rag ready for any stray drops of water that may be forced out.

Once you have water coming out of the vent it is time it is time to swiftly and carefully screw the vent back in. This is done by turning the vent clockwise. Use only sufficient force to stop the flow of water do not over tighten it, it is likely that you (or indeed someone else) will need to vent the radiator in the future.

There the job is done, wipe away any excess water with your old rag and either move on to the next radiator or bask in the glow of a job well done.

Bleeding more then one radiator

If several radiators require bleeding there is a logical order in which to bleed them. Air rises to the top of the system so it is best to start at the lowest radiators that you have and work upwards. Start with the radiator furthest from the boiler is there is several on the same floor or level and slowly work in towards the boiler.

If you have bleed several radiators on a sealed system you will need to check the system pressure is okay. This can normally be found as a small pressure gauge located on the boiler. If it is low it may require topping up via the system's filling loop.


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