Skip to main content

Plumbing Repairs: How to Bleed a Radiator System

Gordon has been an online writer for over nine years. His articles often focus on DIY and home repair.

Radiator of a central heating system

Radiator of a central heating system

When to Vent/Bleed a 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 to) is cold whilst the radiator is warm further down when the heating is on.

This job is easily accomplished and also saves the expense and inconvenience of involving a plumber.

Safety First

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 is 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 beforehand 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.

Two Types of Central Heating: Which Do You Have?

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


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, and it usually consists of a silver braided hose with valves attached.

What You Will Need

At 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 online. I advise getting a brass radiator vent key, as they tend to be more robust.
  • Old rag. This is to soak up any excess water, as heating water is not clean (it is often dirty in colour). It is also likely to contain chemicals.

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.

How to Bleed a Central Heating Radiator

All radiators are fitted with a small square bleed vent. 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.

  1. 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 from losing the vent plug and causing an incident, it is better to vent the radiator more slowly.
  2. Hold the old rag ready for any stray drops of water that may be forced out.
  3. Once you have water coming out of the vent, 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.
  4. 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.

What Order Do You Bleed Multiple Radiators?

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 to 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.

If You Can't Vent the Air Out of Your Radiator

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 recommended to involve a professional plumber or heating engineer both to diagnose the exact fault and rectify it.

Potential Problems With Your Radiator:

  • 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

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: I have bled my radiator, but it is still cold, the other radiators are fine, what else can I try?

Answer: TRV's (Thermostatic Radiator Valve) on radiators often stick. Gently tapping it or opening and closing it several times often frees it off.