With a little knowledge, draining and repressurising a combi boiler heating system is a relatively straightforward task. However, if you are not entirely sure how to carry this out, seek the services of a trained professional.

The following is a basic description of how to drain down and repressurise a combi boiler. System layouts will vary, but basic components will be the same.

Before you begin

Turn off the boiler and disconnect it from the power supply.

Let the water in the system cool down, both to make sure you do not scald yourself and to reduce the overall pressure. When it comes to draining the system, the lower-pressure water will be easier to deal with.

Locate the drainage valve

The drainage point in any system will usually be the lowest point of the lowest radiator in the property. Having located the drainage valve, connect a hose securely, ideally using a jubilee clip. Tighten the clip over the hose to prevent any leaks. It's a good idea to place a bowl under the drainage point to catch any leaked water.

Move the other end of the hose to a drainage point (obviously this will need to be at a lower position to allow the water to run off).

Check that every valve on each of your radiators is open to ensure the system is drained completely. Using a spanner or wrench, open the drainage valve by turning the square head. The water should now be flowing out of the hose.

Open the vents

Speed up the replacement of water with air by opening the vents (bleed valves) at the top of each radiator.

Once the heating system is completely drained of water, close the bleed valves, then you can carry out maintenance/repairs as required.

Repressurising / refilling your combi system

Close all air bleed valves on your radiators, then close the drainage valve and remove the hose. By holding the hose up you will ensure any remaining water drains off.

Opening the filling loop

Beneath your boiler will be a braided filling loop, which connects the cold feed and the return feed. Open this filling loop and allow the system to repressurise to the level recommended by the manufacturer. In most cases this will be approximately 1 bar.

Starting downstairs, open the bleed valve of each radiator and allow the air to escape. This should reduce the chances of air getting stuck in the system.

Having released all the air from the radiators, return to the filling loop and top up the system’s pressure.

Check for leaks

If you have undertaken any repairs or maintenance of the system, go around and check the work to ensure there are no leaks.

Follow your combi boiler user's manual and turn the system back on, then allow the heating to run as usual.

Bleed the radiators

As the system heats up, you will need to bleed the air-bleed valves – a task which may need to be repeated several times over the following days in order to ensure all the air is removed from the system.

Top up the pressure as required.

It is very important to inspect the entire system for any leaks that may have developed; as the water temperature increases connections may expand, causing water to escape.



startrescue.co.uk always recommend seeking the services of a professional tradesperson If you're not entirely comfortable carrying out repairs or maintenance by yourself.