How to Drain a Central Heating System

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Last updated: October 31, 2023

There are some occasions when it is necessary to drain the water out of your central heating system. These can include fitting new radiators or clearing dirt from the system.  This may sound like a pretty daunting task, but is actually very straightforward. All that is usually required is an understanding of what kind of boiler and heating system you have in your home and a few basic tools.

Step 1

Firstly, ensure that the electric power supply to the boiler is switched off. This is all that is required if your home has a combi boiler system.  With an open vented system or a gravity-fed system, the water supply to the boiler needs to be switched off or isolated. This can be achieved by turning off the main stopcock to the house, but this will isolate the entire water supply, so it is possible to deactivate the ball valve in the header tank which fills the system. Find the tank in the loft and use a length of string to tie the ball arm either to a rafter, or a stick placed across the top of the tank, so that no more water enters the system.  It is important to allow the system to cool before any further work is carried out, for safety reasons.

Step 2

Next, locate the drain valve. This is usually on a downstairs radiator or near one. It will have a short, ridged outlet pipe protruding from it. Attach a hosepipe to the drain valve using a jubilee clip for security. Ensure that the hosepipe will be long enough to reach to an outside drain. Be aware that the dirty water which will be draining out may contain chemicals which can harm plants, so be sure to keep it off your flower beds or lawn.

Step 3

Open the valves on all the radiators in the house. Then, open the drain valve using an adjustable spanner and check that the water is flowing freely through the hosepipe.

Step 4

Starting on the uppermost floor, open the bleed valves on the radiators. It is normal to hear a sucking noise as air is drawn into the heating system. Repeat the process on the lower radiators.

Step 5

It can take around 20 minutes for the draining water to stop flowing completely. Whilst you will never get every last drop of water out of the system when the water from the drain off valve has stopped flowing, the system is empty enough to work on quite safely. Once this has happened, it is best to close all the bleed valves so that you don’t forget later. You are now free to work on your heating system.

Step 6

Once the work is complete, you will need to refill the heating system again. Having ensured that all the bleed valves are closed, switch off the drain valve. If you have a gravity fed system, ensure that whatever means you used to prevent water entering the system has been reversed. With a combi boiler system, simply switch the system back on and repressurise the boiler to around 1.5bar. You may need to increase the pressure closer to 2 bar in order to fill the system, then reduce it once filling is complete. With either a combi boiler system, or a gravity fed system you, will probably need to bleed the radiators to release any trapped air, beginning with the lower radiators and finishing with those upstairs.

Can I Drain the Central Heating System and Still Have hot Water?

This is a commonly asked question and the short answer is no. Both combi boiler and gravity fed systems require the central heating system to be filled in order to to provide hot water for the taps. The only exception to this might be if you have an electric immersion heater in your water tank which is then independent of the central heating system.

How Often Should I Drain the Central Heating System?

You should not need to drain your heating system very often at all. Apart from doing so if any work on the radiators is required, it may be that the heating system needs to be drained in order to clean or flush it through.  This should not be the case in a newly installed heating system, but in one over about 5 years old, sludge contamination can start to be an issue. This is a gradual accumulation of rust and dirt particles from inside the pipes and radiators. As it builds up, it prevents the heating system from working efficiently and needs to be flushed out. There is more information on this process elsewhere on the site.

Whilst the thought of having to drain your heating system may seem very technical, it is quite a straightforward process, however, if you have any doubts about it, a plumber or heating engineer can carry out the work.


  1. How to drain a combi boiler – Viessmann