There is rarely a need to drain their combi boiler for most homeowners, but there are specific situations when it might be a good choice. The more common problem is boiler pressure is too low. Or a boiler loses pressure.

A couple of examples might be if you are undertaking some decoration and want to remove the radiators to ensure a full and extensive job, or maybe you are going to be leaving the property vacant for an extended period of time and want to minimize the risk of any leaks that could cause significant damage.

Another common situation would be if you wanted to flush the system to eliminate any sludge that may have accumulated.

Our advice would be to employ the services of a Gas Safe heating engineer unless you have extensive knowledge and experience within the industry.

This is because draining a combi boiler is a complicated procedure with the potential of causing damage to your heating system and even your property in the worst-case scenario.

It is also likely that doing this task yourself will invalidate your warranty. However, the point of this article is to explain the process involved so that you understand the complexities of the processes involved.

There are hundreds of different combi boilers on the market today, and each manufacturer will have a separate process that must be followed, as outlined in the user manual. Regardless of the make and model of your boiler, you must turn off the boiler and disconnect it from the power supply before starting this process.

The last thing to do before connecting any pipes or even touching your tools is to give the boiler time to cool down, which can take anywhere between 30 and 60 minutes; always err on the side of caution.

If your Combi Boiler is leaking then this guide may help.

Drain Your Combi Boiler

The first step involves connecting an appropriately sized hose to the drainage valve securely. This is normally the valve located at the lowest point of the lowest radiator in your home. Once the hose is securely attached, ensure that all radiator valves in the property are fully open, opening the drainage valve last.

Once the water starts exiting the drainage valve, you can speed up the process by opening all of the vents at the top of each radiator. Once all of the water has left the system, ensure that all vents are correctly closed. Now that all of the water is out of the system, you are free to undertake any maintenance work that needs to be completed. 

How To Refill Your System

Once the work has been completed, it is time to refill your system.

Step 1:

Remove the hose from the drainage valve, ensuring that it has been properly closed.

Step 2:

Search and find the filling loop, which is typically under the boiler and can be identified in your user manual.

Step 3:

Open the filling loop while constantly monitoring the pressure until the pressure gets to the appropriate level recommended in the user manual.

If any air is trapped in the system, as a result of this process, you can then bleed the radiators, starting with those radiators located downstairs.

Once all of the radiators have been bled, the pressure may well have dropped to open the filling loop once again to get the pressure back to the correct level.

Before turning the boiler back on again, check all of the joints and radiators where any work was completed for leaks. Once you are satisfied that there are no leaks, the time has come to turn your boiler on once again. 

The Perils of The Process

Having read this far, it is understandable if you are thinking that draining a combi boiler seems like a very simple and straightforward job. However, in reality, numerous things can go wrong. That is why we would always advise paying a Gas Safe engineer to complete the work for you. 

Why Flushing Your Central Heating System is A Good Idea

Over time, every central heating system will end up with a build-up of dust and dirt, often referred to as sludge. This sludge builds up in the bottom of your radiators because it is heavier than the water in the system. In extreme cases, it can even stop hot water from flowing around the system properly, creating cold areas, and making your heating system more expensive to run and less efficient.

Power flushing your central heating system is designed to help remove the sludge, protecting your boiler and reducing system wear and tear in the process. This is another task which a Gas Safe registered engineer should carry out. 

The process involves connecting a machine into your central heating system, which then pushes a flow of liquid through it at a relatively high pressure. This liquid dislodges the rust and dirt, which then flows out of the system. This is a lengthy process that can take a few hours to complete and even longer if your property is larger or the system is badly blocked up. 

Will Turning The Water Supply Off Have Any Adverse Effect On My Combi Boiler?

A combi boiler is supplied directly by the water mains, so the second you turn off the water supply, there will be an instant effect on any appliances within the home. The good news is that you can still use your central heating system with the water supply turned off, but it is essential that you do not use your hot taps at all.

This is really important to understand, and all members of the household must be aware of this because if someone tries to turn on a hot tub, there is a genuine risk of causing significant damage to your boiler. As we have repeatedly said throughout this article, the best and safest course of action is to utilize the services of a Gas Safe Engineer.