James Elston

James Elston

Published February 23, 2024

Boiler water drainage can be a tricky task if you don’t know what you’re doing. While it would be easier to call in a qualified heating engineer to perform the drainage, you can certainly take on the challenge yourself! 

If you’re a homeowner who’s not afraid of a little DIY boiler maintenance, we’ve got your back with a step-by-step boiler draining guide.

Don’t mess it up though – or you’ll be left with bigger problems than just frozen pipes.

How To Drain A Combi Boiler

First things first, you’re going to need:

  • Bucket
  • Hose pipe
  • Spanner
  • Radiator valve key

If you’re not feeling confident in your boiler-draining abilities, consider getting a professional to do it! This isn’t an easy task, and you need to follow each step carefully to avoid mistakes that could damage your combi boiler system permanently.

If you’re adamant about doing this single-handedly, you’ll need to begin by locating the drainage valve and bleed valves. Once you’ve done that, you’re ready to start the draining.

1. Switch off the boiler

Turn off your boiler and power supply. You should also wait a few hours for the water to cool down before you start fiddling with the drainage valve.

2. Attach a hose to the drainage valve

You’ll find the drainage valve at the bottom of the downstairs radiators (most likely at the lowest point of the house). To avoid flooding your home, you need to attach a hose pipe to the drain valve to lead the water outside. It’s recommended to place a bucket underneath the hose and the valve to avoid spilling excess water on the floor. If you don’t have a hose, make sure to have several buckets/containers ready.

3. Open the drainage valve

Time to start draining! Grab your spanner and turn the valve anticlockwise carefully to open it. It should take a few minutes for all the water to drain.

4. Open the bleed valves

Remember, not all the water is out yet. Your upstairs radiators are where the bleed valve will most likely be. Open these with your valve key – you’ll hear a hissing noise once they open.

It’s important to note that sometimes the heating system in certain houses can be split into two, so you then need to drain the water from both sides.

While the bleed valve is open, keep an eye on the boiler’s pressure gauge to check for any changes while draining. If it drops below 1 bar (and especially below 0.5 bars), you might damage the system and must call a qualified heating engineer to assist.

5. Tighten the valves

Once there is no more water coming out (this process usually takes around 30 minutes) you can detach the hose pipe. Finally, tighten the drainage valve and the bleed valves again.

6. Refill the boiler

Once you have drained your combi boiler, you will need to refill it before you can use it again.

  • Locate the filling loop: The filling loop is usually located underneath your boiler. It is a grey or silver pipe with one or two manual valves on either end. 
  • Open the valves: Turn both the valves to open them. You should hear water running through the pipe. 
  • Watch your pressure gauge: While the water is running into your boiler, keep an eye on its digital pressure gauge or pressure gauge dial. The pressure should increase slowly. 
  • Close the valves: When your boiler pressure reaches 1.5 bars, close both the valves on the filling loop to stop the water from flowing into your boiler. 
  • Restart your boiler: You can now turn the boiler on again. 

How Often Should You Drain A Combi Boiler?

Frequent draining might damage the valves. When there’s a blockage or build-up in the pipes it’ll be necessary to drain them, but this won’t be a problem you have to deal with regularly –

especially since there’s another way to deal with blocked pipes: magnetic filters. These filters help collect magnetic debris (like rust) that plays a role in the blockage within a heating system. Having these installed will save you from other issues in the future.

If the magnetic filters weren’t originally installed with your heating system, you’ll need a Gas Safe registered engineer to install them onto the return pipe. Not only will this prevent sludge buildup, but it can extend your boiler warranty!

Why You Need To Drain Your Combi Boiler

Let’s look at three reasons why you would need to drain your combi boiler and the benefits of doing this. 

To prevent frozen pipes 

Are you planning on escaping the winter blues with a tropical vacation? To avoid coming home to frozen or burst pipes, you’ll need to drain your combi boiler.

When water is left in the pipes through your central heating system for a long time, it can freeze during winter, leaving you with issues like a frozen condensate pipe

This can cause your pipes to burst or leak, which can be a costly problem to fix. 

To prevent this, it’s recommended that you drain your combi boiler if you’re planning on going away, especially during winter. 

To replace pipes or radiators 

If you’re planning on removing a radiator to replace it or are upgrading some of the pipework in your central heating system, your boiler will need to be drained before any work can begin. 

Draining your boiler ensures that there is no risk of your house flooding during your maintenance work. 

To clear blockages 

If your radiators are cold at the bottom or take longer than usual to heat up, it could be a sign that you have a blockage along your central heating system. Not only does this reduce the efficiency of your boiler, but you’ll also be wasting energy, causing your heating bills to spike. 

Draining your combi boiler could help clear the blockage or sediment and sludge build-up to ensure your central heating operates optimally and efficiently. 

Alternatively, you could call in a certified plumber to power flush your system. 

Dealing With A Leaking Combi Boiler

So, something went wrong and your boiler is leaking. This is serious and means it’s time for you to admit defeat and contact a professional! Whether it is leaking brown water or clear water, there are numerous possible causes for this, such as:

  • Pump seal failure
  • Corrosion
  • Internal component damage etc.

A qualified heating engineer will be able to pinpoint the problem and advise you on the best way to solve it.


Is it safe to drain a combi boiler?

Draining a combi boiler yourself can be risky and might damage the system. Unlike traditional boilers, combis don’t require regular draining. Unless it’s necessary, leave it to qualified engineers. However, if you want to attempt it yourself, be sure to turn off the power and water supply before tampering with the drainage valve.

How do I release the water pressure from my combi boiler?

Resist the urge to touch that pressure-release valve! Adjusting water pressure in a combi boiler is a delicate task best left to professionals. Incorrect pressure can affect your boiler’s performance, potentially leading to leaks or malfunctions. Leave this tricky procedure to a qualified Gas Safe registered engineer who can accurately diagnose and adjust the pressure if needed.

Final Thoughts

What it all boils down to is that draining a combi boiler is a challenging task, but our step-by-step guide is here to help. It’s vital to know when to drain, especially before extended absences or maintenance tasks. However, the process is tricky, and mistakes could lead to significant system damage.

That is why you should seek professional assistance to avoid voiding warranties or causing irreversible damage to your combi boiler system, especially if you’re unsure of your skills. Remember, a leaking combi boiler requires immediate expert attention.

While DIY drainage is feasible, it’s crucial to follow each step meticulously and read up on all the draining tips for combi boilers. We invite readers to share their experiences or seek further guidance, to create a community of support and knowledge-sharing.

James Elston

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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