Eco Happy (Solar Expert)

Published February 23, 2024

Want to avoid coming home to frozen pipes over the holiday season? Or does your heating system require some work? Well, you’re going to have to drain that heating system to avoid pipes freezing or access different heating components.

Heating system drainage is a challenge most homeowners in the UK will find themselves faced with from time to time.

Boiler water drainage can be a tricky task if you don’t know what you’re doing. Which is why we always recommend hiring a professional. However, if you’re a homeowner who’s not afraid of a little DIY boiler maintenance challenge, we’ve got your back with a step-by-step boiler draining guide.

You don’t want to mess it up though! Or you’ll be left with bigger problems than just frozen pipes.

When Does A Combi Boiler Need Draining?

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. Another reason you might need to drain the combi boiler is for DIY boiler maintenance, like replacing your radiators. If you don’t, you’ll most likely end up flooding your house.

So yes, to avoid causing any serious damage, your boiler unit will need to be drained at some point.

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 even extend your boiler warranty!

Can I Drain A Combi Boiler Myself?

It’s not recommended that you do this yourself. It’s always best to rather leave the draining to a qualified heating engineer. Draining the combi boiler incorrectly could lead to ending your central heating system warranty earlier or damaging the boiler.

However, it is possible to do it on your own as long as you follow the correct steps. We’ve put together a simple 5-step guide below on how to drain your combi boiler, but it’s important to keep in mind that not all models are the same. You should consult your boiler manufacturer’s guidebook for further draining tips.

How To Drain A Combi Boiler: Step-By-Step

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 any mistakes that could lead to permanently damaging your combi boiler system.

However, 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 central heating system

Turn off your boiler and power supply to be extra safe. 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 carefully turn the valve 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. You’ll need your valve key to open those. You’ll hear a hissing noise once it opens.

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 too much you might damage the system and need to call an 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.

Dealing With A Leaking Combi Boiler

So, something went wrong and your boiler is now leaking. This is pretty serious and means it’s time for you to admit defeat and contact a professional! Whether it is leaking brown water or just 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 you fill a combi boiler after draining?

To fill a combi boiler after draining it, locate the filling loop underneath the boiler. It’s a flexible silver hose connecting two pipes, the mains water pipe and the central heating pipe, with valves at each end. Open both valves to let water flow into the heating system. You should close the valves again when the pressure gauge reaches 1.5 bar.

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 and even lead to leaks or malfunctions. Leave this tricky procedure to a qualified Gas Safe registered engineer, like Eco Happy, 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. 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. If you’re unsure about any step in the process, contact us at Eco Happy so we can give you a helping hand.

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