How to drain a central heating system

Draining a central heating system is more simple than you might think. Start by turning off the boiler and allowing it to cool. 

Secondly, locate the drain valve, usually on a radiator, and attach a hose and open the valve 

This allows air in and the water to drain out. Once drained, tighten valves and refill the system.

Draining your central heating system may sound like a daunting task, but with the right steps, it’s a manageable job for anyone with even a little confidence in their DIY skills.

Whether you're preparing to replace a radiator, fix a leak, or carry out some maintenance, knowing how to safely and effectively drain your system is essential.

Lets look into heating system draining in more detail.

Whether you're preparing to replace a radiator, fix a leak, or carry out some maintenance, knowing how to safely and effectively drain your system is essential.

Lets look into heating system draining in more detail.

Table of Contents

1. Prepare for the task
2. Locate radiator with drain-off valve
3. Drain radiators thoroughly
4. Open the bleed valves
5. Finish up the draining process
6. Radiator valves at Trade Radiators

Prepare for the task

To completely drain your central heating system, it’s often worthwhile draining more than one radiator. This helps to ensure that there is no chance of any water leaking out. One thing to bear in mind if you do drain your system completely is that some boilers will not work during this period.

Before you start to drain your radiators, it’s always a good idea to switch your boiler off. This will allow the potentially scolding water to cool down and is therefore much less likely to cause yourself any injury.

• If you have a conventional heating system, you will first need to isolate the water going into the header tank before switching off your boiler.
• If you have a combi-boiler, you can simply switch it off and drain the water once it’s cool enough.

If you feel that you need a replacement valve during or after this process, then have a check through our best radiator valves article, and we have lots of types available on our website.

If your radiators do indeed need draining, follow these simple steps below on how to drain your heating system to make your radiators run more efficiently.

• Further reading: How to Flush Sludge Out of Your Radiator

Locate radiator with drain-off valve

To get started, locate the radiator somewhere downstairs in your house that has a drain-off valve on it.

Attach a long piece of hosepipe to the drain-off valve with a jubilee clip.

If your piece of hosepipe is a tight fit on the valve then you may not need to use a jubilee clip, however the clip is normally advisable as it stops the hosepipe slipping off and avoids dirty water spilling out all over the floor. The jubilee clip can be tightened with a flat-head screwdriver.

Make sure your hose is long enough to reach the outside of your property for the water to drain out, although avoid putting the end in your flower bed as the chemicals inside the inhibitor are likely to kill off any plant life!

• Further reading: A Quick and Easy Guide to Central Heating Inhibitor

Drain radiators thoroughly

Go around your house ensuring that all the radiator valves are open, then go back to your radiator with the attached hosepipe and open up the valve. All the water will start to drain out.

To get the water to drain out more quickly and effectively, go upstairs and open the bleed valves on all of your upstairs radiators.

When you do this, you will hear the air being sucked into the system.

• Further reading: How to Bleed a Radiator - A Complete Guide

Open the bleed valves

Once you've opened the upstairs bleed valves, leave the water to drain for about 15 minutes. After 15 minutes, go downstairs and try slowly opening a bleed valve on one of the radiators.

If water starts to come out, then quickly tighten the bleed valve back up and wait another 10 minutes. However, if you hear the air being sucked into the system, you are safe to fully open the bleed valve and then do the same for the remaining downstairs radiators.

This will ensure that the last remaining bits of water are out of the system.

• Further reading: How to Remove an Airlock in Your Central Heating System

Finish up the draining process

Once the water has stopped coming out of the hosepipe and you are confident that the vast majority of the water is out of the system, go back round all of the radiators and tighten up the bleed valves.

Close the valve on the radiator with the hosepipe, remove the jubilee clip and take off the hosepipe.

Please be aware when removing the hosepipe that there may be a little bit of water waiting to spill out, so prepare with a container to avoid water spilling out.

You will have then successfully drained your central heating system.

• Further reading: How to Balance a Radiator - A Complete Guide

Radiator valves at Trade Radiators

If you’re planning to replace your radiator valves while the system is drained, make sure to explore the wide range of manual and thermostatic valves available on the Trade Radiators website. Whether you’re after a traditional look or something more modern, we’ve got plenty of styles and finishes to suit your space.

For a little inspiration, we’ve included some fantastic photos below submitted by our customers, showcasing how great the right valves can look once installed.