How to Drain a Central Heating System

James Elston
Written by James Elston
Updated on 5th December 2023
Posted on 4th December 2023
Topic: Boiler Guides
How to Drain a Central Heating System

How to drain a central heating system yourself in easy steps

drain valve

So you need to do some work on your central heating system, maybe install a new radiator or do some repair work, but before you can even begin, you need to know how to drain a central heating system.

Don’t let this be a block. Draining your own central heating system is something that you can do yourself with some simple tools that you most likely already own.

Across the UK it is reported that 95% of houses have central heating systems, so if you are in this 95% and need to do some work on your central heating system or your radiators, then this article is for you.

If your’re getting a new gas boiler replacement it may be a good idea to drain your system.

We are going to take a deep dive into exactly how to drain a central heating system for those with or without a radiator drain-off valve, when you might need to drain your central heating system, how long it takes, and how easy it is.

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How do you drain a central heating system?

To drain a central heating system you first need to switch your heating system off for safety reasons as well as cut the water supply, which can be done at the mains or by isolating your boiler. Next, you need to locate the main radiator which has the drain-off valve, this is usually located on the ground floor of your home.

Following this, you then need to securely attach a hosepipe to the drain-off valve, open it and allow the water to run through. You can speed up this process by bleeding the radiators. Then just wait until there is little to no water running through the hosepipe.

Finally, once you have completed any work that you needed to do, you can refill the radiators and central heating system in your home.

Let’s dive into these stages in more detail with a 6 step guide to get you on your way to draining your own central heating system.

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6 steps to drain and refill a central heating system

We rely on our central heating system to heat our homes and often provide hot water. Usually, it ticks along quite nicely in the background. But when there is an issue, getting a replacement boiler, or you need to perform maintenance upkeep, then it is important to know how to drain your own central heating system so that you can proceed with the work.

If you proceed with your job without first draining your central heating system then you risk damaging and flooding your home. Typically hot water and steam run throughout your house through pipes and radiators to heat your property. So before you begin any work, it is important to drain the water out to avoid any damage that this would otherwise cause. It is also worthwhile draining all of the radiators throughout your home, even if you are only working on one.

lockshield drain valve

Before we dive into the step-by-step guide, you will need to gather some equipment.

To drain your central heating system you will need:

  • Adjustable spanner
  • Hosepipe
  • Radiator key
  • Jubilee clip (a clip designed to attach a hosepipe to a rigid pipe with a smaller diameter)
  • Towels

You might also need:

  • Flathead screwdriver
  • Bucket or bowl to catch water

In the next 6 steps, we will break down exactly how to drain and re-fill your central heating system.

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Step 1. Switch the heating system off

First things first, as a safety measure it is important to switch off the heating system. Remember to wait until it has completely cooled down, this can take anywhere from around 30 to 60 minutes.

It is important to allow the pipes and the radiators to cool completely before starting to drain your central heating system. Alternatively, if your boiler operates with solid fuel fire then make sure that the fire is extinguished and cooled before proceeding.

Step 2. Cut the water supply

Next, you need to make sure that the water supply to the central heating system is switched off. There are several ways that this can be done.

You can either completely turn off the water supply at the mains water stopcock. This is usually a separate tap or a valve that you can easily tighten to turn off with a flathead screwdriver. Bear in mind that if you do decide to do this, then it does mean that you will have no running water in your home for the duration of the time period you are working.

If you have a conventional central heating system, you should isolate the central heating tank. This means that there will be no new water running into the tank and the water that is currently in the tank will be drained. There is usually a tap or valve on the water pipe leading into the central heating tank. Then switch off the boiler.

Alternatively, if you use a combi-boiler then the process is slightly different. You only need to switch off the combi boiler and wait for it to completely cool down before continuing with the draining process.

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Step 3. Locate the radiator with the drain-off valve

This is usually one of your larger radiators at the lowest level in your home.

Note: If your radiator does not have a drain-off valve then feel free to jump down to the next section where we break down alternative steps to take.

Next, you need to find where the main radiator is in the house that has the drain-off valve. This is usually located somewhere on the ground floor and looks slightly different from the other radiators as it has a drain-off valve to attach your hosepipe to.

All of the radiators within your house are connected through pipes and will drain out through this main radiator. The drain-off valve (or draincock) looks like a pipe opening and is usually positioned on one of the valves at either end of the radiator.

Take your garden hosepipe and attach it to this outlet. Use your jubilee clip here to make sure that the hose is securely attached to the drain-off valve to prevent leaks. To be on the safe side, it is always worth putting down absorbent towels on the floor just in case.

Trail the hosepipe outside of the house and towards a drain. Remember that quite often there are various chemicals (also known as central heating inhibitors) in this water, so it is important that you don’t just let the water run into any plants or your lawn. Instead, find a main drain that is safe to dispel this water in.

Step 4. Open the drain-off valve

Now that the hosepipe is securely fastened at one end and safely positioned at the other, you can open the valve to begin the draining process.

You will need your adjustable spanner to loosen the nut next to the drain-off valve. Turn it anti-clockwise to loosen. You should hear water rushing through the hosepipe at this point.

Make sure that all the radiator valves in the house are open. Every radiator will have two main valves, one that controls how much water is traveling through the radiator called the regulator, and the other is a thermostatic valve called the lockshield which controls the temperature.

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Step 5. Bleed the radiators and allow water to drain

The next step is to bleed your radiators. To learn more about bleeding your radiators then click here.

In a nutshell, bleeding your radiator is to release air that has become trapped in the system. It is common to need to bleed your radiators as air can get trapped in the system and cause your radiator to distribute heat un-evenly as the areas with trapped air remain cool.

Start the bleeding process at the top of your home with all the upstairs radiators (remember any towel rail radiators or any spare rooms that can be easy to miss) and make your way down to the ground floor. A top tip is to open the ones on the top floor and then wait for 15 minutes before moving on to the downstairs radiators, this will help speed up the water draining process.

Try to drain your radiators as thoroughly as possible before starting any work and keep a spare towel near the drain-off valve just in case there is any delayed water that needs to drain through.

Once your central heating system has drained, which you will know when there is little or no water coming from the hosepipe, you can get started with the work that you need to do.

Step 6. Refill the central heating system

Last but not least, you will want to refill your central heating system. To do this, you will need to close all the bleed valves and the drain-off valve. Double-check that every radiator has its bleed valves closed. When you detach the hosepipe from the drain-off valve, you will be able to see if you have closed it properly.

Switch the mains water supply back on and after a moment your central heating system will start to refill. You will then need to bleed your radiators again. To do this efficiently, you should start from the ground floor of your property and make your way up. It is important to remove all the air in the radiators to ensure that they will work properly.

Have a spare towel on hand to do this. Once the radiator has released all of the air, there may be a small amount of water dispelled just before you tighten the bleed valve back up. Once all the radiators have been bled of air, then they should all be refilled with water.

Before finishing off, give a final check that the valves are all securely tightened. Turn the power supply back on and monitor how they all heat. You may need to bleed some of the radiators again. Use this time to check all the joints and valves for leaks.

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How to drain a central heating system without a drain-off valve

If you do not have a radiator with a drain-off valve then the process is slightly different.

Steps 1 and 2 will be the same. You will still need to turn off your heating system and make sure that all of the radiators and pipes have properly cooled down and that the water supply has been cut off.

Next, you will need to isolate the radiator from the heating system. There are two main valves on a radiator, one to let hot water in (the regulator) and the other to control the temperature (the lockshield). To isolate the radiator, you will need to close the two valves on either side of it.

You close the regulator valve by rotating it clockwise. You close the lockshield valve by removing the plastic cap and using pliers to tighten the valve. A top tip is to take a note of how many turns you have made, as you will need to re-tighten it the same amount when you are refilling it.

Similarly to when you do have a drain-off valve, you will need to bleed all of the radiators to release the air and speed up the draining system.

Of course, without the drain-off valve, you will need an alternative way to release the water. Firstly locate the coupling nut which will be positioned on the regulator valve side of the radiator. Place a bucket and towels underneath this coupling nut and with the support of an adjustable spanner rotate the nut anti-clockwise until the water starts to pour out. Monitor this and tighten the coupling nut when the bucket is full. Empty the water and continue. Alternatively, attach a hose using a speed fit tap to drain the radiator.

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How long does it take to drain a central heating system?

It can take anywhere from 20 minutes to over an hour to completely drain a central heating system. This completely depends on your system, whether or not you have a drain-off valve and if you run into any issues during the process.

Remember to open the bleed valves on all of your radiators to help speed up the process. This will help with the suction of water and encourage it to drain out.

When you are draining a central heating system it is important to keep in mind that your boiler won’t work during that period of time. If you have closed off the water supply at the mains then you won’t have access to any water during this time. Bear this in mind when you start this process to ensure that it doesn’t cause any problems.

When to drain a central heating system?

You may need to drain your central heating system for various reasons. It may be for maintenance reasons such as a noisy central heating pump, basic repair work, or even to install a new radiator. Whatever the reason may be, you will likely need to drain your central heating system at some point.

As with anything, upkeep is hugely important and will ensure that your central heating system continues to perform well and provide long-lasting results. You will want to occasionally drain and clean out the system to remove any build-up of sediment, limescale, and rust. Of course, to do this, you will first need to know how to drain your central heating system.

Additionally, it is recommended to add an inhibitor to your system. An inhibitor is a specific blend of chemicals that will limit corrosion and prolong the lifespan of your central heating system. Over time your central heating system may become clogged with rust, sediment, and limescale, which is perfectly normal but will prevent both your central heating system and radiators from working properly. Using an inhibitor will break this down and even prevent it from building up again. You can add an inhibitor liquid yourself before you refill your system.

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Is it easy to drain a central heating system?

Hopefully, after reading through our step-by-step guide to draining a central heating system you will feel capable and confident in doing this yourself. However, if you come across any issues whilst you are doing it, then both a heating engineer and a qualified gas-safe plumber will be able to support you and drain your central heating system for you.

In the meantime, if you have any questions about draining your central heating system or replacement boilers, then feel free to reach out for advice and information from the boiler central team.

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Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 4th December 2023
Topic: Boiler Guides

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

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