How to drain a combi boiler

There are a number of reasons why you might want to drain your combi boiler and heating system. Perhaps you’re planning on leaving your property empty for an extended period of time, for example, and want to avoid the risk of your pipes freezing. Or maybe you’re planning to make repairs or alterations to your radiators or pipework and don’t want to flood your property in the process.

It’s essential to be aware that draining a combi boiler isn’t something you should attempt yourself unless you have the relevant knowledge and skills to do so properly and safely. If you try to do this without the required knowhow, you could damage your heating system and potentially your home too.

Here, we take a look at the steps involved in draining a combi boiler. We also offer advice on how to repressurise your combi boiler, and explain how cutting off your mains water supply can affect this appliance.

Exactly how you drain a combi boiler system will depend on the model of appliance you have, so you should always follow the instructions set out in the manual. Our steps below can be used as a basic guide, though this task would be better carried out by a professional. 

Step 1: Turn off the boiler and power supply

Before you do anything, it’s really important to switch the boiler off and disconnect it from the power supply completely. Then, wait for the water to completely cool down. This could take up to an hour.

Step 2: Locate the drainage valve and attach a hose

To drain your combi boiler, you’ll need to find the drainage valve. It can usually be found at the lowest point of the lowest radiator in the property. Once located, attach a hose and lead it either outside or into a sink. The idea is that the whole system will be drained via this hose, and you don’t want all the water leaking into your home. Once the hose is attached to the radiator, place a bucket or other container underneath the valve just in case there are any spillages.

Step 3: Open the valves

Opening the radiators’ valves will let the water out. Do this one at a time, starting at the radiator that has the hose attached to it. Use a spanner to turn the valve to the left to loosen it. Wait a few minutes for the water in this section to drain.

Once all the water has drained, you should open the bleed valve at the top of every radiator. You’ll know when the valve is open, as you’ll likely hear a sucking or hissing noise. Again, you may need to wait for a few minutes for all the water to exit the system via the attached hose pipe.

Step 4: Close the valves

When all the water is out of the system, you will need to close the bleed valves. Do this one at a time and make sure you don’t miss a radiator. Finally, disconnect the hose and retighten the drainage valve.

At this point, any planned maintenance of the central heating system can be carried out.

The best way to refill a combi boiler is using the filling loop. This is usually located underneath or near the boiler. When the loop is opened, it allows fresh water from the mains supply to enter the system. Filling loops are generally used to bring more water in when your boiler is showing low pressure on the pressure gauge.

To refill a combi boiler, the filling loop should be opened to re-pressurise the system to the correct pressure, as provided in the boiler’s manual. The loop can be opened using the two valves located on either side of it.

This is the easy part of refilling a boiler. Next, you need to remove all the air that’s trapped in the system. This can be done by bleeding the radiators one at a time, starting downstairs. After the air in the system has been removed, you’ll notice that the pressure on the gauge will have dropped again. Add more water using the filling loop until it sits between about 1 and 1.5 bar of pressure. Continue this process until you’re happy that there’s little to no air in the system and the boiler’s pressure is correct.

If any maintenance or repairs have been carried out to the system, it’s a good idea to check the relevant pipework and radiators to make sure there are no leaks. The boiler can then be switched back on.

Draining and refilling a boiler may seem straightforward in principle, but in practice it can be difficult. So, as stated previously, unless you know exactly what you’re doing, it’s best to get a Gas Safe registered engineer in to do this work for you.

If the pressure in your central heating system is either too low or too high, your boiler may not work properly. Don’t panic though. Adjusting the pressure of your boiler should be a straightforward process and you will find instructions on how to do this in your manual.

When your heating system is cool, the pressure shown on your boiler’s pressure gauge should be between 1 and 1.5 bar. If it falls below this, you will need to repressurise it. The specific instructions on how to do this will vary depending on the model of boiler you have, but you will always start by turning off your heating. You can then locate the filling loop, which is usually a flexible hose with a small valve at each end.

To allow mains water into the system and therefore increase the pressure, open both of the valves on this hose. You should be able to hear water entering into the system. Once the pressure gauge reaches 1.5 bar, close both valves. You can then turn the boiler back on.

For a more detailed explanation, check out our article on what to do if your boiler has low pressure.

If the gauge on your boiler shows that the pressure is too high, the best thing to do is bleed a radiator. This is easier with two people as one person can keep an eye on the pressure gauge. To do this job, you’ll need a radiator key, or a flat head screwdriver for more modern radiators. You’ll also need a bowl or bucket to put under the radiator to catch the water.

Firstly, make sure you switch your boiler off and allow the system to cool down. Making sure you start with a radiator downstairs, locate the valve you need to loosen. You’ll find it on the top of the radiator at one end. Then simply use the key or screwdriver to slowly turn the valve anticlockwise. You’ll hear air escaping first, then water will follow. Make sure you’re ready to catch this in the bowl. Someone should keep an eye on the boiler pressure gauge as you drain the water. Once it returns to a suitable reading, stop bleeding the radiator.

If the pressure dial doesn’t register any change, it may be broken. In this case, it’s best to call an engineer to take a look. Don't panic, however. Your boiler is built with a pressure release valve that means that there is no danger even if the pressure looks much higher than it should be.

Boilers may need repressurising once or twice a year. If you have to top up your boiler more often than this, it could be that there is a problem with your heating system, such as a leak. It’s therefore recommended to get it examined by a Gas Safe registered engineer.

We also recommend reading our article on what to do if your boiler is losing water for more handy tips.

Combi boilers take water directly from the mains, so if you need to switch your water off, this will affect your appliance. You can continue to use your heating while the water is switched off, but you must not use your hot taps at all. If you turn your hot taps on, you risk damaging your boiler.

Regardless of the type of combi boiler you have, make sure you consult your manual before repressurising it or attempting any other fixes. For anything other than simple tasks, we recommend getting a Gas Safe registered engineer to do the work.