There are a number of reasons why you might want to drain your combi boiler and heating system. For example, perhaps you’re going to leave your property empty for an extended period of time 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. You might also want to flush your heating system to get rid of any sludge in your radiators. 

How to drain a combi boiler

It’s essential to be aware that draining or flushing 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 do this without the required know-how, you could damage your heating system and potentially your home too. It is also likely that you will invalidate your warranty.

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

How to drain a combi boiler

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. Before this task is carried out, the boiler must be switched off and disconnected from the power supply. 

Once the water has cooled down, a hose is securely connected to the drainage valve, which is usually the lowest point of the lowest radiator in the property. The valves of all radiators in the property are then opened. Once this has been done, the valve on the radiator with the hose attached is opened. The process of removing the water can be sped up by opening up the vents at the top of each radiator. Once the water has gone, these bleed valves are closed.

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

How to refill

To refill a combi boiler, the drainage valve is closed and the hose removed. The filling loop located underneath or near to the boiler is then opened to allow the system to repressurise to the level recommended in the manual. To get rid of the air trapped in the system, the radiators are then bled, starting downstairs. Once the air has been released from all the radiators, the filling loop can be opened again to get the pressure back to the correct level.

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

A warning

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.

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Flushing a combi boiler

You may wish to flush your combi boiler and central heating system to keep it in good condition. This is because over time, a mixture of rust and dirt can build up in radiators. This sludge gathers at the bottom of radiators and if there is too much of it, it can stop hot water from flowing through your heating system properly, creating cold areas.

Power flushing will help to remove the sludge, increasing the efficiency of your heating system. It can also help to protect your boiler and reduce system wear and tear.

This type of maintenance should be carried out by a Gas Safe registered engineer. They will connect a machine to your heating system that pushes a powerful flow of liquid through it, dislodging rust and dirt from your radiators and your boiler. The process can take several hours, or longer if you have a big property or your central heating system is particularly clogged.

How to adjust the pressure in a combi boiler

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.

Too low

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.

Too high

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.

How often should a combi boiler need topping up?

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 an engineer.

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

Will turning the water off affect a combi boiler?

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, whether it’s a Vitodens 111-W or any other model, 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. 

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