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Last Updated on January 22, 2021

How To Flush A Central Heating System Combi Boiler?

Zac Houghton

Zac Houghton

Construction expert and founder of Loftera.

So, you want to know how to drain a combi boiler? You’re not alone. It is a question asked by thousands of homeowners every year.

The reasons for flushing your boiler system can vary. One of the main reasons why people choose to drain a combi boiler is to remove any unwanted residue in their radiators. This is known as sludge. Sounds unpleasant, doesn’t it? It really is, and you’ll want to get rid of it from your heating system sooner rather than later.

If you are planning certain repairs to your heating system, whether that be through the internal piping or your radiators, you’ll also want to flush your boiler system beforehand.

Finally, if you are planning on spending a long period of time away from your property, you’ll want to avoid your pipes from freezing over. You’ll want to flush your central heating system before departing in order to ensure this.

Whatever your reason, let’s move on from the “why” and focus on the “how”.

How To Flush A Central Heating System Combi Boiler

Table of Contents

How To Drain A Combi Boiler?

First and foremost, let us be clear that we absolutely advise that you contact a professional engineer to perform this job for you. If you have any doubts, or any gaps in your knowledge, you could end up damaging your heating system. Not only that, but any repairs that are required due to damage made by yourself may not be covered in any existing warranty that you have taken out.

If you are confident in your ability to drain a combi boiler yourself, carry on reading below for any tips that you may not have thought of.

To begin with, make sure that the boiler is switched off and the power supply is disconnected. You’ll definitely want to wait a while for the water to cool down, to avoid any unwanted burns or scolds.

From here, connect a secure hose (provided when the system was fitted) to the drainage valve. This will usually be found on the radiator in the property that is at the lowest point, in order for the water to drain properly. Once fitted, the valves of all the radiators in the property will be opened, in turn flushing the water from each one.

Once all the water has been drained, the vales – known as bleed valves – are closed.

How To Drain And Refill Central Heating System With A Combi Boiler

Once the above is complete, it is now time to refill your central heating system. There will be a filling loop nearby to the boiler. This loop will return the pressure in your combi boiler to the required level for your central heating to work properly.

In order to remove the air that is trapped in your heating system, you’ll need to bleed your radiators. Begin downstairs, and work your way up the property. Once the air pressure has been successfully removed, the filling loop can be opened.

At this point, you’ll want to ensure that there is no leaking water coming from your boiler – boiler leaking water post.

Flushing Your Boiler System

Over the years, your radiators can build up rust, dirt, and that unwanted sludge that we discussed earlier. Whenever water and metal combine, rust is an inevitability – so steps need to be taken to deal with this.

Too much sludge can actually negatively impact the performance of your radiators, leading to cold areas within your property. In turn, this will end up costing you more money in bills to heat your home as the energy efficiency won’t be to the required standard.

Flushing your combi boiler helps to eradicate that sludge via a powerful machine that pumps water through your central heating. This water works to dislodge excess dirt and rust, and can take up to three hours, even if performed by a professional engineer.

Adjusting The Pressure In Your Combi Boiler

An incorrect pressure in your heating system can cause your boiler to become faulty. Instructions of how to adjust the pressure should be provided in the user manual that came with the heating system.

If the pressure (1) gauge falls below 1, it is too low and will need repressurising. Similarly, if it is too high, you’ll need to release the excess air and water to bring the pressure back down to between 1 and 1.5.

How Often Should I Refill My Combi Boiler?

We wouldn’t recommend refilling your combi boiler any more than twice a year. If you are required to top it up any more than this, through problems with your pressure or heating, then the time may have come to fully repair (boiler service cost) or even replace your boiler. If this is the case, our blog on new boiler costs may be the best place to start.

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