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Drain A Combi Boiler

How to Drain a Combi Boiler?

The process of draining a combi boiler is a challenging task, therefore, unless you have the relevant knowledge and experience, you should always hire a Gas Safe registered engineer to do the job safely.

When to drain a combi boiler?

There are several reasons why your combi boiler might need draining. Generally, you would drain your system if you are planning to replace the radiators or pipes or to clear any blockages. You might also decide you want to drain your combi boiler to avoid coming home to frozen pipes if you are going to be away from home for a few days during the winter.

How to drain a combi boiler?

The exact way in which you drain your combi boiler depends on the specific model that you have. Therefore, you should always follow the instructions set out in the manual, to get the best results.

Before you go about draining your combi boiler, you should first switch off the unit and disconnect it from the power supply. It is important to be patient and wait before proceeding any further, to ensure that the water in the system has cooled down to a safe temperature.

To drain a combi boiler, you will need several pieces of equipment, which include a hose, a bucket, a spanner and a radiator valve key.

Find the drainage valve

The drainage valve tends to be at the lowest point of your heating system. This is usually located to the side of a downstairs radiator, although there is the possibility that you will find the drainage valve outside. If this is the case, then you can put less emphasis on any potential water spillages.

Attach a hose to the valve

To avoid water damage to your floor or carpet, you need to securely attach a hose pipe to the valve, leaving the other end of the hose over a drain or somewhere you do not mind getting wet. This will take the water that will drain from your heating system outside. It is also recommended to place a bucket underneath the valve, to catch any little dribbles of water.

Open the valve

With the use of a spanner, you can turn the valve to open it. This will cause the water to drain out of the heating system and through the hose pipe.

Open the bleed valves on the upstairs radiators

When you have given the water a couple of minutes to drain from the heating system, you can now move your focus to the radiators situated upstairs.

To begin this part of the process, you need to access the bleed valve, which is usually found at the top left or right of each radiator. You can then take your valve key and turn the bleed valve on each radiator. You will hear a hissing sound to indicate that the valve is open, allowing the water to make its way down to the lowest radiator and eventually out through the drainage valve.

At this point, it is also a good idea to drain the other downstairs radiators to get rid of any excess water.


Tighten up the valves

Your heating system has been successfully drained when the water has stopped running out of the hose. You can now tighten up the drainage valve, along with all the open bleed valves around your home.

How to refill a combi boiler?

After draining your combi boiler, you can then refill the system with water. To do this, you will need to gain access to the filling loop, which connects the mains water pipe to the central heating pipe. It can be located underneath the boiler and looks like a flexible silver hose connecting two pipes.

On the filling loop, there should be a valve at each end. Turn both valves to open them, allowing water to flow into the heating system.

When the valves are open, keep a keen eye on your boiler’s pressure gauge. Your boiler’s water pressure should be somewhere between 1.5 and 2, but always check the manufacturer’s instructions, to ensure you get personalised information for your boiler model.

Worcester Bosch Boiler

Homeowners are still able to get a FREE boiler grant or insulation grant via the ECO scheme, which is still available to households that qualify. Warma UK are currently working throughout the UK helping privately owned and rented homes to improve home energy efficiency and save money on rising energy bills.

For more information email –

Or call 03304600065

Apply For Your Government Backed Heating Grant

Following your successful application, we will arrange a free, no obligation survey to ensure the maximum grant is obtained and advise you on the best products to reduce your homes energy usage.

Michael Cornish

Michael Cornish