How to Drain a Combi Boiler

There are a few reasons why you might want to drain your combi boiler. For example, perhaps you’re going away for a few days in the winter and want to avoid coming home to frozen pipes. 

Or maybe you’re replacing your radiators and don’t want to flood your home in the process. 

Whatever the reason, it’s important to know that draining or flushing a combi boiler isn't something you should try to do yourself unless you have the relevant knowledge and skills to do so properly and safely. 

If you attempt this without the required skills and knowledge, you could damage your heating system and potentially your home, too, so it’s always best to call an engineer.

If your boiler is older or you’re finding yourself carrying out maintenance more often than not, it might be worth exploring a replacement. You can find up-to-date installation prices and advice in our guide to new boiler cost to see what fits your home and budget.

How to Drain Your Combi Boiler

What You'll Need:

• Hose

• Bucket

• Spanner

• Radiator Valve Key

1. Turn Off Your Boiler

To begin with, you’ll need to switch off your combi boiler and disconnect it from the power supply then waiting for the system to cool down. 

You can usually find the power button or master switch on the front of the boiler in its control panel. If you are unable to locate these, please check your manufacturer’s manual to find out how to turn off your boiler.

Make sure to wait a while for the water in the system to cool down fully before doing anything else.

If you’re planning to replace your heating system soon, it’s worth researching the most reliable manufacturers on the market. Our best boiler brands guide compares leading names like Worcester Bosch, Vaillant and Alpha to help you make an informed choice.

2. Locate the Drainage Valve

The drainage valve is usually located at the lowest point of your heating system, typically to one side of a downstairs radiator. However, it may also be located outside your property, so you won’t need to worry as much about any spillages.

3. Connect a Hose to the Valve

If your valve is inside your home, you’ll need to lead the water that will drain from your system outside so you don’t flood your property. 

To do this, connect a hose pipe to the drainage valve and lead the other end outside.

We’d also recommend putting a bucket beneath the valve to catch minor water dribbles as you drain it.

4. Open the Drainage Valve

This is where you’ll need to focus. Use a spanner to open the valve. This will cause the water to drain from the system and through the hose. 

Wait a few minutes for the system to fully drain, keeping a close eye on the hose pipe to ensure it doesn’t become detached from the valve.

5. Open the Bleed Valves on the Upstairs Radiators

Once the water has drained from the system, get your radiator valve key and turn the bleed valve on each radiator until you hear a hissing sound. 

This means the valve is open and allows the water to go down to the drainage valve.

6. Repeat for the Downstairs Radiators

Once you have drained the water from the radiators upstairs, repeat the same process downstairs to eliminate any excess water.

Please note: The heating system in some homes is split into two, meaning you’ll need to drain the water from both sides of the house.

7. Tighten the Valves

Once you’re sure all the water has been drained from your heating system, tighten up the drainage valve and all the open bleed valves around the house.

And that’s all there is to it! Now you can leave home for a few days, flush your system or replace your radiators without worrying.

If you’re thinking about upgrading while your system is drained, take a look at our expert picks for the best combi boilers in 2025 - efficient, compact models ideal for modern homes.