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Last updated: 3rd August, 2023


How to Drain a Combi Boiler

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.

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. 

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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.

How to Drain Your Combi Boiler

What You'll Need:

  • Hose

  • Bucket

  • Spanner

  • Radiator Valve Key

What You'll Need To Drain Combi Boiler

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. 

How To Turn Boiler Off

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.

2. Locate the Drainage Valve

Find Radiator 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

Connect a Hose to the Radiator 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

Open the Drainage Valve On Radiator

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

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

Tighten the Radiator 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.

For more advice and guidance when it comes to your home heating, check out our helpful blogs or get in touch with our friendly team at iHeat today!

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Frequently Asked Questions

What is the process for draining a boiler heating system?

To drain a combi boiler, follow these steps:

  1. Turn off the boiler and power supply, then wait for the water to cool down completely.
  2. Find the drainage valve and attach a hose.
  3. Open the valve and let the water drain out.
  4. Open the bleed valves on upstairs radiators, followed by the downstairs radiators.
  5. Once the system is drained, close all the valves.

How to lower boiler pressure?

Lowering boiler pressure can be done by:

  • Bleeding the radiators: Use a radiator key or a flathead screwdriver to slowly open the bleed valve until air escapes, then close it once water appears.
  • Draining some water from the system.

How to refill a combi boiler after emptying?

To refill a combi boiler, follow these steps:

  1. Close all bleed valves on the radiators and the drainage valve.
  2. Open the filling loop valve to increase water pressure within the system.
  3. Monitor the pressure gauge to ensure it has reached the recommended level, usually between 1.0 and 1.5 bar.
  4. Close the filling loop valve, and restart the boiler.

Where is the drain valve located on a Worcester boiler?

The drain valve on a Worcester boiler can typically be found at the bottom of the boiler unit. It is usually a small valve with a red handle or a brass connection point where a hose can be attached.

How long should it take to empty a boiler?

Draining a boiler can take anywhere from 30 minutes to a few hours, depending on the size of the system and the amount of water inside. It is essential to allow enough time for the water to cool down before attempting to drain it.

Can you partially drain a central heating system?

Yes, it is possible to partially drain a central heating system. This may be necessary when addressing specific issues, such as carrying out maintenance on a single radiator or fixing a leak. To partially drain the system, follow the regular draining process but close the drainage valve once the water level reaches the desired point.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.