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How to Drain Heating Systems

how to drain heating systems

Are you planning to drain your central heating system but need help figuring out how to start? This guide will show you how. including the best times to drain your system and why it’s necessary.

Regular draining and flushing of your heating system can significantly extend its lifespan and reduce the risk of significant faults and breakdowns, ensuring that your heating operates at its optimal level. 

Over time, your central heating systems can accumulate sludge and debris, which can restrict the flow of water and reduce the efficiency of your radiators and boiler. A well-maintained heating system provides consistent warmth, so if you’ve noticed cold spots on your radiators or uneven heating in your home, it could be a sign that your system needs draining. 

It’s not just performance issues that may require you to drain your heating systems. Whether preparing for repairs or just performing regular maintenance, draining your central heating system eliminates the risk of water damage or spills and allows for a more thorough inspection and maintenance service.

Types of Central Heating Systems

Each central heating system has its unique layout and components, which can influence the draining process.

Boilers and Radiators 

Boilers come in different types, including combi, system, and conventional boilers. 

  • Combi boilers are compact and efficient, providing hot water on demand without needing a separate cylinder. 
  • System boilers use a cylinder to store hot water, making them suitable for homes with multiple bathrooms. 
  • Conventional boilers, also known as regular or open vent boilers, require both a cylinder and a water tank, often suited for larger homes with higher water demands.

All of these boilers heat water, which flows through pipes to your radiators, releasing heat and warming your home.

Underfloor Heating

Some homes have underfloor heating, either as the primary source of heat or in addition to radiators. This system involves laying pipes or electric heating mats under the floor to provide uniform heat distribution.

When to Drain Your System

When you drain your central heating system is just as important as knowing how to do it, and the timing can impact its effectiveness, so it’s important to get it right!

Seasonal Changes

The best time to drain and maintain your heating system is towards the end of summer or early autumn before you start using your central heating regularly. Particularly if you’ve had your heating off during summer, draining the system at the end of the season will clear out any accumulated debris. This ensures your system is clean, efficient, and ready to perform optimally during the colder months when you rely on it the most. 

Before Repairs or Upgrades

If you’re planning to repair or upgrade any part of your heating system, whether that’s your boiler, radiators, or pipes, draining the system beforehand ensures that no water is left in the system that could cause issues during the repair or upgrade process.

After Noticing Performance Issues

If you notice issues like uneven heating, strange noises from the boiler, or radiators that don’t heat up properly, it might be time to drain your system. These signs often indicate air or sludge build-up in your system, which draining can resolve.

Following a New Installation

After installing a new boiler or radiator, it’s advisable to completely drain the system. This helps to remove any debris or flux from the installation process, ensuring your new equipment functions efficiently.

Regular Maintenance

Even if you’re not experiencing any issues, it’s good practice to drain and flush your central heating system every few years so your system is running smoothly and extends its lifespan.

How to Drain Your Heating System: Step-by-Step Guide

Before you begin draining your central heating, you’ll need a few tools, including:

  • A hose
  • A flat-head screwdriver
  • An adjustable spanner
  • A bucket or container
  • Old towels or rags for any spills

Once you’re ready, follow these steps to drain your heating system:

  1. Turn Off System and Water Supply: Start by switching off your system and water supply, allowing the boiler to cool down completely to reduce the risk of burns from hot water and prevent any accidental damage to the system while it’s still operational.
  2. Find Radiator Drain Valve: The drain valve is usually found at the system’s lowest point, often near the downstairs radiators closest to the ground floor.
  3. Connect Hose: Remove the plastic cap, attach one end of your hose to the drain valve and ensure the connection is secure to prevent leaks (you can use a jubilee clip for extra security). Run the other end of the hose outside or into a drain, making sure it’s lower than the valve to allow gravity to help the water flow.
  4. Open Drain-Off Valve: Use the flat-head screwdriver to open the drain valve and turn the nut counterclockwise until the water starts flowing out of the system into your hose. Be prepared for the water to be dirty as it contains sludge and debris from inside the system.
  5. Bleed Radiators: Go to the top floor of your house and open the radiator’s bleed valve to allow air into the system, which helps the water drain out faster. Work your way down, bleeding each radiator in turn.
  6. Check Water Flow: Let the water run, but keep an eye on the water flowing through the hose. Once it runs clear, most of the sludge and debris have been removed.
  7. Close Valve & Disconnect Hose: Once the water is clear, close the drain valve and carefully disconnect the hose. Be ready to catch any remaining water with your bucket or rags to protect your floors and furnishings.
  8. Refill System: Close all the bleed valves on your radiators, and then slowly refill your heating system following your boiler’s instructions.
  9. Bleed Radiators (Again): Once the system is refilled, bleed your radiators again to remove any air that might have entered during the refilling process.
  10. Turn On System: Turn your boiler back on, check the pressure gauge to ensure it’s at the correct level, and adjust if necessary.

Safety Tip: Ensure the area is well-ventilated, especially if working in a confined space like a boiler room. Good ventilation helps to avoid any build-up of fumes from the boiler or heating system.

If you feel unsure or unsafe at any point when draining your heating system, stop immediately. It’s best to call a professional who has the expertise to safely drain your heating system.

Troubleshooting Common Problems

While draining your central heating system, you might encounter a few issues. Here’s how to troubleshoot common problems effectively.


Common areas for leaks during the draining process include radiator valves and connections. Once you identify the source, tighten any loose fittings with an adjustable spanner. If the leak persists or the source is unclear, it may be best to consult a professional. At GLP, we have the expertise to accurately diagnose and fix leaks in your heating system.


Blockages can prevent water from draining properly. If the water isn’t flowing or draining very slowly, check for stoppages at the drain valve or the hose. Clearing these can resolve the issue. If the blockage is within the system, a HVAC specialist can provide a professional flush to clear it effectively.

System Checks

After draining and refilling your system, it’s important to check for any operational issues. If the system isn’t heating up as expected, ensure all bleed valves are closed, and the boiler pressure is correct. Air trapped in the system can cause issues, so re-bleeding the radiators might be needed. For complex problems, GLP can offer our skilled services to ensure your heating system functions correctly and efficiently. 

How to Drain a Heating System FAQs

What if the System Doesn’t Drain?

If your system doesn’t drain, it could be due to a blockage or an issue with the drain valve. Check for any visible blockages and ensure the valve is fully open. If the problem persists, contact a professional to assist in identifying and resolving the issue.

How Often Should I Drain My Central Heating System?

It’s generally recommended to drain and flush your system every few years. However, this can vary based on your system’s age, type, and usage, which is why it’s best to schedule regular maintenance checks that can help determine the need for draining.

Will Draining Affect My Boiler’s Warranty?

Draining your system shouldn’t affect your boiler’s warranty as long as it’s done correctly. Always check your boiler’s warranty terms and, if in doubt, consult with a professional.

Do I Need to Refill the System After Draining?

Yes, after draining, it’s essential to refill the system and bleed the radiators to remove trapped air.

How Long Does it Take to Drain a Central Heating System?

The time it takes to drain a central heating system can vary. Generally, it takes a few hours, but this depends on the size and complexity of your system.

Need Help with Your Central Heating System? Contact GLP. 

At GLP, we understand that maintaining your central heating system can be a challenging task. Whether you’re facing difficulties draining your system, encountering unexpected issues, or prefer a professional job, we’re here to help.

Our experienced heating engineers are equipped to handle all your heating system needs. From routine maintenance to troubleshooting complex problems, we offer a range of services to ensure your home stays warm and comfortable all year round.

Don’t hesitate to reach out to us for expert advice and assistance. Contact GLP today, and let us take the stress out of maintaining your central heating system. 

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