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How to drain a central heating system

hand adjusting radiator dial - how to drain a central heating system

Draining your central heating system can reduce your heating bills by up to 10% per year. Here's how you can rejuvenate your system

Keeping up with your central heating’s maintenance is important if you want to extend the life of your system. If left untouched over the years, rust, sludge and other debris can build up, reducing energy efficiency and potentially leading to costly boiler breakdowns. If your radiators aren’t chucking out what they used to, it’s easy to become reliant on alternatives, such as electric heaters. While this might be an ideal short term solution, for the long term, it would be far more cost effective to sort your central heating out.

We spoke to Patrick Garner, Gas Safe engineer and installation manager at Heatable, to learn how to drain your central heating system and the benefits of doing so, while Ben Gallizzi, an energy expert at, offers his thoughts on how optimising your central heating can help lower your bills.

Why drain your central heating system?

The most significant reasons to drain your central heating system are to:

  • Remove built-up sludge, rust and other debris to mitigate corrosion
  • Helps prevent boiler breakdowns

Over time, sludge can build up in central heating systems leading to corrosion. “Corrosion is one of the leading causes of boiler repairs, as a result of boiler component damage. Draining the system can therefore help extend the life of the system and prevent boiler breakdowns”, Patrick Garner tells us.

“In terms of improving efficiency and reducing heating costs, draining helps to prevent blockages that can reduce the efficient flow of hot water through the system, leading to uneven heating, cold spots on radiators and increased energy consumption.” By cleaning your central heating system and ensuring that water is flowing freely, you can reduce your annual heating bills by around 10%.

E.ON recommends annual boiler services to keep your appliance running smoothly and efficiently. Regular maintenance throughout the year will help to bring problems to light early, before they become more significant issues if they’re discovered in the winter months.

What equipment is needed to drain a central heating system?

  • An adjustable spanner or wrench
  • Radiator bleed key
  • Drainage hose
  • Bucket, towels and cloths

READ NEXT: Best oil-filled radiators

How to drain your central heating: Step-by-step guide

You should only attempt this task if you’re confident in your abilities and are sure your heating system isn’t already showing signs of damage, such as leaks or error codes. If you’re unsure, speak to a heating engineer who can complete the job for you.

Patrick Garner talks us through his step-by-step guide:

1. Turn off the heating system

Ensure your boiler and any electrical pumps are turned off to avoid heating while you drain the system. Allow the system to cool completely to prevent scalding during the draining process.

2. Prepare the area and equipment

Place towels or cloths near the work area to guard against any spills. Attach one end of your hose to the drain valve, which is usually located at the lowest point of the system, often at a radiator on the ground floor. Ensure the hose is secure, using your adjustable spanner, to avoid leaks.

3. Run the hose to a bucket

Place the other end of the hose in a suitable drain or a large bucket.

4. Open the bleed valves on radiators

Start from the topmost radiators in your home and open the bleed valves using the radiator bleed key. This action allows air into the system to replace the draining water. We’ve got a full length guide on how to bleed a radiator if you need more advice.

hands opening radiator valve with a screwdriver - how to drain a central heating system

5. Open the drain valve

Open the drain valve where you have connected the hose. Water should start flowing out of the system through the hose.

6. Drain the radiators systematically

Gradually work your way down to the lower floors, opening the bleed valves on each radiator. This ensures that air pushes the water out of the radiators, speeding up the drainage process.

7. Check for complete drainage

Once the flow of water stops, check each radiator to ensure no water remains. If a radiator still contains water, open the bleed valve to let out any trapped water and air.

8. Close the drain valve and disconnect the hose

Once all water has been drained, and there’s none running out of the hose, close the drain valve and carefully remove the hose. Ensure all connections are securely closed to prevent leaks when the system is refilled.

9. Clean and flush the system (optional, but recommended)

To remove any sediment or sludge, flush the system with clean water before refilling. This can be done by connecting the hose to the drain valve and running clean water through the system, opening and closing each radiator valve sequentially.

10. Refill the system

Reconnect your filling loop and open it to allow water back into the system. Watch the pressure gauge to ensure the system isn’t over-pressurised.

11. Bleed radiators

Bleed all radiators once again to ensure no air is trapped in them, which could cause uneven heating or noise.

12. Turn on the boiler and check for leaks

man checking boiler with tablet and adjusting settings - how to drain a central heating system

Restart your boiler and check around all radiators and connections for any signs of leaks.

If you notice any leaks or issues following drainage and re-filling, get in touch with a heating engineer.

READ NEXT: Best infrared heaters

How do you drain a heating system without a drain valve?

We asked Patrick Garner how to drain a heating system without a drain valve. Here are his instructions:

Switch everything off and wait for the radiators and pipes to cool. Then, find a small radiator on the ground floor of your home.

1. Turn off the heating system and stop the water supply

Switch everything off and wait for the radiators and pipes to cool. Then, find a small radiator on the ground floor of your home.

2. Isolate both sides of the radiator at the valves

If the radiator has a TRV, make sure this goes all the way to zero, rather than just to a frost setting.

3. Open up the bleed valves

Allow the water to run into a bowl to relieve the radiator’s pressure.

4. Loosen nuts and drain into a bowl

When the water stops flowing, gently loosen one of the nuts that attached the radiator to the valve at the bottom of the radiator – if your radiator valve has a three-quarter-inch union connection, this is usually the best place from which to drain.

Once the radiator is completely drained, gently spin one of the valves to point into the room/towards you (you may need to remove the radiator to do this). Connect a small piece of 15mm copper to the valve using a nut, olive oil and the “tail of the radiator valve”, if you have a three-quarter-inch valve.

5. Attach hose pipe and drain

Push a hose pipe over the 15mm copper and secure it with a jubilee clip or cable tie. Then, take the other end of the hose pipe to a suitable drain. Open the radiator valve to which the hose is connected, then open the upstairs radiator bleed valves to allow the system to drain.

How can I optimise my central heating usage?

man checking radiator heat with screwdriver on floor in living room - how to drain a central heating system

“Households can save money on their heating by keeping the warmth in their home and by keeping an eye on their thermostat and heating timer”, Ben Gallizzi shares. “Every degree by which you increase the temperature of your thermostat is estimated to hike up your heating bill by about 10%; it can make a big difference.”

Taking a close look at your boiler’s settings can also herald savings: “If you have a combination boiler, check the flow rate setting. Reducing the flow rate on combi boilers to around 60°C can cut heating bills, and you won’t notice the difference.”

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