How to Properly Drain a Pressurised Heating System

Draining a pressurised heating system is essential for carrying out maintenance, replacing radiators or fixing leaks. If done incorrectly, it can cause water damage or introduce airlocks into the system. In this guide, we’ll walk you through the correct steps to safely drain your heating system.

Why Drain a Pressurised System?

A pressurised heating system, unlike an open-vented system, operates with a sealed loop and relies on a pressure gauge rather than a feed-and-expansion tank. Draining may be necessary when:

• Replacing or moving radiators

• Fixing leaks in the pipework

• Installing new valves or components

• Performing a power flush

Step-by-Step Guide to Draining a Pressurised System

1. Turn Off the Boiler

Before starting, switch off the boiler and allow the system to cool down. Draining hot water can be dangerous and may cause burns.

2. Locate the Drain Valve

The drain-off valve is typically found at the lowest point of the system, often near a radiator or at the bottom of the pipework.

3. Attach a Hose

Securely connect a hose to the drain valve and direct it to a safe drainage area such as an outside drain or bucket.

4. Open the Drain Valve

Using an adjustable spanner slowly open the drain valve. Water will begin to flow out through the hose. Ensure that the hose remains securely in place to avoid leaks.

5. Open Radiator Bleed Valves

To speed up the draining process open the bleed valves on all radiators using a radiator key. This allows air into the system, helping the water drain more efficiently.

6. Monitor the Pressure Gauge

Keep an eye on the pressure gauge on your boiler. As the system drains the pressure will drop to zero.

7. Check for Remaining Water

Once the water stops flowing, check for any residual water in the system by gently tilting radiators. If necessary open additional bleed valves to remove trapped water.

Refilling the System

After completing maintenance you will need to refill and repressurise the system. Follow these steps:

1. Close all drain and bleed valves.

2. Open the filling loop on the boiler and allow water to enter.

3. Monitor the pressure gauge and stop filling when it reaches the recommended level (typically 1-1.5 bar).

4. Bleed radiators to remove trapped air.

5. Restart the boiler and check for leaks.

Final Checks

Once the system is fully drained and refilled, check all radiators for even heat distribution. If any remain cold, they may need additional bleeding.

By following these steps you can safely drain and refill a pressurised heating system without causing damage. Regular maintenance ensures efficiency and longevity, keeping your home warm and energy efficient.

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Safety Notice

Important: Working with a pressurised heating system carries risks, including potential water damage burns and incorrect system repressurisation. If you are unsure or inexperienced it is strongly recommended to seek professional assistance from a qualified heating engineer or plumber. Improper handling can lead to costly repairs or system inefficiencies.

FAQ: Pressurised Heating Systems

Q: How do I get air out of a pressurised heating system?
A: To remove air from a pressurised heating system, start by switching off the boiler and allowing it to cool. Then use a radiator key to open the bleed valves on each radiator, starting from the ground floor and moving upwards. As air escapes, you’ll hear a hissing sound followed by water. Once water starts flowing steadily close the valve. After bleeding all radiators check the boiler pressure and top up using the filling loop if it’s below the recommended level (usually 1–1.5 bar).

Q: How do I flush a pressurised central heating system?
A: Flushing a sealed central heating system involves draining the system completely, then refilling it with clean water mixed with a central heating system cleaner. Run the boiler with the cleaner circulating for a few hours (or as directed by the product), then drain again. Refill with clean water and add an inhibitor to prevent corrosion. If your system is heavily sludged, consider hiring a professional to perform a power flush using specialist equipment.

Q: How do I drain a pressurised hot water tank?
A: To drain a pressurised hot water cylinder (such as an unvented system), switch off the boiler and isolate the cold water supply to the cylinder. Open a hot water tap to relieve pressure, then locate and open the drain valve at the base of the tank. Attach a hose to the valve and direct it to a safe drainage area. Open the drain valve slowly, allowing the tank to empty fully. Always consult the manufacturer’s instructions or a qualified heating engineer before draining an unvented cylinder, as incorrect steps can be hazardous.

https://en.wikipedia.org/wiki/Boiler
https://en.wikipedia.org/wiki/Central_heating
https://www.quora.com/What-is-the-correct-way-of-draining-a-central-heating-system