How to Drain Your Central Heating System

Draining your central heating system is occasionally necessary — for major repairs, radiator replacement, or before a power flush. It's a straightforward process but requires care to avoid burns and water damage.

When you might need to drain the system

• Replacing a radiator
• Replacing a section of pipework
• Before a power flush (the engineer will usually do this)
• Replacing a pump, zone valve, or other component

What you need

• Garden hose (to carry water to a drain)
• Radiator bleed key
• Container or towels for residual water

Step by step

1. Turn off the boiler and let the system cool for at least 1 hour. Never drain a hot system — the water can be 70°C+.
2. Locate the drain valve — a tap-like fitting usually at the lowest point of the system, often near the boiler or on a ground floor radiator pipe.
3. Connect a hose to the drain valve outlet and route it to a drain or large container.
4. Open the drain valve by turning the square fitting anti-clockwise with a spanner.
5. Open bleed valves on the highest radiators to allow air in — the system drains much faster with air admitted at the top.
6. Allow the system to drain fully — typically 20–40 minutes depending on system size.
7. Close the drain valve and all bleed valves before refilling.

Refilling the system

Refill via the filling loop until pressure reaches 1–1.5 bar. Bleed all radiators to remove air. Dose with inhibitor. Check pressure after the system heats up. For major repair work, we recommend calling a Gas Safe engineer.