How to Drain a Central Heating System — UK Method
Draining a UK central heating system is a 30-minute job with the right hose routing. Here is the trade method for sealed and open systems.
Step-by-step method
1. Turn off boiler and cool
Programmer off, wait 30 minutes. Turn off gas at emergency valve.
2. Locate drain-off cock
Usually on the lowest radiator return, or a dedicated drain-off point on ground floor pipework. Small brass square nipple with hose barb.
3. Attach hose and route outside
Garden hose over drain-off. Route to outside gully or drain. Downhill flow all the way.
4. Open drain-off + bleed screws
Open drain-off with a spanner. Open bleed screws on top-floor radiators to admit air. System drains in 15–25 minutes.
5. Refill and dose
Close drain-off. Open filling loop until pressure 1.2 bar. Bleed rads top down. Dose Fernox F1 inhibitor via a radiator or F&E tank.
Frequently asked questions
Do I need to drain to change one radiator?+
No — closing both radiator valves is enough. Full drain only for pipework changes, boiler swap, or full flush.
How long to refill a system?+
25 minutes for a 3-bed house. Watch pressure gauge, bleed each rad, top up between rounds.