How do I drain down a heating system safely?

Draining a heating system is straightforward mechanically and easy to get into difficulty with, because the water is filthy, the refill traps air, and the job is only finished once everything is bled and repressurised.

Safety

What you'll need

• Hose to reach an outside drain, and a jubilee clip
• Radiator bleed key and a drain cock key or spanner
• Towels, buckets and old sheets
• Corrosion inhibitor for the refill

Step by step

1. Decide whether you actually need to.Removing one radiator only needs that radiator isolated at its valves. A full drain-down is for work on the pipework, replacing a component, or a full system flush. Draining unnecessarily introduces fresh oxygenated water, which accelerates corrosion.
2. Turn the boiler off and let it cool.Off at the programmer and at the isolating switch, then leave it for several hours. System water runs at 60 to 80°C and will scald.
3. Identify what kind of system you have.A sealed system has a pressure gauge and a filling loop. An open-vented one has a small feed and expansion tank in the loft. They are drained and refilled differently, and knowing which matters.
4. Isolate the water feed.Close the filling loop valves on a sealed system. On an open-vented system, tie up the ball valve in the loft tank or turn off its supply, so it does not keep refilling as you drain.
5. Find the lowest drain cock.Usually on a ground-floor radiator or at the lowest point of the pipework, sometimes outside. There may be more than one on a system with separate circuits.
6. Attach a hose properly.Onto the drain cock spigot, clipped with a jubilee clip, running to an outside drain lower than the drain cock. System water is black, stains permanently and does not come out of paving.
7. Open the drain cock, then the bleed valves.Open the drain first, then work from the highest radiators downwards opening bleed valves to let air in. Without air entering, the system will barely drain.
8. Expect it to take a while.A typical house holds a substantial amount of water and it drains slowly. Water will keep coming from radiators when you disconnect them however long you have waited.
9. Protect floors before disconnecting anything.Towels, a tray and a bucket under every joint. There is always residual water and it stains carpet and timber permanently.
10. Close everything before refilling.Drain cocks closed, all bleed valves closed, all radiator valves open. Check every one — a bleed valve left open floods a room when you refill.
11. Add inhibitor as you refill.Corrosion inhibitor is what protects the radiators and boiler from sludge and pinholing. Refilling without it is a genuine mistake, and it goes in during the refill.
12. Refill slowly from the bottom up.Open the filling loop gently, or untie the loft tank ball valve. Filling too fast traps air pockets that then take a long time to clear.
13. Bleed as it fills, lowest first.Work upwards, bleeding each radiator until water runs steadily. Repeat the whole circuit once the system is full, because air migrates upwards as it settles.
14. Set the pressure on a sealed system.Around 1.0 to 1.5 bar cold, or as the boiler states. Then close both filling loop valves and disconnect the loop if it is the removable type.
15. Fire it up and check everything.Run it to temperature, check every joint you disturbed, then bleed again once it has been hot and cooled. Check the pressure again the next day.
16. Call an engineer for anything you are unsure of.Unvented cylinders, boiler internals and anything gas-related are not DIY. A drained system that will not refill or repressurise is a call-out, and that is the normal outcome rather than a failure.

Tips

• Open the bleed valves after the drain cock, working from the top down. Without air entering, the system barely drains at all.
• Add inhibitor as you refill, every time. It is what stops sludge and pinholed radiators, and it is easily forgotten.
• Bleed the whole system twice — once as it fills and again after it has been hot and cooled.

Common mistakes

• Refilling without corrosion inhibitor — Fresh oxygenated water without inhibitor causes sludge and internal corrosion, leading to cold radiators and eventually pinhole leaks.
• Draining a system you cannot refill or repressurise — You are then without heating and hot water until an engineer attends. Check you understand the filling loop and pressure requirements before opening a drain cock.

If it doesn't work

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