How to drain a hot water cylinder
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Never drain an unvented (pressurised) cylinder yourself — it requires a G3 qualified engineer. For vented cylinders, always let the water cool first.
Draining a hot water cylinder is a task most homeowners face at some point — whether you're replacing the cylinder, flushing out sediment, or carrying out maintenance work. It's one of those jobs that looks more complicated than it actually is, but getting it wrong can lead to water damage in your airing cupboard or, worse, a flooded kitchen. The good news is that if you've got a vented cylinder (the most common type in older UK homes), it's a perfectly manageable DIY task with the right approach and a bit of patience. However, if your system is unvented or pressurised, this isn't a job for a homeowner — you'll need to call in a Gas Safe registered engineer with G3 qualifications to do it safely. This guide walks through identifying your cylinder type, preparing your system properly, and executing the drain safely. If at any point you're unsure about your setup or lack confidence with the process, it's always worth ringing a qualified plumber rather than risking water damage or personal injury.
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First, identify your cylinder type. An unvented cylinder has a pressure relief valve (a small pipe running to an outside drain) and is fed directly from the mains — do not drain this yourself.
If you have a vented cylinder (fed from a cold water tank in the loft), turn off the boiler and immersion heater and wait for the water to fully cool before doing anything.
Draining a cylinder involves attaching a hosepipe to a drain-off cock and opening it — it's straightforward but fiddly in an airing cupboard. If you're not confident, call a plumber rather than risk flooding.
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Frequently asked questions
How long does it take to drain a hot water cylinder?
Draining typically takes 1–3 hours depending on your cylinder size and water pressure. The waiting time for the water to cool after switching off heating is often longer than the actual draining process itself. Keep the drain-off cock open slowly rather than fully — this reduces mess and gives you more control.
Can I drain a cylinder while the heating is still on?
No — always switch off your boiler and immersion heater first and let the water cool completely. Draining hot water is dangerous (scalding risk) and inefficient, and cold water drains far more easily without creating steam in your pipework.
What's the difference between a vented and unvented cylinder?
A vented cylinder is fed from a cold water tank in your loft and has no pressure relief valve — these are safe for DIY draining. An unvented cylinder is fed directly from the mains, sits under pressure, and has a small relief pipe running outside — these must only be drained by a qualified G3 engineer or you risk serious injury and system damage.
Where should the water drain to when I open the drain-off cock?
Attach a hosepipe to the drain-off cock and run it to an outside drain, gully, or soak-away if possible. If you must drain indoors, position towels and a bucket underneath as a precaution — the water will be dirty from sediment and rust even after cooling.
Do I need to turn off the water supply as well as the heating?
For a vented cylinder, turn off the boiler and immersion heater, then close the stop valve on the feed pipe from the cold water tank — this prevents the tank refilling as you drain. You don't need to turn off the mains stop valve unless you're also draining the cold water tank itself.
What should I do if the drain-off cock is stuck or won't open?
Apply penetrating oil (like WD-40) and leave it for 30 minutes, then gently tap the valve with a hammer to loosen it — never force it, as you could snap the valve body. If it's still stuck after two attempts, call a plumber rather than risk damaging the cylinder or creating a leak.