How to Drain a Hot Water Cylinder Safely

Updated July 2026 · Emergency Plumber Chelsea

Why you might need to drain it. Draining a cylinder is not a routine yearly job for most UK homes. You need to do it when an immersion element has to be replaced, when the cylinder itself or a valve on its outlet is being changed, when there is a leak from the tank or a fitting low down, before pipework alterations, or when a property is being left empty over winter. If you are simply short of hot water, draining will not fix it, and our guide to hot water that is not hot enough is the better starting point.

Step one: work out which type you have. Look for a cold water storage tank in the loft or on a high shelf feeding the cylinder, and a vent pipe from the top of the cylinder that bends over that tank. If you find those, it is a vented cylinder and the steps below are well within reach of a careful householder. If instead the cylinder has a label saying unvented, a cluster of valves on the cold inlet, a small expansion vessel nearby and a tundish on a discharge pipe, it is unvented. Our guide to how a hot water cylinder works explains the parts if you are not sure.

Step two: turn off every source of heat. Switch the immersion heater off at its wall switch, and ideally at the consumer unit as well so nobody switches it back on. Set the programmer so the boiler is not calling for hot water, or turn the boiler off. This matters for two reasons. The stored water can be hot enough to scald, and an immersion element that switches on in an empty or half-empty cylinder will burn out within moments, because it relies on being surrounded by water to shed its heat.

Step three: let the water cool. A full, well-insulated cylinder holds its heat for many hours, so plan ahead. Turning the heat off the night before and draining in the morning is the easy way. If you are in a hurry, you can run the hot taps for a while to draw off the hottest water, which replaces it with cold from the feed and brings the temperature down faster.

Step four: shut off the cold feed to the cylinder. On a vented cylinder there is usually a gate valve, often with a red wheel handle, on the cold pipe running from the loft tank into the bottom of the cylinder. Turn it clockwise until it stops. If there is no valve, or it will not close fully, you can stop the tank refilling by tying the float arm up or closing the isolating valve on its supply, then drain the tank as well. On an unvented cylinder the cold inlet usually has its own isolating valve before the pressure-reducing valve; if you cannot find it, the main stopcock turns off the supply to the whole property.

Step five: open a hot tap to drain the pipework. Open the hot tap nearest to the cylinder, and on a vented system a hot tap upstairs as well. Water will run for a short while and then stop once the pipes above the cylinder are empty. Leave the tap open. It lets air into the system as the cylinder empties, and without it the water will glug out slowly or not at all because of the vacuum forming inside.

Step six: find the drain cock and attach a hose. The drain cock is a small valve with a ridged hose outlet, usually on the cold feed pipe close to where it enters the bottom of the cylinder. Push a length of garden hose firmly onto the outlet, secure it with a jubilee clip if you have one, and run the other end outside to a gully or drain. The end of the hose must be lower than the drain cock, because the water drains by gravity alone. Put an old towel under the drain cock in case it drips.

Step seven: open the drain cock slowly. Most drain cocks are turned with a small spanner or an adjustable spanner on a square spindle, and some take a flat screwdriver. Open it a turn at a time and check the hose is not leaking at the connection. A full cylinder can take a long time to empty through a narrow drain cock, so be patient. If the flow slows to a trickle early on, the drain cock may be choked with limescale or sediment, which is common with London's hard water. Do not force a seized or old plastic drain cock, because they snap, and then you have an open pipe you cannot close.

Draining an unvented cylinder. The principle is the same, but the cylinder is sealed, so air has to be admitted in a controlled way. Many manufacturers' instructions have you close the cold inlet, open a hot tap, open the drain cock and then let air in by opening one of the relief valves manually as the level falls. The exact sequence varies by make, so follow the instructions in the cylinder's manual, which are often on a label on the cylinder itself. Adjusting, removing or replacing the safety valves, the pressure-reducing valve or the expansion vessel is work for a G3-qualified engineer, and so is anything you would need to do if the cylinder will not refill correctly afterwards.

The Chelsea flat problem. In many mansion block flats and conversions the cylinder sits in a cupboard on the same level as, or below, everything around it, with no outside drain within reach of a hose and nothing lower to drain to. Gravity draining simply does not work there. The options are a small submersible or drain pump, draining slowly into buckets, or leaving it to a plumber who brings the kit. Be careful too with a cylinder fed from a shared tank in the roof of a block: close the valve on your own cylinder's feed, not a valve that serves the whole building, and speak to the managing agent before touching anything in a communal area. Our guide to shutting off water in a mansion block flat covers which valves are yours.

Refilling afterwards. Close the drain cock and remove the hose. Leave the hot taps open. Open the cold feed valve slowly and let the cylinder fill; on a vented system you will hear the loft tank refilling. When water runs from the hot taps without spluttering, the air is out and you can close them. Check the drain cock and any joints you have disturbed for drips. Only once the cylinder is completely full should you switch the immersion or the boiler back on. If a tap keeps spitting air or the flow stays weak, our guide to clearing an airlock should help.

Leaving a property empty over winter. Draining the hot water cylinder is one part of protecting an empty home from frozen and burst pipes, alongside draining the cold water tank and the pipework. Our checklist for winterising an empty property sets out the full sequence, and it matters in Chelsea where many homes stand empty for long stretches.

When to call a plumber instead. Call us if the cylinder is unvented, if the drain cock is seized, missing or plastic and brittle, if there is no drain point lower than the cylinder, if the cylinder is leaking, or if you are draining it to replace an immersion or the cylinder itself, since the work that follows is ours anyway. Emergency Plumber Chelsea drains down, repairs and replaces vented and unvented cylinders as part of our plumbing repairs and plumbing installation work across SW3, SW10, Knightsbridge, South Kensington and Fulham. Call 020 3002 6667. There is no call-out fee on jobs over 3 hours.

Quick tips

• Switch the immersion off at the consumer unit as well as the wall switch; an element that fires in an empty cylinder burns out almost instantly.
• Turn the heat off the night before so the water has cooled by the time you drain it.
• Keep a hot tap open throughout; it lets air in so the cylinder drains instead of glugging to a stop.
• The hose end must be lower than the drain cock, or the water will not flow.
• Never force a seized or plastic drain cock. If it snaps you have an open pipe you cannot close.

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