How to Drain a Hot Water Cylinder Safely, Step by Step

Do you actually need to drain it?

Unlike some water heaters sold abroad, a UK hot water cylinder does not need draining as routine maintenance. People usually need to drain one for a specific reason.

If you are only replacing a tap or a shower valve, you normally do not need to drain the cylinder at all; isolating the supply to that outlet is enough. Draining a full cylinder takes time and puts air into the system, so it is worth being sure it is needed before you start.

If you only need to stop the hot water flowing, for a leak or a repair on the hot pipes, you can isolate the cylinder without draining it. Our guide on how to turn off hot water supply from a cylinder shows where the valve is on vented and unvented cylinders.

• Replacing an immersion heater element, which sits below the water line
• Replacing or removing the cylinder
• A leak from the cylinder or a fitting low down that cannot be isolated
• Leaving a property empty and unheated through the winter
• Moving or altering pipework connected at the base of the cylinder

Which cylinder do you have?

Check this before you touch anything, because the method and the risks are not the same.

A vented cylinder is usually copper, often covered in yellow foam or a red jacket, and fed by a cold water tank in the loft. A pipe rises from its top and bends over the loft tank. Nothing on it is under mains pressure.

An unvented cylinder has no loft tank. It is usually a white or grey casing with a data badge, a cluster of brass valves on the cold inlet, often a small round expansion vessel nearby, and a clear funnel called a tundish in a pipe running outside. It is sealed and runs at mains pressure. Our unvented vs vented cylinder guide shows the differences in more detail.

What you need

Gather everything first so you are not leaving a half drained cylinder to go and find a spanner.

• A garden hose long enough to reach an outside drain or gully lower than the cylinder
• A hose clip or jubilee clip to hold the hose on the drain cock
• A drain cock key or small adjustable spanner
• Old towels and a bowl for drips
• A torch, because airing cupboards are dark

Step by step: draining a vented cylinder

Allow an afternoon. The draining itself is slow, because it relies on gravity through a small valve, and a full cylinder can take an hour or more depending on the hose run.

• Switch off the immersion heater at its wall switch and set the boiler so it is not heating hot water. Leave it long enough for the water to cool, ideally a few hours. You will be handling a hose full of the cylinder's contents.
• Shut off the cold feed to the cylinder. Look for a gate valve on the pipe coming down from the loft tank into the base of the cylinder and turn it clockwise until it stops. If there is no valve, or it will not close, you can stop the tank refilling by tying the ball valve arm up in the loft, then run the cold bathroom taps that are fed from the tank to empty it.
• Open a hot tap upstairs. This lets air into the top of the cylinder so the water can flow out of the bottom.
• Find the drain cock. It is a small brass valve with a ribbed spout, usually on the cold feed pipe close to where it enters the cylinder at the base.
• Push the hose firmly onto the spout, clip it, and run the other end outside to a drain that sits lower than the cylinder.
• Open the drain cock slowly with the key or spanner. Water should start to flow down the hose. Check the connection for leaks in the first few minutes.
• Leave it until the flow stops. Close the drain cock before you remove the hose.

Do not switch the immersion heater on at any point while the cylinder is empty or part filled. An element that is not covered by water burns out within moments. Tape over the switch as a reminder if anyone else is in the house.

If the water will not come out

A few problems come up again and again, and most are simple to get past.

If nothing flows, or it slows to nothing after a minute, air is not getting in at the top. Open another hot tap, or check that the one you opened is actually on the cylinder's hot supply.

If the drain cock is seized, do not force it with a long spanner. Old drain cocks snap, and a snapped drain cock on a full cylinder is exactly the leak you were trying to avoid. That is the point to stop and call someone.

If there is no drain cock at all, which happens on some older installations, the cylinder is drained by disconnecting the cold feed at the base or by siphoning from the top. Both are best left to a plumber, because they involve breaking a joint on a vessel full of water.

If the drain cock drips after closing, the washer has perished. A bowl under it will do until it is changed, but tell whoever next works on the cylinder.

Draining an unvented cylinder: why it is different

Physically, an unvented cylinder drains in a similar way, through a drain valve at the base. The difference is everything around it. The cylinder is under mains pressure and has to be isolated and depressurised through its own valves first. The brass inlet group on the cold feed, the expansion vessel and the two relief valves are its safety set, and the pressure in the expansion vessel is normally checked while the cylinder is empty, because that is the only time it can be read accurately.

Refilling needs the same care in reverse, with the air purged, the relief valves proven to reseat, and the tundish checked. That is why draining an unvented cylinder is normally done as part of work by a G3 qualified plumber rather than as a separate job for the homeowner. If a part on yours needs changing, our unvented cylinder repair service covers the drain down and recommissioning as part of the visit.

Refilling the cylinder afterwards

Close the drain cock and remove the hose. Leave the hot tap you opened earlier running, then open the cold feed valve, or untie the ball valve in the loft, so the cylinder starts to fill.

Water will come out of the hot tap in spits and bursts at first as air is pushed out. When it runs steadily with no spluttering, the cylinder is full. Run each of the other hot taps in turn until they also run clean, which clears air from the rest of the pipework.

Only when every hot tap runs without air should you switch the immersion or boiler hot water back on. Check the drain cock and any joint you disturbed for drips over the next hour, and again the next day. A cylinder refilled with cold mains water takes a while to come back up to temperature; our guide on how long does hot water take to heat up gives realistic timings.

Draining down a property for the winter

If a flat or house will be empty and unheated for weeks through the coldest months, draining the hot and cold water system is the most reliable way to stop pipes and the cylinder freezing and splitting. Some insurance policies also set conditions for unoccupied properties in winter, so check yours before you go.

For a full drain down, turn off the stopcock (if you are not sure where it is, our where is my stopcock guide will help), open every tap in the house, drain the cylinder as above, and drain the loft tank. Flush the toilets once the supply is off. Leave a clear note on the stopcock and the immersion switch that the system is empty, so nobody switches anything on before it has been refilled.

If the property has an unvented cylinder, a combi or a sealed heating system, the heating circuit usually needs separate treatment and draining it is not a homeowner job. Leaving the heating on a low frost setting is often the simpler option, provided the boiler is reliable and someone checks on the property.

When to call us instead

Call if the cylinder is unvented, if the drain cock is seized or missing, if you are draining because of a hot water cylinder leaking from a joint you cannot isolate, or if the reason for draining is a job you would rather not do, such as an immersion heater repair or a hot water cylinder replacement. We will drain down, do the work and refill and test in one visit. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.