How to Turn Off the Hot Water Supply From Your Cylinder

Why you might need to turn the hot water off

Most people search for this in a hurry: a hot pipe has started leaking, a hot tap will not stop running, the cylinder is dripping, or there is water on the floor of the airing cupboard. It is also worth doing before any work on hot pipes or taps, and it is part of shutting a property down if it will be empty through the winter.

Turning off the hot side only, rather than the whole house, means you keep cold water for drinking, flushing and washing while the problem is sorted. That is usually what you want.

Step one: switch off the heat

Do this before closing any valve. A cylinder that keeps heating while its supply is off can cause problems of its own. On a tank fed cylinder, if the level drops, an immersion heater can end up running partly out of water and burn out. On an unvented cylinder, heating a sealed tank with its inlet closed pushes the pressure up until a relief valve opens.

• Switch the immersion heater off at its wall switch, usually a switch marked "water heater" near the cylinder.
• Set the hot water to off on the boiler programmer or timer. If the boiler has a separate hot water setting, turn that off too.
• If you have a combi boiler, there is no cylinder, and the boiler heats mains water as it passes through. Skip to the section on combi boilers below.

Step two: close the cold feed valve on a tank fed cylinder

If you have a tank in the loft and no expansion vessel or brass valve set beside the cylinder, it is tank fed, also called vented. Look at the cylinder's cold feed: a pipe, usually 22mm, coming down from the loft and entering the cylinder near the bottom. Somewhere on it, normally in the airing cupboard, there is a gate valve with a round wheel handle, or on newer work a lever or a slotted quarter turn valve.

Turn a wheel valve clockwise until it stops. Turn a lever or slotted valve a quarter turn so the lever or slot sits across the pipe. Then open a hot tap. It will run for a short time as the pipework empties, then stop. The cylinder itself stays full, because hot water leaves from the top and nothing is replacing it from below.

Gate valves that have not moved in years can be stiff, or can turn without actually closing. If the hot tap keeps running at full flow after you have shut it, the valve is not sealing. In that case, go to the loft, tie the arm of the ball valve on the cold water tank up so it stops refilling, and let the tank run down. Or use the main stopcock.

Step two: close the inlet valve on an unvented cylinder

An unvented cylinder runs straight off the mains. You will see a cluster of brass valves on the cold pipe coming into the cylinder, usually an expansion vessel beside it, and a tundish on a pipe nearby. Before that cluster, on the incoming mains pipe, there is an isolating valve, usually a lever or a quarter turn valve with a slot for a screwdriver.

Turn it a quarter turn so it sits across the pipe. Then open a hot tap to let the pressure off. You will get a short burst of water as the pressure drops, and then the flow will fade to nothing. That is normal on a sealed cylinder.

Leave the other brass valves, the relief valves and the expansion vessel alone. They are the cylinder's safety set and are not for isolating it. If the only valve you can find is part of that set, or the isolating valve will not turn, use the stopcock instead, and book an unvented cylinder repair visit to have the valve sorted. If the cylinder is under the stairs or in a cupboard in a flat and you are not sure what you are looking at, our unvented vs vented cylinder guide shows how to tell them apart.

If water is coming out of the tundish or a pipe outside while the cylinder is hot, the heat is the priority, not the water. Switch the immersion and the boiler off and keep people away from the discharge. Our guide on a tundish dripping explains what that discharge means.

If you cannot find the valve, use the stopcock

The main stopcock turns off all the cold water into the property, and with it the supply that pushes hot water out of the cylinder. In most London houses it is under the kitchen sink, and in flats it can be in a cupboard, a bathroom or a communal riser. Our guide, where is my stopcock, covers the usual places. Turn it clockwise until it stops, then open a hot tap to drain the pressure.

You lose all the cold water too, so it is a stopgap. Once the leak is contained, you or an engineer can find and close the cylinder's own valve and turn the cold back on.

On a combi boiler there is no cylinder to isolate

A combi heats mains water as it passes through the boiler, so the hot supply starts at the boiler. Underneath it is a row of valves. The one on the cold mains pipe going in, usually marked or labelled cold, turns off water to the boiler and so to every hot tap. Switch the boiler off before closing it. If you are not sure which pipe is which, the stopcock is the safer choice.

Going away this winter: turn off, or leave on?

For a normal week or two away in winter, most homes are better left with the heating on a low setting and the water on, so the pipes and the cylinder stay above freezing. Switch the immersion off if you do not need it, and on your return run each hot tap for a couple of minutes before using it.

If a property will be empty and unheated for a long spell, isolating the water is sensible, and draining it down is the most reliable protection against frozen and burst pipes. If a pipe has already split, turn off the stopcock and see our burst pipe repair service. Our guide on how to drain a hot water cylinder covers a winter drain down, including why unvented cylinders and sealed heating systems are best left to an engineer. Check your home insurance too, as some policies set conditions for empty properties in winter.

Once the water is off

Keep the heat off until the supply is back on and the cylinder is full. Put a note on the immersion switch and the boiler so nobody turns them back on in the meantime.

If you turned the water off because of a hot water cylinder leaking, or a leak on the relief valves or the pipework around it, call us on 020 4652 5111 or send an enquiry with a photo of the cylinder and where the water is coming from. We have 20 engineers across London working on hot water systems every day, including G3 qualified engineers for unvented cylinders. Our rate is on the pricing page and agreed before we attend. If water has reached a ceiling or electrics, our guide to water escaping in the house explains what to do first.