How Long Does It Take to Replace a Hot Water Cylinder?

What decides how long a cylinder replacement takes?

Five things set the pace, and the first four are visible before anyone arrives. The type matters: a direct unvented cylinder with two immersion heaters is a simpler swap than an indirect cylinder with a coil, motorised valves and a boiler connection. The change of type matters even more. Swapping a vented cylinder for an unvented one removes the loft tank and vent pipe and adds a valve group, expansion vessel and a discharge route, so it is a different job, not a longer version of the same one.

Access is the third. A cylinder in an airing cupboard with a door that opens wide is a different task from one under a loft hatch, up a flight of stairs or in a room where the new cylinder has to be carried through a narrow door. Fourth, the existing pipework: whether the connections are in good condition, whether the old valves and stopcock nuts come apart cleanly, and whether limescale has locked anything. The fifth only shows when the old cylinder is out: what was hidden behind or below it.

Because those factors differ so much, any claim of a fixed number of hours belongs in a quote for your specific cylinder, not in a general guide. The useful question to ask is what the survey found and which of those five factors apply. The quote checklist lists questions worth asking before you accept.

What is checked before the old cylinder comes out?

Most of the avoidable delay on a replacement day comes from something not checked beforehand. A competent installer works from a short list.

• The new cylinder’s size, type and connections against the old one, and whether the flow, return and cold feed can reach them without extending pipework.
• The discharge route for an unvented cylinder: the pipe size, the run length and fall, and where it terminates safely outside. See the moving a cylinder guide.
• The floor or shelf: a full cylinder is heavy. Water weighs a kilogram a litre, so a larger cylinder can put well over 150 kg on the base once filled.
• The electrical supply, the isolator and the immersion wiring, including whether the cable is in good condition.
• Whether the old stopcock and isolation valves hold and can be shut off. A stopcock that does not close properly changes the whole plan.
• What the household needs: a time when the water can be off, and clear access to the cupboard.

What is the order of work on the day?

The sequence below is the one a like-for-like swap follows. Each step must be finished before the next starts, and none of them is skipped on a quick job.

Isolate the heat source and the water first. The immersion heaters go off at the isolator, proven dead before any wiring is touched, and any boiler connection or motorised valve is isolated. The cold feed is closed at the stopcock or isolation valve. Next the cylinder is drained, from a drain point to a safe outlet, with a hot tap open to let air in. Hot water in a drained cylinder is a real scald hazard, so the water is allowed to cool first where it can be, and the draining guide shows the sequence.

The old cylinder is then disconnected and removed. On an unvented swap the valve group, expansion vessel and tundish assemblies are usually renewed with the cylinder. Old compression nuts and olives are replaced with new ones. It is poor practice to reuse an olive that has been compressed once. The like-for-like cylinder replacement page covers the swap in more detail.

The new cylinder is positioned, level and secure, and connected: cold feed through the valve group, hot outlet, secondary return if there is one, the immersion heaters wired by an electrician or competent installer, and the discharge pipe run to the outside.

How is a replacement cylinder commissioned?

Commissioning is where a cylinder swap is finished properly. The cylinder is filled with the vent open or a hot tap running until the pipework is free of air, and every joint is checked for weeping. On an unvented cylinder the expansion vessel is checked and set to the right charge before the water goes in, because a vessel charged wrongly makes the relief valve lift on every heat cycle.

The power is restored and the thermostat set. The safety devices are then proved, not assumed: the relief valve and the tundish are seen to work, the discharge route is checked, and the over-temperature cut-out is confirmed in circuit. The heating is checked, with all the connections inspected again once the cylinder is up to temperature and the metal has expanded.

A newly filled cylinder needs time to heat. Recovery time depends on the cylinder size and the heat source, so allow for it. Nothing more specific can be promised without knowing your cylinder and its heating.

What can the household use meanwhile?

Cold water to the taps is cut when the stopcock is closed and returns once the cylinder is refilled and the cold feed reopened. Where the cold feed to the cylinder can be isolated separately, the kitchen cold tap and other mains-fed outlets may stay available for much of the work. This depends on whether the installer can isolate the cylinder only, and whether the old isolation valve holds.

Hot water is off from the moment the cylinder is isolated until the new one has filled and heated. Heating may continue where the boiler is separate from the cylinder and the heating circuit is not disturbed. On an indirect cylinder the boiler connection is isolated and the heating is usually fine once that is done, but confirm this with the installer rather than assuming it.

Ask in advance about the toilet and any appliance fed from the same stopcock. A dishwasher, washing machine and boiler fed from the mains will be off if the main stopcock is closed.

What should you be left with at handover?

A new cylinder is not finished when the water is hot. Any competent installer should show you the isolator, the thermostat setting and the cut-out reset, tell you which valves belong to the cylinder, and hand over the manufacturer’s documents. Ask for notification paperwork for an unvented installation, and ask what servicing the warranty needs. The unvented cylinder service check page explains what that servicing covers.

It also helps to ask what was found when the old cylinder came out. A good engineer can tell you whether the old valve group, tundish or stopcock were failing, which informs whether a repair would have been worth trying. The repair or replace decision guide is the place to go if you are still deciding.