Hot Water Cylinder Jacket: Do You Need One and What Size?

Does my cylinder need a jacket?

Open the airing cupboard and look at the cylinder. What you see tells you the answer.

If the cylinder is covered in rigid, sprayed foam, usually yellow or green and bonded to the copper, it is factory insulated. Most cylinders fitted in the last few decades are made this way. A jacket on top gains little.

If the cylinder has a white, grey or coloured smooth outer casing with a data badge, a set of brass valves on the cold feed and a clear funnel in a nearby pipe, it is almost certainly an unvented cylinder. These are insulated inside the casing and do not need a jacket.

If you can see shiny or tarnished bare copper, or an old quilted jacket, often red, that has thinned, split or slid down to leave the top exposed, the cylinder is losing heat it does not need to lose. That is where a new jacket earns its keep.

• Bare copper: fit a jacket
• Old quilted jacket, thin, torn or slipped: replace it
• Rigid factory foam, intact: no jacket needed
• Unvented cylinder in a casing: no jacket needed, and do not cover its valves

Why a jacket makes a difference

A hot water cylinder holds its water at around 60 degrees for hours at a time, in a cupboard that is much cooler than that. Heat escapes from every uninsulated surface, all day and all night, and the boiler or immersion has to put it back to keep the water at temperature.

The loss is quiet, which is why it gets ignored. The usual clues are an airing cupboard that is noticeably warm even when the heating is off, a cylinder that takes longer to heat than it used to, and hot water that has gone lukewarm by the morning. A properly fitted jacket reduces all three. Our guide to how long does hot water take to heat up explains how much insulation affects heat up times.

How to measure for the right size

Cylinder jackets are sold by the height and diameter of the cylinder they fit, in millimetres. Buying the wrong size is the commonest mistake, because a jacket that is too short leaves the hottest part of the cylinder, the top, exposed.

Measure the height of the cylinder body from the base to the start of the domed top, not including any pipes. Then measure around the widest part with a tape and divide by a little over three to get the diameter, or measure straight across if you can reach. Pick the jacket that matches both figures. If you are between sizes, go for the larger one, as a slightly generous jacket can be tied in snugly but a small one cannot be stretched.

Thickness matters as much as fit. Look for a jacket at least 80mm thick. Thin budget jackets are better than bare copper, but not by as much as a proper one.

How to fit a cylinder jacket

Fitting takes about ten minutes and needs no tools. Do it with the cylinder at its normal temperature; you are only touching the outside.

• Remove the old jacket if it is falling apart, and check the copper underneath for green staining or damp, which can mean a slow leak.
• Fit the top cap first, threading the pipes through the slits or the central hole so the dome is fully covered.
• Wrap the side segments around the cylinder so the edges meet or overlap slightly, with no gaps showing copper.
• Tie or buckle the straps firmly, top, middle and bottom, so the jacket cannot slide down over time.
• Leave the immersion heater cover, its wiring and the cylinder thermostat dial accessible. Tuck the jacket around them rather than over them.
• Keep the jacket clear of the drain cock at the base and any valves, so they can still be reached.

The immersion heater head and its flex should never be buried under insulation. They need to be visible and reachable, and the flex should not be trapped or kinked by the straps.

Do not forget the pipes

The pipe leaving the top of the cylinder carries the hottest water in the house, and on many older systems it runs bare for a metre or more before it disappears under a floor. Hot water can circulate up and down that pipe by itself, radiating heat into the cupboard or the loft all day.

Foam pipe lagging from any DIY store slips over the pipe and costs very little. Insulating the first metre or two of the hot pipes from the cylinder, and any pipes running through a cold loft, is worth doing at the same time as the jacket. In the loft it also helps protect the pipes from freezing.

What about drying clothes in the airing cupboard?

A lot of households rely on the warmth from an uninsulated cylinder to air clothes, and a good jacket will make the cupboard cooler. That warmth is heat you are paying the boiler or immersion to produce, and much of it is wasted when the cupboard is shut and empty.

If you still want an airing space, it is usually cheaper to heat the water efficiently and dry clothes another way than to leave the cylinder unlagged. Some people compromise by leaving the jacket on and using the shelves above, where a little heat still rises.

Keep the water at 60 degrees

Insulating the cylinder is not a reason to turn the temperature down. Stored hot water is normally kept at around 60 degrees to control bacteria, and the saving comes from losing less heat at that temperature, not from storing the water cooler. If the water at the taps feels too hot, that is a separate issue, and turning the thermostat down is the wrong fix.

When a jacket is not the answer

A jacket solves heat loss. It does not fix a cylinder that is failing. If your hot water runs out much sooner than it used to, takes hours to heat even with good insulation, or the copper is showing green or white crusting around the joints and the base, the cylinder itself is the problem.

Old copper cylinders with a jacket are almost always vented, fed from a tank in the loft. Many are well past the age where replacement makes more sense than repair, especially in London's hard water, where scale builds on the coil and the immersion element; our guide on how long does a hot water cylinder last gives realistic ages. Our guide to the signs your cylinder needs replacing covers what to look for. If it is time, a hot water cylinder replacement with a factory insulated cylinder, vented or unvented depending on your mains, deals with the heat loss and the age together. Our unvented vs vented cylinder guide explains that choice.

If you find damp or staining under the old jacket when you take it off, stop and look more closely. A weeping joint or a pinhole in the copper is much easier to deal with before it lets go, and our page on a hot water cylinder leaking explains what to do.

Want a second opinion on your cylinder?

If you are not sure whether your cylinder is worth insulating or worth replacing, call us or send an enquiry with a photo of the cylinder and the pipes around it. We can usually tell from that what type it is and roughly how old it is. We have 20 engineers across London, hot water systems are our core work, and our rate is on the pricing page and agreed before anyone attends.