Insulating Hot Water Pipes: Which Ones and How to Do It

Which hot water pipes to insulate first

Heat escapes wherever a hot pipe is exposed to cooler air, and it escapes fastest where the pipe is hottest and the surroundings coldest. That gives a clear order of priority.

• The hot draw-off pipe from the top of the cylinder, for at least the first metre or two. It holds the hottest water in the house, and on many systems hot water circulates up and down it on its own, leaking heat into the cupboard all day.
• The primary pipes between the boiler and the cylinder coil. These run at boiler temperature whenever the hot water is heating.
• Any pipe, hot or cold, in the loft, a garage, a cellar or under a suspended timber floor. These lose the most heat and are the ones that freeze.
• A pumped secondary return, if your house has one. This is a loop that keeps hot water moving round the house so taps run hot straight away, and it is hot around the clock. Uninsulated, it is a radiator you did not ask for.
• Long runs to distant taps, such as a kitchen at the far end of a house from the cylinder.

What insulation actually changes

Insulating the pipes near the cylinder reduces standing heat loss, which is the heat a full tank loses while nobody is using it. On a cylinder heated by an immersion, that is electricity. On one heated by the boiler, it is gas. Either way, the saving comes every hour of every day, which is why it is worth doing even though each individual pipe loses only a little.

Insulating the run to a tap keeps water in that pipe warmer for longer between uses. If you run the hot tap, stop, and come back ten minutes later, the water arrives hot sooner. If the pipe has been sitting for hours, it will have cooled regardless, and you will still run off the cold water in it first. The wait for hot water after a long gap is decided by the length and size of the pipe, not by its insulation. Our guide on how long does hot water take to heat up covers the waiting that happens at the cylinder rather than in the pipes.

In a loft or under a floor, insulation slows freezing. It buys time in a cold night; it does not stop a pipe freezing if the space stays below zero for days. Keeping the heating on a low setting through a freeze is still the main protection.

Choosing the right pipe insulation

Pipe insulation is sold by the pipe diameter it fits. Most household hot and cold pipes are 15mm, the pipes to and from the cylinder and the main runs are often 22mm, and some older or larger systems use 28mm. Measure across the pipe, or check a marking on the copper, and buy the matching size. A loose tube lets air circulate inside it and does much less.

Thickness matters more than brand. Thicker walls insulate better, and pipes in a loft or other cold space deserve the thickest you can fit. Standard polyethylene foam is fine for most pipes. Close to a boiler, or on pipes that run very hot, a higher temperature rated rubber type is a better choice, as ordinary foam can soften and shrink.

How to fit it properly

Fitting pipe insulation is a straightforward DIY job, and most people do it themselves. Work systematically from the cylinder outwards, so you do not miss short bare sections between lengths.

• Turn the heating and hot water off and let the pipes cool, so you are not handling hot copper.
• Open the split along the tube and push it over the pipe, split facing down or to the back.
• Butt the ends of each length tightly together. Gaps at joints are where most of the benefit is lost.
• At bends, cut the tube into a mitre or cut small notches on the inside of the bend so it follows the pipe without kinking open.
• Seal the split and every joint with the tape made for pipe insulation, or cable ties at regular intervals.
• In the loft, cover the pipes all the way to the tank and make sure nothing is left bare where it passes through the loft insulation.

What to leave uncovered

Some parts of a hot water system must stay visible and accessible. Covering them hides the warning signs you need to see.

• The immersion heater head, its thermostat and its flex
• On an unvented cylinder, the relief valves, the tundish and the discharge pipe. The tundish has to stay visible so a discharge can be seen.
• Valves you may need to turn in a hurry, such as the cold feed valve to the cylinder and any isolating valves
• Pipes close to a boiler flue, and anything the boiler manufacturer says to keep clear
• Compression joints and fittings that are weeping, until they have been repaired. Insulation over a slow leak hides it and holds the water against the pipe.

Pair it with the cylinder itself

If your cylinder is an older copper one with a thin red jacket, or no jacket at all, insulating the cylinder saves more than insulating the pipes. Do both at once. Our guide to the hot water cylinder jacket explains how to measure for the right size and fit one without covering the immersion. Modern factory insulated cylinders do not need a jacket, but the pipes leaving them still benefit from insulation.

If you have a cold water tank in the loft, insulate its lid and sides at the same time as the loft pipes. Our guide to the cold water tank in the loft covers the right way to do that without cutting off the warmth it needs from below.

When insulation is not the answer

If your hot water runs out much faster than it used to, the usual cause is a failing element, thermostat or coil, or a heavily scaled cylinder, not heat lost from the pipes. Our guide on hot water runs out quickly explains how to tell which. If the cylinder is getting hotter than it should, or the pipes near it are too hot to touch, that is a control fault to be looked at, and hot water cylinder overheating covers the signs.

If a pipe does freeze, do not use a naked flame to thaw it. Turn off the stopcock if you see any sign of a split, and our burst pipe repair service covers what to do next. For anything around the cylinder, including leaks, a hot water system that will not heat, or a cylinder you want checked before winter, call us on 020 4652 5111 or send an enquiry. We have 20 engineers across London, including G3 qualified engineers for unvented cylinders. Our rate is on the pricing page and agreed before we attend.