How much energy does a hot water pipe lose?

Any object that is hotter than its surroudings will transfer energy to the surroundings. So, if you have a pipe carrying hot water from one place to another (e.g. from a hot water tank or boiler to your taps) some of the heat energy the water contains will be lost.

Imagine an uninsulated copper pipe of 15mm diameter carrying water at 60C, passing through an unheated space at 15C (for example, through a roof space). The pipe will be losing heat to its surroundings at a rate of almost 50 watts for every metre of pipe. This means that, over the course of a year, a 2.5m length of uninsulated pipe will lose more than 1 MWh (1,000 kWh) of energy. This is enough energy to boil a kettle nearly ten thousand times!

Whenever hot water pipes pass through unheated spaces, it is therefore absolutely essential to insulate them. Even pipes passing through living areas should be insulated. It is true that, in winter they will help to keep the living area warm, but in the summer months when space heating is not wanted, uninsulated hot water pipes will still be wasting heat to their surroundings, and will make the room uncomfortably warm.

How can this energy loss be reduced?Specially made pipe insulation is available in the form of tubes of foam that are split along their length and can be easily slipped over a pipe. Using this insulation can reduce the heat loss from a hot water pipe by over 80%. This means the copper hot water pipe from the previous example, with insulation applied, would lose heat at only about 8 watts per metre of length. In case you are worried about the cost of the foam pipe insulation, it is worth remembering that the insulation costs about the same as the amount of energy that an uninsulated pipe loses in just two weeks.

As an alternative to the foam tubes described above, pipe insulation is also sometimes available in a long strip that is wound in a spiral around the pipe to be insulated. The amount of heat that an uninsulated hot water pipe loses is huge, so even if proper pipe insulation is totally unavailable, it is worth trying to improvise with whatever insulating materials are available. Even a thick layer of old rags wrapped around a pipe is better than nothing!

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