A worked example of how much a metal pipe lengthens when heated, and why it must be allowed for.
Hot pipes get longer, and over a long run the movement is enough to cause creaking, strain on joints, or damage if it is not allowed for. This worked example shows how to estimate it.
The formula. Expansion = original length x coefficient of linear expansion x temperature rise. The coefficient is a small number that says how much each metre grows per degree, and it depends on the material.
Step 1 - inputs. Take a 10 metre run of copper pipe that goes from a cold 10 °C to a hot 70 °C, a temperature rise of 60 °C. Use an example coefficient of 0.000017 per °C for copper, and confirm the correct value for the material in use.
Step 2 - apply it. Expansion = 10 x 0.000017 x 60 = 0.0102 metres.
Step 3 - make it real. That is 0.0102 x 1000 = about 10.2 millimetres of growth along a 10 metre run, which is easily enough to make a pipe creak against a joist or stress a rigid joint.
Step 4 - allow for it. Movement like this is handled by good practice: clipping that lets the pipe slide rather than pinching it, expansion loops or offsets on long runs, and sleeves where pipe passes through the building fabric, so the pipe can grow and shrink without noise or damage.
Step 5 - take it further. The longer the run, the hotter the water, or the higher the material coefficient, the more movement you get, so plastic pipework, which moves more than copper, needs even more generous support and provision. Use the correct coefficient and the method in the current guidance for the actual installation.
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