On Wednesday, March 4, 2020 at 9:10:11 AM UTC, Chris B wrote:

I understand why a large expansion tank is required to accommodate the
expansion of 200lt of water as it is heated from say 5C to 60C.

However on the cold side the entire volume of the cold supply pipe
inside the house (2 or perhaps 3 litres) will be warmed from 5C to house
ambient - say 20C. This must cause some expansion (volume increase may
only be an egg-cup full) and I am curios as to how this is dealt with.

As you touch on, the expansion to be accommodate is a function of initial volume and temperature rise which, compared to the hot side, is relatively small - particularly given that the expansion is volumetric and hence is exponential.

There are a few graphs, with the formulae behind them, at https://www.engineeringtoolbox.com/e...nks-d_885.html that illustrate it quite well.