From the Alpha combi boiler instructions:
"Note: If the mains is fitted with water meter, check valves or loose jumper stop c*ck, then a DHW expansion device must be fitted."
and there's a similar note in the W-B Si instructions, but not in, for instance the Glow-worm equivalents. I can see that there is a risk of trapping a quantity of water in a length of pipe, which then gets warmer, but doesn't this apply to all cold water systems?
Does anyone fit these devices in real life?