For context, in my home, I'm adding an air-to-air in one room (that isn't currently heated) on a separate exchanger.
But for the rest of the house, when I already have radiators and underfloor heating, I don't want to run 100mm ducting, or fit split units throughout the building.
When I do eventually go to a heat pump for the main system (eventually) I'll need to install a new tank funnily enough, as currently I just have on-demand gas-heated water.