That's when we are speaking of an individual system installed in a flat. When we are speaking of a building as a whole, almost all of the cost of installing eit

That's when we are speaking of an individual system installed in a flat. When we are speaking of a building as a whole, almost all of the cost of installing either a heat pump or just a ventilation system is modifying a building to install air ducts. Which is sometimes a highly invasive, destructive procedure equalling to a near total refurbishment of a building (so this is typically combined: heat insulation + ventilation ducts + general refurbishment, as doing any of the 3 separately won't be much cheaper than doing all 3, anyway). Then you have a single large pipe where a large mass of air enters and and another one where it exits the building. Then it's comparatively not at all hard or expensive to add a massive heat pump so the inlet pipe passes through it (and a heat exchanger between outlet pipe and inlet before, so-called regenerative heating, which will achieve some of the same result, but for free). That's why i think that buiding-scale ventilation systems without heat pumps make no economic sense. They mean leaving a very low hanging fruit, hanging.