Ranked by heat-recovery effectiveness - how much warmth the unit reclaims from outgoing air instead of losing it. A central ducted system recovers 80-90% (Centre for Sustainable Energy, cse.org.uk); per-room units recover measurably less, as each unit sits in an external wall and loses heat to the cold fabric around it (Selfbuild.ie). The direction is well established; the exact decentralised range is not independently sourced here. As with every efficiency figure in this catalogue, these are class-level theoretical ranges - real performance depends on the specific product, the install, and commissioning quality.