Lowtech Graph lists low-tech options for every need of a home. Here you can compare them, find tutorials to build them, and link them together into a circular system. Why low-tech?
Moving air with a fan, so the rate is known rather than hoped for. In a cold climate the point is to do it without throwing the heat away, which is what a heat exchanger is for. The options differ in whether one unit serves the house or each room has its own, and in how much warmth they reclaim.
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.