Pipe coil through an active compost pile pre-heats water before the main heater. Passive where the loop can thermosiphon or sits on an existing pressurised feed; a separate circulating pump is needed if the geometry does not allow either.
Note: Cost, carbon and footprint are rated against the other options for this same need, not in absolute terms.
An active compost pile large enough to maintain thermophilic temperatures
No sizing or quantities: how much a technique yields, and how much a household needs, both depend on the build and the people, so the catalogue compares options rather than dimensioning them. It carries no bill of materials and no build sequence, and it is written for an oceanic, clay-soil climate (Normandy, France). Regulatory notes are indicative and need confirming locally.