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?
Compost-coil water pre-heater
Passive
One of 7 ways to meet this need. Cost, carbon, footprint and rankings are relative to other solutions for the same need, not absolute.
What your site needs
An active compost pile large enough to maintain thermophilic temperatures
A coil of pipe buried in an active compost heap, with water passing through it on the way to the water heater. The heap stays warm as it breaks down and warms the water in the coil. The water heater then has less work to do, so hot water costs less.
Copper coil itself: 15-25 years buried in an active compost pile (corrosion from the compost environment is the limiting factor, not the pipe wearing out)
Levels are relative to the other options for this need, not absolute. Fewer dots is lighter.
Compost-coil water pre-heater (This page) Can I use it? Not in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Basic · Upkeep: Light · Change to daily life: Occasional · Works without the grid: Yes Choose it if you already keep a big compost heap and want it to pre-heat your water.
Thermosiphon solar water heater Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Basic · Upkeep: Light · Change to daily life: Occasional · Works without the grid: Yes Choose it if you want solar hot water kept in its own insulated tank, with no pump.
Electric resistive water heater (cumulus) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Intermediate · Upkeep: Light · Change to daily life: None · Works without the grid: No Choose it if you want the cheapest heater to buy, with parts you can replace yourself.
Batch solar water heater (integrated collector-storage) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Intermediate · Upkeep: Light · Change to daily life: Daily · Works without the grid: Yes Choose it if you want the simplest solar hot water, panel and tank in one box.
Wood-stove flue water heater (thermosiphon coil) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Intermediate · Upkeep: Light · Change to daily life: Daily · Works without the grid: Yes Choose it if you already run a wood stove and want hot water from it in winter at no extra fuel.
Gas water heater Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Specialist · Upkeep: Light · Change to daily life: None · Works without the grid: No Choose it if you are on the gas mains and want hot water on demand, without keeping a tank hot.
Heat-pump water heater (chauffe-eau thermodynamique) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: High · Skill to build: Specialist · Upkeep: Heavy · Change to daily life: None · Works without the grid: No Choose it if you want electric hot water for a third of the power, and have an unheated room for it.
We don’t list sizing or quantities, as the yield of each technique depends on the quality of its build. The catalogue compares options but doesn’t dimension them. Regulatory notes are indicative, with a French example, and need to be confirmed locally.