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?
Wood-stove flue water heater (thermosiphon coil)
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
Requires an existing wood stove with an accessible flue pipe; soldering competence to install the coil. Two requirements are often confused. THE THERMOSIPHON ITSELF has no minimum temperature: water circulates on the density difference between the hot leg and the cold one, so any differential drives it. What it needs is GEOMETRY: the tank above the coil, a continuous rise with no horizontal runs or high points where air can lodge, and generous pipe bores, because the driving pressure is small. HOW MUCH HEAT MAY BE TAKEN is limited by the flue rather than the coil: extracting heat lowers the flue gas temperature, and below roughly 120 degC creosote condenses in the chimney, becoming thick and tar-like lower still. That is a wood-specific limit, and it is measurable with a flue thermometer, a standard stove accessory.
A copper coil wrapped around the flue of a wood stove, linked to a water tank. The hot flue heats the water, which rises to the tank on its own. It gives hot water from a stove already lit for heating, at no extra fuel, and works best in winter when the sun gives least.
15-25 years for the copper coil before flue-gas corrosion or soot fouling degrades performance
Embodied carbon
Low
Space / material use
Low
Output timing
Weather/time-dependent
Skill to repair
Intermediate
Regulation (FR example)
Flue installation follows DTU 24.1. Chimney sweeping is compulsory (typically once or twice a year) under the departmental sanitary regulation, and insurers generally require proof. Some communes restrict wood burning under an air-quality plan.
Levels are relative to the other options for this need, not absolute. Fewer dots is lighter.
Wood-stove flue water heater (thermosiphon coil) (This page) 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.
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.
Compost-coil water pre-heater 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.
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.
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.