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?
Heat-pump water heater (chauffe-eau thermodynamique)
Needs the grid
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
Needs a sufficient volume of unheated air to draw from (a garage or cellar is the usual answer) or ducting to outside; it cools that space, which is a benefit in summer and a cost in winter
A water heater with a small heat pump on top of the tank. It takes warmth from the surrounding air and moves it into the water, giving two to three and a half times more heat than the electricity it uses. It needs a large unheated room or a duct to the outside, and it cools the room it draws from.
Build it
This solution is not buildable as a DIY project.
Cost
High
Upkeep
Seasonal
Skill to build
Specialist
Lifespan
10-15 years, following the heat pump rather than the tank
Levels are relative to the other options for this need, not absolute. Fewer dots is lighter.
Heat-pump water heater (chauffe-eau thermodynamique) (This page) 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.
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.
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.
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.