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?
Slate-clad solar air heater (wall-mounted thermosiphon collector)
Passive
One of 2 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
South-facing wall area with two through-wall penetrations (low intake, high return) into the conditioned space
A glazed box on a south wall with a dark slate plate inside. Room air enters at the bottom, is warmed by the sun-heated slate, rises and flows back into the room at the top, with no fan. It can raise a small room by 5 to 7 °C on a winter day, and a flap turns it into a cooling vent in summer.
20-25 years for the collector box (glazing and slate absorber are the longest-lived parts); the wax-thermostat damper mechanism may need earlier replacement
Levels are relative to the other options for this need, not absolute. Fewer dots is lighter.
Slate-clad solar air heater (wall-mounted thermosiphon collector) (This page) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Medium · Skill to build: Basic · Upkeep: Light · Change to daily life: Occasional · Works without the grid: Yes Choose it if you want to warm one small room on sunny winter days with a box you can build on a south wall.
South-facing glazing sized for winter gain Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: None · Upkeep: None · Change to daily life: None · Works without the grid: Yes Choose it if you are building or replacing south windows and want a real share of the heating for free.
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.