Earth Tube (Ground-to-Air Heat Exchanger)

Soil below the frost line holds a fairly stable temperature close to the region's annual average year-round — noticeably cooler than summer air and noticeably warmer than winter air. An earth tube is a buried pipe that runs incoming fresh air across that stable soil temperature before it reaches the house, pre-tempering it for free before any mechanical heating or cooling has to do the rest of the work.

Materials

• Solid (not perforated) HDPE or PVC pipe, 4–6" diameter, sized to the run length and desired airflow
• Gravel bed, for the trench bottom — supports the pipe and helps drainage
• Intake screen/filter and pest guard, for the outdoor air-intake end
• Low-power inline duct fan (optional) — passive convection works over short runs, but most real installs use a small fan to guarantee airflow

The Build

1. Trench below the local frost line along the planned run — this is the whole mechanism, so don't shortcut the depth.
2. Slope the trench slightly toward the outdoor intake end (roughly 1–2% grade) so any condensation that forms inside the tube drains back out rather than pooling — this single detail is the difference between a working system and a mold problem, see the caution below.
3. Lay a gravel bed, then the pipe, then backfill.
4. Fit the outdoor end with a screened, pest-proofed intake cap, positioned away from anything that would draw in exhaust, dust, or standing water.
5. Connect the indoor end to a fresh-air intake point — ducted into an existing HVAC return, or a dedicated vent into a room, with or without an inline fan depending on run length and desired airflow.

Caveats

• Real, well-documented failure mode: warm humid air cools as it travels through the tube, and if the tube doesn't drain, that condensation sits and grows mold — which then gets blown straight into the house. The condensate slope in the build steps above isn't optional; it's the fix for the single most common thing that goes wrong with these systems.
• This is a load-reducer, not a full HVAC replacement — it pre-tempers incoming air, it doesn't heat or cool a house on its own. Frame it paired with an existing furnace/AC or another MM off-grid appliance (a thermal mass heater, for instance), the same way it's been scoped in earlier MM design sessions, not as a standalone system.
• Actual soil temperature and achievable pre-tempering range depend on local frost line, soil type, and burial depth — the "65–68°F" range cited in the original brain-dump note is a reasonable regional estimate, not a guaranteed number for every specific property. Verify against the real site conditions before promising a customer a specific temperature.