The greenhouse climate battery
Blow the greenhouse's own hot afternoon air underground and get it back at 4 a.m.
What it is
Perforated pipe buried in gravel under the greenhouse floor, with a fan that pushes warm daytime air down through it. The soil mass takes up the heat and gives it back overnight. Also called ground-to-air heat transfer, or GAHT.
The numbers
- Working installations commonly report 5–15°F of overnight lift versus an identical unassisted greenhouse — enough to change what you can grow.
- Fan power is small — on the order of 50–150 W running intermittently — so the energy return is genuinely favorable.
- It must be installed before the floor goes in. Retrofitting means excavating the greenhouse you already built.
Why it's here
It's a battery made of dirt, with no chemistry, no degradation, and no replacement cost. For season extension it's one of the better dollar-per-degree investments in a greenhouse, provided you're building anyway.
The automation angle
This one genuinely wants control: soil temperature at depth, air temperature high and low, and a fan rule with hysteresis. The whole point is running the fan only when the air is warmer than the ground, and that's a trivial rule with two sensors.
The catch
Condensation. Warm humid air hitting cold pipe drops its water in the ground loop, and a loop that holds standing water grows things you then blow into the greenhouse. Slope the pipes to a drain, plan a condensate exit, and give yourself access to inspect. Do that wrong and you've buried a mold generator under your floor.