Full engineering calculator for ground-to-air heat transfer (GAHT) greenhouse heating. Size your fans, tubing network, calculate heat storage capacity, costs, and energy payback. Based on the Cruickshank/CRMPI methodology. For the technical background, read our guide to climate batteries.
A climate battery stores daytime solar heat underground and releases it overnight -- no boiler, no propane, no electricity bill. A small fan pushes warm greenhouse air down through buried 4" ADS tubing. The soil absorbs the heat. At night, stored heat radiates back up through the floor. In Colorado, this technology delivers 11 months of frost-free Mediterranean climate in Zone 4. On Vancouver Island (Zone 8a), it extends the growing season significantly.
Three layers of 25-35 ft tubes are buried parallel under the greenhouse floor, spaced to allow 10-14 inches of soil between adjacent tube walls. Manifolds at each end connect all tubes so a single fan moves air through the entire network. The goal: cycle the full greenhouse air volume through the underground network at least 5 times per hour.
This calculator follows the original Cruickshank/CRMPI methodology. Fill in the yellow cells for your greenhouse dimensions, then adjust the green cells for fan and tubing design. Watch the blue results update live.
Calculate the total air volume from your greenhouse dimensions. The "box section" is the straight-wall portion. Add the roof volume above it.
0 if flat roof or hoop
180 for full half-circle hoop. 0 if not a hoop roof.
Usually half the greenhouse width
Enter 0 to use calculated value above
Enter 0 to use calculated value above
Ballpark tubing estimate: ~1.33 ft of 4" tubing per sq ft of greenhouse floor for a 3-layer design.
Decide how many fans and how much airflow. Start with one fan per 2,000 sq ft of floor. Aim for at least 5 air changes per hour. Field trials show up to 20 exchanges/hr with favorable results.
Min 5 recommended. Up to 20 trialed.
Standard design: 3 layers of parallel 4" ADS tubing, 25-35 ft long. Tube spacing is set by manifold rib spacing. Play with tube count and length, watching the air speed result.
Equal to number of fans
25-35 ft recommended
Enter your local material costs. Prices below are reasonable CAD estimates for Vancouver Island. Adjust for your supplier quotes.
4" ADS with sock
18" N-12 or barrel
15" N-12 ADS
Thermostat, wiring, misc
Estimate how long the system pays for itself in avoided heating costs. The heatsink stores heat every sunny day -- how many sunny heating days do you get per year?
1 therm = 100,000 BTU. Check BC Hydro / FortisBC rates.
BC Hydro residential ~$0.10-0.14/kWh
Vancouver Island: 120-160. Colorado: 180. Florida: 200+.
Estimate your greenhouse heat loss and solar heat gain to understand how your climate battery compares to the heating demand.
Single poly: 0.83. Twin-wall 8mm: 1.43. Triple-wall: 2.5
2" XPS: 10. 4" XPS: 20. Straw bale: 30.
Minimum you want to maintain (40F = 4C frost-free)
Campbell River: 25F. Victoria: 30F. Interior: 0-10F.
Tight build: 1.5. Standard: 2.0. Leaky: 3.0+
25% typical for thermal curtain
This calculator is based on the Climate Battery Calculator originally developed by John Cruickshank, designer and inventor who partnered with Jerome Osentowski to develop the climate battery technology at the Central Rocky Mountain Permaculture Institute. Version 2.0 maintained by Eco Systems Design (Jerome Osentowski, Michael Thompson, Will Lennox). Adapted for web by Swell Farms.
These steps assume a new greenhouse build or a major retrofit where you can access the full floor area.
Remove soil to the depth calculated in Step 3 across the full greenhouse footprint. Grade the bottom with a 1-2% slope toward one end for condensation drainage.
Use a plate compactor or hand tamper to firm the excavated base. Check for level with a string line.
Rigid XPS foam (minimum R-10, 50mm) vertically around the inside perimeter from grade to the bottom of the excavation. Prevents heat leaking sideways.
Lay parallel runs of 4" ADS perforated tubing at the calculated horizontal spacing. Connect all tubes to a manifold pipe at each end.
Cover tubes with soil, compact lightly. Maintain the vertical spacing between layers (typically 12 inches).
Lay the next layer of parallel tubes offset from the layer below. Backfill and compact between each layer.
Run solid pipe from each manifold up through the foundation to above floor level. Inlet riser connects to fan. Outlet riser returns air to greenhouse.
Cover the top layer with soil to the depth specified (typically 12 inches of cover above top tube wall). This soil is your growing medium.
Mount inline fan at inlet riser. Wire to thermostat set to activate at 25-28C. Fan speed controllers allow seasonal tuning.
Cap both risers with rodent screen. Run fan and verify airflow. On a sunny day, you should see a 10-25F temperature drop between intake and exhaust.
Available at Home Depot, Slegg Building Materials, and agricultural supply yards. Buy with factory-installed filter sock to prevent soil migration into tubes.
Large-diameter ADS available from drainage supply specialists. Check local aggregate/drainage suppliers. 55-gallon barrels with holes work as a budget alternative.
Inline duct fans (AC) from Home Depot or HVAC suppliers. Solar DC fans from greenhouse or off-grid solar suppliers. Match CFM to your Step 2 calculation.
2" XPS (blue or pink board) from any building supply. Slegg, Home Depot, Windsor Plywood. For Interior BC, double up to 4".
Swell Farms designs and builds passive solar greenhouses with climate batteries on Vancouver Island and the Gulf Islands.