Climate Batteries Explained: Heating a Greenhouse with Soil

A climate battery stores excess daytime heat in the soil beneath a greenhouse and releases it back at night. No propane. No electric heaters running all winter. Just a fan, some buried pipes, and a gravel bed. The system captures heat that would otherwise be vented out during warm days and banks it underground for when you need it.

The technical name is Ground-to-Air Heat Transfer (GAHT) system. The concept is simple. The execution needs to be right or it does not perform. This guide covers how the system works, the materials involved, sizing for a production greenhouse, and the installation sequence we follow on Vancouver Island.

How a Climate Battery Works

The principle is straightforward:

1. Charging (hot days): When greenhouse air temperature exceeds approximately 25 degrees Celsius, a fan pushes that hot air down through a network of pipes buried 1.2 to 1.8 metres below the greenhouse floor. The surrounding soil and gravel absorb the heat. Air exits the pipes cooler than it entered -- cooling the greenhouse in the process.
2. Discharging (cold nights): The warmed soil radiates stored heat upward through the greenhouse floor. Ground temperature stays elevated for days or weeks after charging, providing a steady baseline warmth without any energy input.
3. Seasonal cycling: Over a full season, the soil mass beneath the greenhouse accumulates heat from spring through fall. By December, even without daily sun, ground temperature remains 4 to 8 degrees Celsius above what it would be without the system.

The fan is the only mechanical component. It runs when conditions trigger charging (thermostat control) and can be powered by a small solar panel if you want the system fully off-grid.

The Physics: Why Soil Stores Heat

Wet soil holds approximately 1.5 kJ per kilogram per degree Celsius. That is lower than water (4.18 kJ/kg/C) but soil is dense, cheap, and already under your greenhouse. A cubic metre of moist soil weighs roughly 1,800 kg and can store approximately 2,700 kJ per degree of temperature rise.

The gravel bed beneath the pipes serves two purposes: it stores heat (rock holds 0.7 to 1.0 kJ/kg/C) and it provides air space for the warm air to disperse from the pipes into the surrounding soil mass. Without gravel, the pipes only heat the soil immediately touching them. With gravel, the heat distributes through a larger volume.

Ground Temperature Baseline

In Campbell River at 1.5 metres depth, undisturbed ground temperature sits at approximately 10 to 12 degrees Celsius year-round. A climate battery raises this to 15 to 20 degrees Celsius in the zone beneath the greenhouse after a full season of operation. That 5 to 10 degree increase in ground temperature translates directly into warmer overnight air in the greenhouse above.

When a Climate Battery Makes Sense

A climate battery is not appropriate for every greenhouse. Here is when it works and when it does not:

Good Fit

• Greenhouse floor area 200 square feet or larger (our 14x20 production unit at 280 sq ft is the minimum practical size)
• Year-round production goal -- you want the greenhouse to never freeze
• Site has well-drained soil and water table below 2 metres
• Building during initial construction (excavator already on site)
• Combined with passive solar design -- the climate battery supplements, not replaces, good orientation and insulation

Poor Fit

• The 9x12 backyard greenhouse (108 sq ft) -- not enough floor area for an effective pipe network
• High water table (above 1.5 metres) -- saturated soil cannot store heat effectively
• Retrofitting an existing greenhouse -- excavation cost is prohibitive
• 3-season greenhouse where occasional freezing is acceptable -- thermal mass (water barrels) and insulated blinds are simpler and cheaper
• Rocky ground where excavation to 1.5+ metres is impractical

Battery-to-Greenhouse Volume Ratios

The thermal storage volume (soil + gravel beneath the floor) needs to be proportional to the greenhouse air volume it is heating. Too small a battery and it discharges in one cold night. Too large and you waste money on excavation that never gets fully charged.

Greenhouse Size	Air Volume	Minimum Battery Volume	Pipe Runs
14x20 ft (280 sq ft)	~65 m³	30-40 m³ (gravel bed)	4 runs, 6m each
16x40 ft (640 sq ft)	~150 m³	70-90 m³ (gravel bed)	8 runs, 10m each

Rule of thumb: the gravel bed volume should equal 40 to 60 percent of the greenhouse air volume. More is better -- you are never penalized for over-sizing the thermal storage.

Pipe Layout

The pipe network is the distribution system. It moves hot air from the greenhouse down into the soil mass and spreads it evenly across the gravel bed.

Specifications

• Pipe material: 100mm (4-inch) HDPE drainage pipe -- solid or perforated
• Depth: 1.2 to 1.8 metres below finished floor
• Spacing: 450 to 600 mm (18 to 24 inches) between pipe centres
• Run length: 6 to 10 metres per pipe (longer runs lose too much heat to friction and back-pressure)
• Slope: Slight fall (1:100 gradient) toward a drainage point to handle condensation
• Header: Manifold at the fan end connects all pipe runs to a single inlet duct
• Outlet: All pipes converge to a return plenum on the opposite end, with the air returning to greenhouse level

Layout Pattern for 14x20 Production Unit

Four parallel pipe runs, each approximately 6 metres long, spaced 500 mm apart. The pipes run east-west (parallel to the long axis of the greenhouse). A manifold header on the west end connects to the fan. A return plenum on the east end brings air back up to greenhouse level through a floor grate.

Total pipe: approximately 24 linear metres of 100mm HDPE.

Gravel Bed Construction

The gravel bed surrounds the pipes and extends to fill the excavated volume beneath the greenhouse floor.

Specifications

• Material: 19mm (3/4-inch) clean crushed gravel -- no fines
• Depth: 600 to 900 mm total bed depth (pipes sit in the middle of this)
• Coverage: Full greenhouse footprint -- edge to edge
• Drainage: Perimeter drain tile at the base of the gravel bed, sloped to daylight or a sump
• Separation: Landscape fabric on top of gravel before backfilling with growing soil

For the 14x20 greenhouse: approximately 16 cubic metres of clean gravel (21 cubic yards). That is roughly two tandem dump truck loads from a local aggregate supplier.

Fan Sizing

The fan moves greenhouse air through the pipe network during charging. It needs to be large enough to move the full greenhouse air volume every 2 to 3 minutes.

Calculation for 14x20 Production Unit

Air volume: 280 sq ft x 8.5 ft average ceiling = 2,380 cubic feet (67 m³)
Target air changes: 1 every 2-3 minutes
Required CFM: 2,380 / 2.5 = 950 CFM (approximately 27 m³/min)

An 8 to 10-inch inline duct fan rated at 750 to 1,150 CFM handles this. Look for EC motor fans -- they are quieter, more efficient, and last longer than AC motor units.

Control

The fan runs when greenhouse air temperature exceeds 25 degrees Celsius (charging mode). A simple thermostat wired to the fan does the job. More sophisticated controllers can also run the fan in reverse at night to actively pull warmth from the soil, but passive radiation from the warm ground handles most of the heating without this.

Power

A 950 CFM inline fan draws approximately 150 to 250 watts. On a sunny day in shoulder season, it might run 4 to 6 hours. Daily energy use: 0.6 to 1.5 kWh. A modest solar panel (300W) and battery can run this entirely off-grid.

Growing Degree Day Increase

A climate battery increases annual growing degree days (GDD) by approximately one-third. In practical terms for Campbell River:

• Overnight lows stay 4 to 8 degrees Celsius above outdoor temperature, even on overcast days when the battery has not been actively charging
• The shoulder seasons (March-April and October-November) become fully productive -- plants keep growing instead of stalling
• Deep winter (December-January) lows stay above freezing in a well-insulated greenhouse with climate battery, even when outdoor temperatures hit -10 to -12 degrees Celsius

This is what makes year-round production of cold-hardy crops realistic at 50 degrees north without burning propane.

Seasonal Heat Cycling

The climate battery operates on two timescales:

Daily Cycle

Hot afternoon air charges the gravel bed. Overnight, the stored heat radiates back up. This handles typical spring/fall temperature swings where days are warm (15 to 25 degrees Celsius) but nights drop to 2 to 5 degrees.

Seasonal Cycle

From March through October, the soil mass accumulates heat continuously. Ground temperature beneath the greenhouse rises from its baseline 10 to 12 degrees Celsius up to 15 to 20 degrees by late fall. This elevated ground temperature persists through December and January, even with minimal solar gain, because soil at 1.5 metres depth changes temperature very slowly.

By February, the seasonal charge has mostly dissipated. March solar gain begins recharging the system. This is why the climate battery works best as part of an integrated passive solar design -- it needs those sunny shoulder-season days to charge.

Installation Steps

Install the climate battery during initial greenhouse construction. Retrofitting is technically possible but costs 3 to 4 times more because you are excavating inside an existing structure.

Sequence

1. Excavate: Dig the full greenhouse footprint to 1.8 metres below finished floor level. Set spoil aside for later backfill (topsoil) or removal (subsoil if clay-heavy).
2. Perimeter drainage: Install 4-inch perforated drain tile around the base perimeter of the excavation. Slope to daylight or sump. This keeps the gravel bed drained -- critical on the wet coast.
3. Base gravel: Spread 200 mm of clean 19mm gravel across the excavation floor. Level.
4. Lay pipes: Position the HDPE pipe runs on the gravel base at 500 mm spacing. Connect to the manifold header and return plenum. Confirm slope for condensation drainage.
5. Surround with gravel: Fill around and over the pipes with clean gravel to a total bed depth of 600 to 900 mm.
6. Landscape fabric: Lay geotextile fabric over the gravel bed to prevent soil migration into the gravel.
7. Backfill: Add 300 to 600 mm of growing soil on top of the fabric. This is your greenhouse floor -- plants grow directly in it.
8. Install fan and ductwork: Mount the inline fan at greenhouse level. Connect to the manifold header with insulated duct. Install the return grate on the opposite end.
9. Wire thermostat: Set charging threshold at 25 degrees Celsius. Wire to fan.
10. Test: Run the fan and confirm airflow at all return points. Check for leaks or blockages.

Materials List (14x20 Production Unit)

Item	Quantity	Notes
100mm HDPE pipe (solid)	24 linear metres	4 runs x 6m each
100mm pipe fittings (elbows, tees)	12-16 pieces	Manifold + return connections
19mm clean crushed gravel	16 m³ (21 cubic yards)	No fines -- must drain freely
4" perforated drain tile	25 linear metres	Perimeter drainage
Geotextile landscape fabric	30 m² (1.5x footprint for overlap)	Prevents soil intrusion into gravel
Inline duct fan (8-10")	1	750-1,150 CFM, EC motor preferred
Insulated flex duct (8-10")	3-4 metres	Fan to manifold connection
Floor return grate	1-2	Where air returns to greenhouse
Thermostat (cooling mode)	1	Set to 25°C trigger
Growing soil (backfill)	8-10 m³	Quality topsoil for greenhouse floor

Integration with Passive Solar Design

A climate battery does not work in isolation. It is one component of a complete passive solar greenhouse design. The system depends on:

• South-facing glazing: Generates the excess heat that charges the battery. Without adequate solar gain, there is nothing to store. See Passive Solar Greenhouse Design for Coastal BC.
• Insulated north/east/west walls: Prevent the stored heat from escaping sideways. R20 minimum on all non-glazed surfaces.
• Thermal mass (water barrels): Handles the daily cycle (day-to-night). The climate battery handles the longer seasonal cycle. Both are needed for year-round operation.
• Insulated blinds: Reduce nighttime heat loss through glazing by 30%. This means the climate battery does not have to work as hard.
• Ventilation: On hot summer days, vent excess heat outdoors rather than overcharging the battery. Ground temperature above 25 degrees Celsius starts to harm plant roots.

Common Mistakes

• Building too small: The 9x12 greenhouse does not have enough soil volume beneath it for a functional climate battery. Use water barrels instead.
• Ignoring drainage: A gravel bed that sits in water does not store heat -- it conducts it away. Perimeter drain tile is not optional on the wet coast.
• Undersizing the fan: A weak fan cannot push air through 24 metres of pipe at adequate volume. Measure the static pressure of your pipe layout and size accordingly.
• Pipes too shallow: At 600 mm depth, heat dissipates within hours. At 1.5 metres, it holds for days. Deeper is better (to a point).
• No thermostat: Running the fan constantly wastes energy and can overcool the greenhouse on mild days. Charging should only happen when air temperature exceeds 25 degrees Celsius.
• Skipping the insulation: A climate battery in a poorly insulated greenhouse is like heating a house with the windows open. Fix the envelope first.

Use Our Climate Battery Calculator

Our free Climate Battery Sizing Tool calculates pipe layout, gravel volume, fan size, and expected temperature lift for your specific greenhouse dimensions and location. Pair it with the Passive Solar Greenhouse Planner for the complete design picture.

Sources and References

• Rob Avis, P.Eng. -- Verge Permaculture (GAHT system design principles)
• Natural Resources Canada -- EATEX earth tube design tool (Kelowna case study: 4 pipes x 23-29m, 100mm HDPE, 1.6-1.8m depth, 13.2°C maximum delta-T measured)
• BC ground temperature data -- Campbell River area baseline 10-12°C at 1.5m depth