A 55-gallon water drum holds about 0.87 megajoules of heat per kelvin of temperature change, so a 10 K swing stores 8.7 MJ, and dividing that by the greenhouse’s night-time heat loss, glazing area times U-value times temperature difference times the hours, tells you how many drums you need. With an illustrative single-layer U-value of 6 W/m²K, one drum covers about 3.4 m² of glazing through a 12 hour night at a 10 K indoor-outdoor difference. That means a small tunnel needs dozens of drums, so water mass smooths day-night swings and will not by itself hold a hard freeze at bay.
Thermal mass heating stores solar heat by day in water, stone or soil inside a greenhouse and releases it at night, evening out the temperature without fuel.
Stored Heat Is Mass Times Specific Heat Times The Swing
The carrier guide (Four-Season Growing & Season Extension) recommends black-painted 55-gallon drums filled with water along the north wall (about 460 BTU per degree Fahrenheit each), a stone or brick north wall 8-12 inches thick, raised beds, and a hot-compost pile with water pipes through it, and says a well-designed passive solar greenhouse can stay above freezing down to 0°F outside. The arithmetic below checks the first claim and tests the last.
Worked Example 1: What One Drum Stores
Inputs: 55 US gallons × 3.785 L/gal = 208 L, which is 208 kg of water. The specific heat of water is 4.18 kJ/(kg·K), a standard physical constant.
- Heat per kelvin: 208 kg × 4.18 kJ/(kg·K) = 870 kJ/K = 0.87 MJ/K.
- Check against the carrier: 460 BTU/°F × 1,055 J/BTU × 1.8 °F per K = 873,000 J/K = 0.87 MJ/K. The carrier’s figure is correct.
- For a 10 K swing the drum releases 0.87 × 10 = 8.7 MJ. Spread over a 12 hour night (43,200 s), that is 8.7 × 10⁶ ÷ 43,200 = about 201 W.
Worked Example 2: What A Night Costs
Heat loss through the cover is Q = U × A × ΔT × time, where U is the heat loss coefficient of the cover in W/(m²·K), A the glazed area in m², and ΔT the inside-outside temperature difference.
Illustrative input, not a sourced value: U = 6 W/(m²·K) for a single layer of glazing or film. Measure it for your own structure: on a still night, with the house closed and no heat source, the temperature fall over a few hours against a known stored heat gives it.
- Per m² of cover at ΔT = 10 K: 6 × 10 = 60 W; over 12 hours (43,200 s): 60 × 43,200 = 2.59 MJ.
- One drum at a 10 K swing (8.7 MJ) covers 8.7 ÷ 2.59 = 3.4 m² of cover.
- At ΔT = 17.8 K (holding 0°C inside when it is 0°F, or −17.8°C, outside): 6 × 17.8 × 43,200 = 4.6 MJ/m². One drum covers 8.7 ÷ 4.6 = 1.9 m².
Worked Example 3: A Small High Tunnel
The carrier’s smallest tunnel is 20 ft wide (6.1 m) and 50 ft long (15.2 m). For a semicircular arch, the half circumference is π × 3.05 m = 9.6 m, so the arched cover has area 9.6 × 15.2 = 146 m². The two end walls are half-discs of area π × 3.05² ÷ 2 = 14.6 m² each, 29 m² together. Total cover: about 175 m².
- At ΔT = 10 K: 175 ÷ 3.4 = about 52 drums, which is 52 × 208 L = 10,800 L of water.
- At ΔT = 17.8 K: 175 ÷ 1.9 = about 92 drums.
- A drum about 0.6 m across along the 15.2 m north wall gives 15.2 ÷ 0.6 = 25 drums per row.
So drums alone cannot supply this tunnel at a 10 K difference, and far less at 17.8 K, with a single-layer cover. Other mass (soil, a stone wall), double glazing, an insulating night cover and the daytime heat the house gains all change the result, and none of them is quantified here. What the arithmetic supports is narrower than the carrier’s claim: water drums reduce the daily swing and take the edge off a mild frost.
What Must Be True For Stored Heat To Be Released
A drum gives up heat only while it is warmer than the air, and it can store only what the sun delivers to it. Put the drums where the winter sun strikes them directly, paint them dark, and keep the swing real: a drum that never warms by more than a few kelvin by day cannot deliver the figures above. A second layer of cover or a night blanket lowers U, which cuts the number of drums needed in proportion; test it by measuring.
Compost As A Heat Source
The carrier says a fresh hot-compost pile inside the greenhouse generates 100-160°F for weeks and can warm water pipes. The pile’s heat output depends on its size, materials and moisture and is not given; measure it with a probe thermometer. See Hot Composting.
Common Mistakes
- Counting a drum as heating the house — it is a buffer, not a source.
- Putting drums where they never see sun — they stay at air temperature and store nothing.
- Skipping the loss estimate — without U, A and ΔT there is no number to size against.
- Assuming a double wall or a night cover without testing — measure.
Safety
Weight, gas and fire
A full 55-gallon drum holds 208 L of water and weighs over 200 kg: set it on a firm base, chock it, and do not roll it up a slope or stack it. A composting pile in a closed greenhouse gives off carbon dioxide and, from fresh manure, ammonia; ventilate before you enter. Do not burn fuel inside without a flue and ventilation: combustion produces carbon monoxide.
What’s Next
- Learn next: Crop Selection for Protected Growing — what the heated space should grow
- Combine with: Thermal Mass for Night Warmth — the same principle in buildings