Every greenhouse heating conversation eventually arrives at this question: can water barrels replace a heater? The short answer is no — but they can dramatically reduce how often the heater runs and how much fuel it burns.
What Water Barrels Actually Do
Water barrels store daytime solar heat and release it at night, buffering the overnight temperature drop by 10 to 15°F in a well-insulated greenhouse. They do not generate heat — they redistribute solar energy from day to night. On a night when outdoor temperatures drop 30°F below the daytime greenhouse peak, water barrels cover the first 10 to 15 degrees of that drop. The heater covers the rest.
What a Backup Heater Does
A propane, natural gas, or electric heater generates heat on demand, maintaining whatever minimum temperature you set on the thermostat. It is reliable, controllable, and unlimited in capacity (as long as fuel or electricity is available). The downside is cost — running a heater as the sole heat source in a cold-climate greenhouse burns significant fuel.
The Hybrid Approach
The most cost-effective strategy combines both. Water barrels handle the first 10 to 15 degrees of overnight cooling passively and for free. The heater kicks in only when temperatures drop below the barrel's buffering capacity — typically on the coldest 20 to 30 nights of winter. This hybrid approach reduces heater run time by 50 to 70 percent compared to heater-only operation, cutting fuel costs proportionally.
For growers who keep greenhouses only frost-free (minimum 35 to 40°F), water barrels alone may be sufficient in zones 7 and warmer. For growers maintaining 50 to 55°F minimums for warm-season crops, the heater is essential backup in any zone below 8. The barrels reduce fuel consumption; the heater provides insurance.
Sizing Your Thermal Mass
A useful rule of thumb: 2 to 3 gallons of water per square foot of greenhouse floor area provides meaningful overnight temperature buffering. A 200 square-foot greenhouse benefits from 400 to 600 gallons of water — approximately 8 to 11 standard 55-gallon drums. Paint drums black or dark blue to maximize solar absorption during daylight hours. Position drums along the north wall (in the Northern Hemisphere) where they absorb and store the sunlight that passes through the greenhouse without shading plants on the south side.
Water is not the only thermal mass option. Concrete, stone, brick, and phase-change materials (PCMs) all store and release heat. Water is the cheapest and most effective per unit cost — it stores approximately 62 BTUs per cubic foot per degree of temperature change, which is higher than concrete (22 BTUs) or stone (20 BTUs). Phase-change materials store dramatically more heat per unit volume but cost 10 to 20 times more and are practical only for commercial installations.
Heater Sizing
To size a backup heater, calculate the heat loss rate of your greenhouse at your desired minimum temperature. A simplified formula: BTU/hr needed = total glazing area (sq ft) × temperature differential (desired minimum minus expected outdoor low) × the heat loss factor of your glazing (1.2 for double-poly, 1.0 for polycarbonate, 0.8 for insulated panels). A 200 square-foot greenhouse with double-poly walls maintaining 50°F when outdoor temperature is 20°F needs approximately: 400 sq ft glazing × 30°F differential × 1.2 = 14,400 BTU/hr. A standard 20,000 BTU propane heater covers this with margin to spare.
With water barrel thermal mass handling the first 10 to 15°F of cooling, the heater only needs to cover the remaining differential. This effectively allows a smaller heater to maintain the same minimum temperature — or the same heater to maintain a higher minimum temperature — saving both equipment and fuel costs.
For growers in zones 7 and warmer who maintain only frost-free minimums (above 32°F), a robust thermal mass system may eliminate the need for a backup heater entirely in most winters. Monitor overnight lows for several weeks during the coldest period before committing to heater-free operation — one winter with unusual cold can teach you whether your thermal mass is sufficient or whether a small backup heater belongs in your emergency preparedness kit.
Frequently Asked Questions
Can water barrels replace a heater?
In mild climates (zones 7+), possibly — for frost-free minimums only. In cold climates, no. Water barrels buffer 10 to 15°F of overnight temperature drop. Any additional heating requires an active heat source.
How many water barrels do I need?
2 to 3 gallons of water per square foot of greenhouse floor area. A 200 sq ft greenhouse needs 400 to 600 gallons — roughly 8 to 11 standard 55-gallon drums.
Is a propane or electric heater better?
Propane heaters are cheaper to run at scale but produce moisture and combustion byproducts. Electric heaters are cleaner and easier to control but cost more per BTU. For small greenhouses, electric is simpler; for larger ones, propane is more economical.
Do water barrels work in cloudy climates?
Less effectively. Barrels charge from solar heat — cloudy days produce less solar gain and therefore less stored heat. In consistently cloudy climates, supplemental heating is more important than thermal mass.