The cost to heat a greenhouse is one of the highest operational expenses, often second only to labor, making it a major factor in profitability. Heating expenses are highly variable, changing dramatically based on the specific structure, local climate, and fuel prices. Calculating the cost depends on the complex interplay between the heat demand of the structure and the price of the energy source used. Understanding the factors that determine heat loss and the cost of various fuels is the first step in managing this expense.
Physical and Environmental Variables
The physical characteristics of a greenhouse determine the heat energy required to maintain a desired temperature. The total surface area of the structure—including the roof, walls, and ends—is directly proportional to the amount of heat lost. A larger surface area increases the total heat demand. The difference between the desired indoor temperature and the average outdoor temperature, known as the Delta T (\(\Delta T\)), is a primary driver of heat loss.
The glazing material covering the structure plays a substantial role in determining heat loss. Glazing materials are rated by their U-factor, which measures how easily heat transfers through them; a lower U-factor indicates better insulation. Materials like single-pane glass or single-layer polyethylene film have higher U-factors and lose heat faster than double-layer inflated polyethylene or twin-wall polycarbonate sheets. The specific temperature setpoint chosen for the crops also impacts the cost, as a small increase of just one degree can lead to a measurable rise in variable costs.
Understanding Fuel Types and System Efficiency
The financial cost of meeting a greenhouse’s heat demand depends on the fuel source and the efficiency of the heating equipment. Fuel costs are best compared using the British Thermal Unit (BTU), the standard measure of energy content. Natural gas is often the most economical fuel where pipeline access is available, and it burns cleanly, requiring less equipment maintenance.
Propane (LPG) is a clean-burning gaseous fuel, similar to natural gas, but it is typically more expensive and requires on-site storage tanks. Fuel oil is another common option, though its price can fluctuate widely and it also necessitates on-site storage. Electricity is usually the highest-cost option per BTU, making it impractical for large-scale heating.
The efficiency rating of the heating system, often expressed as an Annual Fuel Utilization Efficiency (AFUE) percentage, determines the amount of usable heat produced per unit of fuel. Older heaters may operate around 80% efficiency, meaning 20% of the fuel’s energy is wasted, while modern condensing heaters can achieve efficiencies of 90% or higher. Selecting a high-efficiency heater directly lowers the cost of the energy required to maintain the indoor temperature.
Estimating Heat Loss and Monthly Costs
Calculating the peak heat requirement is the first step toward estimating monthly heating costs. Total heat loss, measured in BTUs per hour (BTU/hr), is composed of transmission loss through the glazing and infiltration loss through cracks and gaps. Transmission loss is calculated by a simplified formula: Heat Loss (Q) equals the product of the glazing’s U-factor, the total exposed Surface Area (A), and the Temperature Difference (\(\Delta T\)).
The formula used is \(Q = U \times A \times \Delta T\), where \(Q\) is the heat loss in BTU/hr. For example, a 400 square foot greenhouse with a single-layer polyethylene cover (U-factor around 1.2 BTU/hr/sq ft/°F) maintaining a 50°F difference (\(\Delta T = 50°F\)) would lose approximately 24,000 BTU/hr (\(1.2 \times 400 \times 50\)). This value represents the maximum heating capacity needed by the furnace under worst-case conditions.
To estimate monthly costs, this peak heat loss must be multiplied by the number of hours the heater runs and the fuel cost per BTU, adjusted for the heater’s efficiency. Significant heat loss can occur due to air infiltration, where cold air leaks in through unsealed doors, vents, and joints. Infiltration losses can account for a considerable portion of the total heat loss, highlighting the need for an airtight structure.
Strategies for Minimizing Heating Expenses
Implementing energy-saving measures involves reducing the structure’s heat demand and optimizing the heating system’s performance. The most effective strategy for an existing structure is to minimize uncontrolled air leaks by sealing gaps around doors, fans, and vents with weatherstripping or caulk. Even a small crack can result in heat escaping the building.
Insulating non-glazing parts, such as the foundation and kneewalls, reduces heat transfer to the cold ground. Installing foam board insulation around the perimeter, extending below ground level, can reduce heat loss from the soil inside the greenhouse. Inside the structure, thermal blankets or curtains—retractable layers of insulation—can reduce night-time heat loss by 20% to 70% when closed.
Heating costs can also be managed by optimizing temperature setpoints, such as lowering the thermostat at night when plants require less heat. Installing circulation fans, often called heat-saver fans, can push warm air that rises to the ceiling back down to the plant level, reducing stratification and improving heating efficiency by as much as 20%. Utilizing thermal mass, such as large water barrels or rock beds, helps absorb heat during the day and slowly release it at night, moderating temperature swings.