A 50 kW heat generator creates sharp temperature swings — from +28°C near the source to +9°C in the opposite corner — resulting in uneven plant growth across zones.
Quick Glossary
- Heat loss — the amount of heat leaving the greenhouse through the cladding and ventilation gaps per unit of time; measured in watts or kilojoules per hour
- Heat transfer medium — the substance through which heat is transferred: air, water, or substrate
- Temperature uniformity — the goal of quality heating: no more than ±2°C across the entire growing area
Calculating Heat Loss
Formula: Q = U × A × ΔT
Where Q = heat loss (W); U = thermal transmittance of the cladding; A = cladding surface area (m²); ΔT = temperature difference.
U-values for common materials:
- Glass: 5.8 W/m²·K
- Twin-wall polycarbonate: 3.5
- Triple-wall polycarbonate: 2.5
- Double-layer film: 4.0
Example calculation: greenhouse 6×12 m, height 3 m, twin-wall polycarbonate. Cladding area ≈ 200 m². At outdoor temperature −20°C and indoor +18°C: Q = 3.5 × 200 × 38 = 26.6 kW. Adding 10–15% for infiltration — actual requirement 30–31 kW.
Heating System Types
Air (convective) heating. Fastest to install, lowest upfront cost. Main drawback: heated air rises, creating temperature differentials of 3–5°C. Solution: place at least two heaters at opposite ends, or use circulation fans.
Hot water heating. The most uniform heat distribution. Pipes are placed along the perimeter (where heat loss is greatest) and between plant rows at canopy level. Higher upfront cost but precise temperature control. Standard for commercial greenhouses.
Sub-floor (substrate) heating. Pipes in the ground warm the root zone to 18–22°C, allowing air temperature to be kept lower. Effective for crops with root temperature sensitivity in winter.
Infrared heaters. Heat surfaces and plants directly. Efficient in greenhouses with high ventilation, but create uneven distribution when too few units are installed.
Placement of Heating Elements
The logic: compensate for heat loss at the point where it occurs. Pipes or heaters along the perimeter at 20–40 cm above floor level offset heat loss through the side walls. Central placement works well for the centre but poorly for the perimeter where losses are greatest.
Three Mistakes That Cost the Most
Choosing capacity without calculating heat loss. Often leads to overheating and uneven microclimate. Correct approach: calculate heat losses and specify 110–120% of that figure.
Not measuring temperature at multiple points. Minimum three measurement points: near the heater, in the centre, and at the far end. A single sensor gives a misleading picture of the microclimate.
Ignoring airtightness. Loose joints and gaps cause significant heat loss. Sealing is the first step before increasing heating capacity.
How to Know the Setup Is Correct
- Temperature difference between any two points is no more than ±2°C during stable operation
- The system maintains the target temperature at the design outdoor minimum without being overloaded
- Plants across the entire growing area develop uniformly with consistent marketable appearance and no cold corners