"I bought a 50 kW heat generator, that will definitely be enough." It is, but the heat is uneven: 82°F (28°C) next to the generator, 48°F (9°C) in the far corner. Plants in different zones grow at different speeds and fall ill in different ways. The problem is distribution, and more power will not fix it. A greenhouse where heat pools at one point holds several microclimates, and none of them is your target.
Quick glossary: Heat loss is the amount of heat that leaves the greenhouse through the cladding and ventilation gaps per unit of time; it is measured in watts or kilojoules per hour and sets the minimum capacity of the heating system. The heat transfer medium is the substance that carries heat from the source to the growing area: air (convective heating), water (hot-water heating), or soil and substrate (under-bench or root-zone heating). Temperature uniformity is the spread of temperature between different points of the growing area; the target for good heating is no more than ±4°F (±2°C) across the whole floor.
Calculating Heat Loss: Where Every Decision Starts
Without a heat loss calculation, choosing capacity is guesswork. Heat loss sets the minimum capacity that holds the target temperature at the design outdoor minimum.
Formula: Q = U × A × ΔT
Where:
- Q is heat loss (W)
- U is the thermal transmittance of the cladding (W/m²·K): glass 5.8, twin-wall polycarbonate 3.5, triple-wall polycarbonate 2.5, double-layer film 4.0
- A is the area of the cladding, walls and roof included (m²)
- ΔT is the difference between the target indoor temperature and the design outdoor temperature (°C)
Example: a greenhouse of 6×12 m, 3 m high, clad in twin-wall polycarbonate. The cladding area is about 200 m². With -4°F (-20°C) outside and 64°F (18°C) inside, ΔT is 68°F (38°C). Q = 3.5 × 200 × 38 = 26,600 W, or 26.6 kW.
Add 10-15% for infiltration through gaps and leaky joints, and the real requirement comes to 29-31 kW. A 50 kW generator will cope, but it will run in short on/off bursts with sharp temperature swings, as the mistakes section below explains. A 25 kW unit will fall short at -4°F (-20°C). Capacity is chosen for the design minimum temperature, and an average winter is the wrong reference.
Types of Heating Systems: What to Use and When
Air (convective) heating covers heat generators, heat cannons and heat exchangers that warm the air. It is the fastest to install and the cheapest upfront, and its main drawback is that warm air rises, leaving the roof warmer than the plant zone.
Without forced circulation you get an uneven spread of 5-9°F (3-5°C) by height, and uneven temperature across the floor when the heaters are grouped in one place.
To spread the heat evenly with air heating, use at least two heaters at opposite ends of the greenhouse, or heaters with a fan that directs the flow horizontally at plant level. Circulation fans help even out the temperature whatever the type of heating.
Hot-water heating (pipes carrying hot water) gives the most even heat distribution when it is designed properly. The pipes run along the perimeter, where heat loss through the walls is greatest, and/or between plant rows at the level of the growing zone. It costs more to install, but gives precise control and minimal difference between zones. This is the standard for commercial greenhouses.
Root-zone (substrate) heating uses pipes in the soil or substrate to warm the root zone. With the roots at 64-72°F (18-22°C) and the air kept cooler, the plant stays comfortable and less energy goes into heating air. It works especially well for crops whose root system is the limiting factor in winter.
Infrared heaters warm surfaces and plants directly without heating the air. They suit heavily ventilated greenhouses, where heating the air makes little sense. With only a few units the distribution is uneven, because each heater creates a "hot zone" beneath itself.
Where to Place the Heating Elements
Heat loss is greatest where the cladding area is largest relative to volume, which means the gable ends and the perimeter. The logic of placement is to offset heat loss at the point where it happens.
Pipes or heaters along the perimeter at 20-40 cm above the floor cover the loss through the side walls and warm the air from below at the same time, so the heat rises through the plant zone. A heater at one gable end blowing along the length of the greenhouse spreads heat well from end to end, provided the fan has the right capacity.
A heater in the middle of the greenhouse works well for the center and badly for the perimeter, where heat loss is highest.
Three Mistakes That Cost the Most
Buying capacity "with plenty to spare" without calculating heat loss. "50 kW will definitely be enough" often means overheating most of the time and an uneven microclimate: a powerful heater that switches on only now and then produces sharp temperature swings. Calculate the heat loss and choose 110-120% of that figure, which is a different thing from simply "buying bigger".
Not measuring temperature at several points and assuming "it is all even". One sensor next to the heater shows 72°F (22°C) while the far corner sits at 50°F (10°C). Use at least three measuring points: near the heater, in the center, and at the end farthest from the heater. Different temperature and humidity at different points of the greenhouse mean different microclimates and different product quality.
Ignoring airtightness while adding heating capacity. If the greenhouse is draughty through loose joints and open gaps, any amount of heating will end up heating the street. Sealing leaks is the first step before adding capacity. To check, run your hand along the joints and vent frames on a frosty day with the heating on: the draft is felt at once. For the full cold-season picture, see winter growing in Ukraine.
How to Tell the Heating System Is Set Up Right
The temperature difference between any two points of the growing area stays within ±4°F (±2°C) while the heating runs steadily. The system holds the target temperature at the design outdoor minimum without being overloaded. Plants across the whole area grow evenly and look equally marketable, with no "cold corners" where growth lags behind.
If you want to go deeper: The Greenhouse as a System: Structure, Cladding, and Microclimate explains how the choice of cladding and the design of the structure directly determine heat loss, and with it the heating capacity you need and the cost of holding temperature through the winter.