Formula used
Air-infiltration heat loss: Q = 0.33 × ACH × V × ΔT
Recommended output: Net load × safety factor ÷ distribution efficiency
U measures heat transfer through each greenhouse surface. Area and temperature difference determine conductive demand. Air leakage and allowances are then added separately.
Internal gains reduce the calculated design heating requirement. Heater efficiency changes fuel input and operating cost. Safety allowances protect crops during severe weather events.
How to use this calculator
Choose the greenhouse shape and enter its dimensions. Add manual areas when drawings provide better measurements. Select coverings matching the actual installed materials.
Enter indoor targets and the coldest outdoor temperature. Review air leakage, wind, insulation, and heater settings. Submit the form and examine every warning carefully.
Compare an alternative covering or heating system. Enter upgrade cost for simple payback estimates. Confirm final equipment sizing with a qualified professional.
Example greenhouse data
| Input | Example value | Purpose |
|---|---|---|
| Shape | Rectangular | Calculates envelope area and air volume. |
| Dimensions | 12 m × 6 m × 3.5 m | Defines the greenhouse geometry. |
| Covering | Double polyethylene | Sets the heat-transfer coefficient. |
| Indoor and outdoor temperatures | 18°C and -5°C | Creates a 23°C temperature difference. |
| Air changes | 1.0 hourly | Represents average construction leakage. |
| Heater | Natural gas, 90% efficient | Estimates fuel demand and cost. |
Frequently asked questions
What is a greenhouse U-value?
A U-value measures heat passing through one square metre. Lower values indicate stronger insulation and slower heat loss. Use manufacturer data whenever reliable values are available.
Which outdoor temperature should I enter?
Use the local winter design temperature for equipment sizing. It should represent severe weather, not the seasonal average. Local engineering guidance provides dependable design temperatures.
How do air changes affect heating?
Air changes replace warm air with colder outdoor air. Leaky structures therefore need much larger heaters. Seal gaps before purchasing additional heating equipment.
Does double polyethylene reduce heat loss?
Double inflated film usually lowers heat transfer substantially. Correct inflation also limits condensation and wind movement. Actual performance depends on installation and maintenance.
Should heaters be oversized?
A modest safety factor supports recovery during severe conditions. Excessive oversizing causes cycling and uneven temperatures. Most projects need careful professional equipment selection.
How are heat pumps calculated?
Heat pumps divide delivered heat by their operating COP. Higher COP values reduce electricity use and cost. Cold weather may reduce the published COP significantly.
Can thermal curtains reduce heater size?
Thermal curtains reduce exposed covering losses during closed periods. Their benefit depends on coverage, insulation, and operating hours. Include reliable product performance data for sizing.
Why include internal heat gains?
Lights, equipment, people, and stored solar energy add heat. These gains can reduce the steady heating requirement. Avoid overstating temporary gains during critical nights.
Is ground heat loss important?
Ground losses vary with insulation, soil, and perimeter exposure. They matter more in small or poorly insulated structures. Perimeter insulation can improve winter efficiency.
Can this replace professional heater design?
This calculator provides a detailed planning estimate only. Large systems need combustion, electrical, and ventilation checks. Final designs should follow local safety requirements.