Example Data Table
| Scenario | Size | Covering | Inside | Outside | ACH | Efficiency | Estimated useful load | Estimated input |
|---|---|---|---|---|---|---|---|---|
| Seedling greenhouse | 50 ft × 20 ft × 8 ft | Double polyethylene | 65°F | 15°F | 1.0 | 80% | 118,526 BTU/hr | 148,157 BTU/hr |
| Small hobby house | 24 ft × 12 ft × 7 ft | Twin-wall polycarbonate | 60°F | 25°F | 0.8 | 90% | 25,000 BTU/hr | 27,778 BTU/hr |
| Leaky glass house | 40 ft × 18 ft × 9 ft | Single glass | 62°F | 10°F | 2.0 | 78% | 145,000 BTU/hr | 185,897 BTU/hr |
Formula Used
Temperature difference: ΔT = inside target temperature − outside design temperature.
Covering heat loss: Qcover = surface area × U value × ΔT.
Floor heat loss: Qfloor = floor area × floor loss factor × ΔT.
Air leakage heat loss: Qinfiltration = 1.08 × CFM × ΔT.
Airflow estimate: CFM = greenhouse volume × air changes per hour ÷ 60.
Adjusted load: Qbase = (Qcover + Qfloor + Qinfiltration) × exposure factor.
Useful load: Quseful = Qbase × (1 + safety margin ÷ 100).
Rated heater input: Qinput = Quseful ÷ heater efficiency.
How to Use This Calculator
- Select imperial or metric units.
- Choose the greenhouse shape that best matches your structure.
- Enter length, width, sidewall height, and roof rise.
- Add your inside crop target and outside design temperature.
- Select a covering type, or enter a custom U value.
- Enter air changes, exposure, heater efficiency, and margin.
- Press calculate, then review useful load and rated input.
- Download the result as a CSV or PDF file.
Greenhouse Heat Planning
A greenhouse loses heat through covering, floor edges, cracks, doors, and moving air. This calculator joins those paths into one BTU per hour estimate. It is useful for hobby houses, nursery bays, seed rooms, and small production tunnels. The goal is not guesswork. The goal is a heater size that protects crops during the coldest expected night.
Why BTU Heating Matters
BTU per hour shows how much heat must be supplied each hour. A small house with double poly may need modest heat. A tall glass house with leaks may need far more. The temperature difference drives most of the load. A larger gap between inside target and outside design temperature always raises the demand.
Inputs That Shape the Load
Dimensions create surface area and air volume. Covering type sets the U value. A lower U value means better insulation. Air changes per hour estimate leakage. Wind exposure adds a practical correction. Heater efficiency converts useful heat into required fuel input. A safety margin covers drafts, aging plastic, open doors, and weather surprises.
Reading the Result
The useful heating load is the heat the plants need. The rated input is the heater capacity before efficiency losses. Compare the rated input with equipment nameplate data. Electric heaters are often chosen by kilowatts. Gas and propane heaters are often sold by BTU input. The fuel use line estimates hourly consumption at full load.
Better Greenhouse Decisions
The result can guide more than heater shopping. Try a lower U value to test double layers. Reduce air changes to value weather stripping. Add thermal curtains to lower the night load. Change the safety margin for high value plants. These tests reveal where insulation, sealing, and heater upgrades save the most money.
For final design, use local weather records and equipment manuals. Check code rules for combustion air, venting, clearances, and thermostats. Keep spare heat available for sensitive crops.
Use the table to compare typical scenarios before changing real settings. One row can represent a mild night. Another can represent a record low. This makes planning clearer. It also helps explain fuel budgets to owners, growers, or maintenance staff before the season begins. Review numbers again after repairs, storms, or covering changes.
FAQs
What is a BTU heating load?
It is the heat needed each hour to hold the greenhouse at the target temperature. The calculator reports that need in BTU per hour.
Why is outside design temperature important?
Heating load depends on the gap between inside and outside temperature. A lower outside value creates a larger temperature difference and a bigger heater requirement.
What does U value mean?
U value measures heat flow through the covering. Lower values mean better insulation. Double layers usually lose less heat than single layers.
How do air changes affect the result?
Air changes estimate leakage and ventilation. Higher air changes pull more cold air inside. That raises the heating load quickly.
Should I include a safety margin?
Yes. A margin helps cover wind, small leaks, aging materials, open doors, and colder nights. Many growers use 10% to 25%.
Is rated input the same as useful heat?
No. Useful heat warms the greenhouse. Rated input includes heater efficiency losses. Lower efficiency requires more input fuel.
Can I use this for metric measurements?
Yes. Select metric units. The calculator converts dimensions and temperatures internally, then reports heat load in BTU per hour.
Does this replace a professional heating design?
No. It is a planning estimate. Use local weather data, manufacturer guidance, and code requirements before final equipment selection.