Dial in heater power for tender winter crops. See coverage limits before buying new units. Adjust insulation and airflow, then save reports instantly here.
| Space | Area (ft2) | dT (F) | Heater (BTU/hr) | Assumptions | Estimated coverage (ft2) |
|---|---|---|---|---|---|
| Greenhouse | 240 | 35 | 30,000 | Average insulation, medium leakage, average exposure | ~230 |
| Hoop house | 384 | 25 | 40,000 | Average insulation, medium leakage, windy exposure | ~520 |
| Shed | 200 | 30 | 20,000 | Good insulation, low leakage, sheltered | ~260 |
| Grow room | 120 | 20 | 5 kW | Average insulation, low leakage, average exposure | ~530 |
This estimator uses practical heating load factors for garden structures.
Space heating demand rises with floor area, ceiling height, and temperature difference. A 30°F (17°C) delta is a common planning baseline. Taller spaces stratify heat, so circulation fans help reduce wasted output. Greenhouses lose energy through glazing and frames, so identical floor areas can require very different heater capacities depending on materials and sealing quality.
This calculator starts from practical BTU/hr·ft² intensities at a 30°F delta and then scales to your delta. Good insulation uses lower intensities, while thin covers and gaps use higher ones. The space-type factor further adjusts for typical surface losses. Use the result as a planning number, not a replacement for engineered heat-loss calculations.
Infiltration often dominates at night when doors open and wind gusts hit. Medium leakage can add roughly 5% to demand, while high leakage can add about 20% in this model. Windy exposure increases convective losses and can drop effective coverage even if the heater rating is unchanged. Sealing vents and adding vestibules improves reliability.
Nameplate input is not delivered heat. Fuel heaters depend on combustion efficiency, and ducted systems lose some heat before it reaches plants. Electric resistance is near 100% at the unit, but distribution still matters. Place heaters to avoid hot spots, keep sensors away from direct blast, and aim airflow along benches for uniform canopy temperatures.
A safety margin helps when outside temperatures dip below forecasts or when irrigation adds latent cooling. Ten to twenty percent is typical for drafty greenhouses. If you need fast warm-up after venting, a larger margin reduces recovery time. Pair adequate capacity with thermal curtains or water barrels to smooth temperature swings and reduce cycling. Track runtime and fuel use weekly; if the heater runs continuously and still falls short, increase capacity or reduce losses now.
Coverage is the floor area a heater can support under your selected delta temperature, insulation, leakage, exposure, and height factors. It assumes continuous heat delivery at the effective output, not short bursts.
Enter the rating you know, then set efficiency to reflect delivered heat. For electric resistance units, use about 100%. For combustion heaters, use the manufacturer efficiency or a conservative 80–90% range.
Wind strips heat from surfaces and increases infiltration through gaps. The exposure factor models that extra loss, reducing coverage even when the heater rating stays the same.
Improve sealing, add double layers or thermal curtains, repair door gaps, and use circulation fans to cut stratification. Windbreaks and ground insulation also reduce losses and improve stability overnight.
Bench systems heat plants directly and can reduce air temperature needs. Use this calculator for space-air heating. For mats, size by wattage per tray area and target media temperature rather than room delta.
Follow clearance rules, provide ventilation for fuel heaters, and use grounded circuits for electric units. Add carbon monoxide detection where applicable, keep cords off wet floors, and never block airflow around heaters.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.