Keep seedlings warm when nights drop below freezing. Plan heater capacity before buying or equipment. Dial in settings for stable growth, then save money.
| Scenario | Size | Inside / Outside | Glazing | ACH | Efficiency | Suggested capacity |
|---|---|---|---|---|---|---|
| Small hobby greenhouse | 4m × 2m × 2.2m | 18°C / 2°C | 90% double glazing | 1.5 | 0.90 | ≈ 7,000–10,000 BTU/hr |
| Insulated grow room | 12ft × 10ft × 8ft | 70°F / 35°F | 10% glazing, insulated walls | 0.7 | 0.95 | ≈ 4–6 kW |
| Leaky tunnel house | 8m × 3m × 2.5m | 16°C / 0°C | 95% thin film | 3.0 | 0.85 | ≈ 25,000–35,000 BTU/hr |
The calculator models heat loss from transmission through surfaces and infiltration from air exchange. It then applies safety and efficiency.
The constants 0.33 and 0.018 approximate air heat capacity with unit conversions. ΔT uses your inside and outside temperatures.
Heater demand rises with surface area, poor insulation, and higher temperature difference. Glazing usually dominates because its U-value is higher than solid panels. Wind also matters because it increases infiltration through gaps, vents, and doors, raising ACH and pushing the required capacity upward during winter.
Use a conservative outside design temperature that matches your coldest typical nights, not the seasonal average. For seedlings, stable minimums reduce stress, so pick an inside setpoint that reflects the crop, humidity control, and condensation risk. A small change in ΔT can shift heater size noticeably. If you heat only at night, keep daytime gains out of the setpoint.
If you know the cover material, start with a preset U-value and adjust only if you have measured performance. Enter glazing percentage for walls plus roof so the model splits heat loss between glazing and insulated surfaces. Better covers reduce transmission, so you can often downsize while maintaining the same setpoint. Add interior curtains to reduce effective glazing losses.
ACH captures leakage plus planned ventilation. Tight greenhouses may run near 0.5–1.0 ACH, while tunnel houses can exceed 3.0 in wind. If doors open often, increase ACH or apply a larger safety factor. Consider a vestibule, weather seals, and controlled intake to reduce losses. Fans that mix air can reduce cold pockets and improve thermostat accuracy.
The recommended output already includes safety margin, while the adjusted nameplate accounts for heater efficiency. If you use electric resistance heat, efficiency is close to 1.0, so nameplate and output match closely. For combustion heaters, verify rated output, ventilation needs, and safe clearance around plants. Distribute heat with circulation fans, and consider zoning so you heat propagation benches instead of the full volume. Thermal mass, like water barrels, can reduce cycling and lower peak demand.
Use the minimum temperature your crop needs overnight. Many seedlings prefer steady warmth, but humidity and disease risk matter too. Pick a setpoint you can maintain without heavy condensation.
Start with 1.0 ACH for an average greenhouse, 0.5 for tight rooms, and 2–3 for drafty tunnel houses. Increase the value if wind exposure, vents, or frequent door openings are common.
Presets are reliable for planning when materials are typical. Use custom values only if you have manufacturer data or tested performance. Incorrect U-values can overstate savings or oversize equipment.
Glazing usually transfers heat faster than insulated panels. A larger glazed share increases transmission losses, especially at high ΔT. Improving covers or adding inner curtains can reduce this penalty.
Common values are 1.10 to 1.25 to cover wind spikes, door use, and uneven heat distribution. If your structure is exposed or you cannot tolerate drops, choose the higher end.
It is a planning estimate for electric heating using your duty cycle, hours, and rate. Real use varies with weather and controls. Compare scenarios to see which upgrades reduce ongoing costs.
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.