| Scenario | Structure | ΔT | ACH | U-value | Estimated capacity |
|---|---|---|---|---|---|
| Cool night protection | 10 m × 4 m × 2.5 m tunnel | 17 °C | 1.0 | 3.5 W/m²·K | ~6–9 kW (with margin) |
| Sunny winter day | Same tunnel, moderate sun credit | 12 °C | 0.7 | 3.5 W/m²·K | Often 20–40% lower heating need |
| Summer cooling check | Ventilated structure with shade | 8 °C | 8.0 | 6.0 W/m²·K | Fan capacity dominates total load |
- Qcond = U × Aenv × ΔT
- Qvent ≈ 0.33 × ACH × V × ΔT
- Qsolar = Afloor × S × fsun × (1 − shade)
- Qbase = Qcond + Qvent ± Qsolar (add solar for cooling, subtract for heating)
- Qfinal = Qbase × (1 + safety%)
The ventilation constant (0.33) approximates air density and heat capacity per hour. The model focuses on sensible heat for temperature control in growing spaces.
- Select units and choose heating or cooling mode.
- Enter length, width, and height for volume accuracy.
- Use a cover preset, or enter a verified custom U-value.
- Set inside and outside temperatures for your target period.
- Estimate ACH based on sealing, vents, and wind exposure.
- Set solar input, sun factor, and shading to match conditions.
- Apply a safety factor, then calculate and download reports.
Why heat load matters for protected cropping
Heating and cooling capacity influences germination rates, flower set, pest pressure, and irrigation timing. This calculator estimates sensible load for tunnels, hoop houses, and greenhouses by combining conduction, ventilation, and solar effects. Use the result to choose heaters, fans, or evaporative systems with a realistic margin for windy nights and door openings. In most gardens, a 10–25% safety factor keeps performance stable during abrupt weather shifts.
Key inputs that typically change the answer most
Surface area and cover quality drive conduction: a higher U-value and a larger envelope area increase losses in winter and gains in summer. Air changes per hour drives ventilation load; doubling ACH nearly doubles the ventilation component. Solar input, sun factor, and shading adjust daytime balance, often lowering heating needs but raising cooling demand.
How to read the breakdown and final capacity
Conduction load represents heat transfer through glazing and structure, while ventilation load represents heat carried by exchanged air. The solar component is subtracted in heating mode and added in cooling mode. The base load is the combined requirement before adding the safety factor. The final recommended capacity is the equipment target in watts, kilowatts, and BTU per hour.
Practical calibration using site observations
If overnight temperatures fall faster than predicted, increase ACH to reflect leaks, vent cracks, or frequent access. If daytime peaks are lower than expected, increase shading or reduce the sun factor to reflect haze, dust, or low-angle winter sun. For extremely humid houses, remember that latent loads are not included; add extra cooling capacity when dehumidification is required.
Seasonal planning and recordkeeping
Save multiple runs for different months, crops, and setpoints, then compare capacities against your installed equipment. Track changes after repairs, new films, or added screens by re-running the same scenario. Consistent inputs make results comparable, helping you justify upgrades and reduce fuel and electricity use across the season. Store your assumptions in a logbook so next season’s tuning takes minutes, not hours.
1) What should I use for envelope area?
Use the structure’s heat‑exchanging surface: walls plus roof. If you do not know it, use the envelope multiplier option and start around 1.8 for tunnels, then refine using drawings or measured panels.
2) How do I estimate ACH for my setup?
Start with 0.5–1.5 for well‑sealed houses, 2–6 for leaky covers, and 6+ for active venting. Wind exposure and frequent door use raise ACH. If your temperature drops quickly at night, increase ACH.
3) Which cover preset is most appropriate?
Pick the preset that matches your outer layer and air gap. Double film or twinwall materials usually reduce U-value versus single film. If you have a manufacturer datasheet, select custom and enter the rated value.
4) Why does solar decrease heating but increase cooling?
In heating mode, solar gains provide free heat, so it is treated as a credit. In cooling mode, that same sun adds heat that must be removed. Shading lowers the solar term for both modes.
5) What safety factor should I choose?
Use 10–15% for steady conditions and well‑sealed structures. Use 20–25% for windy sites, frequent access, or uncertain inputs. Larger factors can oversize equipment and increase cycling, so keep it practical.
6) Is this calculator suitable for open gardens?
It is best for enclosed or semi‑enclosed spaces where insulation and airflow can be estimated. For open beds, wind and radiant exchange dominate and the model can under‑predict variability. Use it only for covered frames or row tunnels.