This estimator uses a simplified sensible heat balance to approximate the hourly HVAC load for a greenhouse envelope.
- Envelope UA ≈ U × (FloorArea × ShapeFactor)
- Conduction load = UA × ΔT
- Infiltration load ≈ 0.018 × ACH × Volume × ΔT (BTU/hr)
- Solar gain = W/ft² × FloorArea × 3.412 × (1 − Shade)
- Internal gains = Watts × 3.412
- Net load (heating) = Conduction + Infiltration − Solar − Internal
- Net load (cooling) = Conduction + Infiltration + Solar + Internal
- Runtime fraction = NetLoad ÷ Capacity (clamped for reporting)
- Runtime per day = RuntimeFraction × ControlHours
- Pick Heating or Cooling based on your concern.
- Enter greenhouse area, height, and your temperatures.
- Choose shape and insulation to estimate heat transfer.
- Set ACH to represent vents, leaks, or frequent door use.
- Add solar gain, shade, and internal watts for lights and fans.
- Enter HVAC capacity and the efficiency metric from the nameplate.
- Click Estimate Runtime to see runtime, energy, and cost.
- Download CSV or PDF to save results for your grow logs.
| Scenario | Area | Height | Mode | Outside | Target | ACH | Capacity | Efficiency | Estimated runtime/day |
|---|---|---|---|---|---|---|---|---|---|
| Cold night | 240 ft² | 10 ft | Heating | 35 °F | 65 °F | 1.5 | 12,000 BTU/hr | COP 3.0 | ~8–16 hours |
| Sunny noon | 240 ft² | 10 ft | Cooling | 92 °F | 78 °F | 2.0 | 18,000 BTU/hr | EER 10 | ~4–10 hours |
| Lights on | 120 ft² | 8 ft | Cooling | 82 °F | 75 °F | 1.0 | 12,000 BTU/hr | SEER 18 | ~2–6 hours |
Greenhouse load drivers and runtime sensitivity
Runtime rises fastest when the temperature difference widens and air leakage increases. The calculator estimates conduction through glazing and structure, then adds infiltration using air changes per hour. For a 240 ft² house at 10 ft height, moving from 1.0 to 2.0 ACH can add thousands of BTU/hr during cold snaps. A 20°F change on a windy evening can double runtime compared with a calm night, so capture worst-case inputs rather than seasonal averages.
Insulation choices and practical U-values
Cover materials change the overall U-value used in the envelope term. Single-layer film behaves like a high U surface, while twinwall panels reduce heat flow and smooth temperature swings. Better insulation lowers the net load, shortens run minutes, and often stabilizes root-zone temperatures for propagation benches.
Solar gain, shade cloth, and crop protection
Solar input is entered as watts per square foot and converted to BTU/hr. Shade factor reduces that gain linearly, supporting safer mid-day leaf temperatures. Use nighttime values near zero, then test sunny-hour values such as 10–30 W/ft² to see how quickly cooling runtime can climb in spring.
Capacity, efficiency, and operating cost signals
Capacity is compared against the calculated load to estimate a duty cycle. When load exceeds capacity, the calculator flags that setpoint may not hold and assumes full scheduled runtime. Efficiency inputs convert rated capacity into estimated electrical kW, producing daily kWh and cost based on your utility rate. For heat pumps, a higher COP reduces kW draw for the same BTU output, while older resistance heaters operate near COP 1 and raise costs sharply in cold weather.
Using results for irrigation and plant health planning
Long heating cycles usually dry the air, increasing transpiration and watering frequency, especially for seedlings under fans. Extended cooling can increase condensation risk on leaves and glazing. Use the load breakdown to decide whether sealing leaks, adding thermal curtains, or adjusting shade offers the biggest runtime reduction.
FAQs
1) What does “runtime minutes per hour” mean?
It is the average duty cycle converted to minutes. For example, 30 minutes per hour indicates the system is expected to run about half of each hour during the selected control period.
2) Why can the status say the setpoint may not hold?
If the estimated net load is higher than the listed capacity, the equipment cannot meet demand continuously. The estimator assumes it will run the full scheduled time and still struggle to maintain temperature.
3) How should I choose an ACH value?
Start with 0.5–1.5 for a tight small house and 2–4 for leaky structures or frequent door use. If winds are strong or vents stay open, test higher values to see sensitivity.
4) What solar gain should I enter?
Use 0 for night estimates. For sunny hours, many growers test 10–30 W/ft² depending on glazing and season. Shade cloth reduces this input, so you can model different coverings quickly.
5) Which efficiency type should I pick?
Use COP or HSPF for heating equipment labels, and EER or SEER for cooling labels. The calculator converts that rating into approximate electrical input power for daily energy and cost estimates.
6) Is this suitable for precise engineering design?
No. It is a planning tool focused on sensible loads. Humidity, latent heat, wind effects, and equipment modulation can change results. Use it to compare scenarios and prioritize upgrades.