Calculator Inputs
Example Data Table
| Scenario | Size | Inside / Outside | Insulation | ACH | Suggested Output |
|---|---|---|---|---|---|
| Hobby greenhouse | 12×10×8 ft | 65°F / 35°F | Double-layer | 1.0 | ~18,000–25,000 BTU/hr |
| Seedling grow room | 4×3×2.4 m | 22°C / 5°C | Well insulated | 0.5 | ~2.0–3.0 kW |
| Shed for tender plants | 10×10×8 ft | 55°F / 20°F | Light insulation | 2.0 | ~25,000–35,000 BTU/hr |
Formula Used
This estimates heat escaping through walls and roof.
- Qtrans = U × Aexposed × ΔT × Shape
- U depends on insulation/glazing selection.
- Aexposed reduces for floor-on-ground contact.
This covers warm air replaced by cold outside air.
- Imperial: Qvent = 0.018 × ACH × V × ΔT
- Metric: Qvent = 0.33 × ACH × V × ΔT
- Total: Q = (Qtrans + Qvent) × (1 + Safety%)
- Fuel input: Input = Output ÷ Efficiency
How to Use This Calculator
- Choose your unit system and enter length, width, and height.
- Set the target inside temperature for your plants.
- Use a realistic cold-night outside temperature for your location.
- Select the insulation/glazing level that best matches your structure.
- Enter ACH based on drafts, vents, and wind exposure.
- Set a safety margin if you expect frequent door openings.
- Pick a fuel type and enter your local energy price.
- Press calculate, then export the results as CSV or PDF.
Heating Load Inputs That Matter
Heater sizing starts with the structure’s volume and exposed surface area. This calculator uses length, width, and height to estimate total envelope area, then adjusts for floor contact to reduce exposed losses. The temperature difference (inside minus outside) drives the load, so a 30°F (17°C) gap typically needs far more capacity than a mild 10°F (6°C) gap.
Insulation And Glazing Impact
Transmission loss is modeled with overall U-value selections that represent common garden builds. Planning U-values range from about 1.20 BTU/(hr·ft²·°F) for single-layer glazing to about 0.20 for tight, well-insulated structures. In metric terms, that’s roughly 6.8 down to 1.1 W/(m²·K). Better insulation reduces required output and improves temperature stability for tender crops.
Air Exchange And Vent Strategy
Drafts and venting can dominate winter heat loss. The tool accepts Air Changes per Hour (ACH) from 0 to 25. Typical tight rooms may run 0.3–1.0 ACH, while leaky greenhouses can reach 2–6 ACH on windy nights. Because ventilation loss scales with volume and ΔT, sealing gaps, using double doors, and managing night vents can cut demand quickly.
Selecting Heater Type And Efficiency
The recommended “heater output” covers heat delivered to the space, then the calculator estimates “heater input” by dividing by efficiency. For electric resistance, efficiency is effectively near 100%. Combustion heaters vary; 80–95% is common depending on venting and design. Use a larger safety margin when you expect frequent door openings, wet floors, or intermittent fan mixing.
Operating Cost Planning
To help budgeting, the calculator converts required input to fuel rates: kWh for electric, therms for natural gas, and gallons for propane or diesel/kerosene. Enter your local energy price to estimate cost per hour at steady-state conditions. For overnight planning, multiply by runtime hours and remember that real use cycles with thermostat control, solar gain, and changing outdoor temperatures.
FAQs
1) What outside temperature should I use?
Use a realistic cold-night design value for your garden site. Many growers choose the typical seasonal low, or a conservative percentile low, so plants stay protected during brief cold snaps.
2) How do I estimate ACH for my greenhouse?
Start with 1.0 ACH for average builds. Use 0.3–0.7 for tight, sealed rooms, and 2–6 for drafty structures in wind. If you run exhaust fans, consider higher values during operation.
3) Should I include solar gain?
This tool sizes for cold conditions when solar gain is minimal. If you get strong winter sun, actual heater runtime may drop, but sizing should still cover cloudy nights and early mornings.
4) Why add a safety margin?
A margin helps handle wind gusts, door openings, uneven insulation, and wet surfaces that increase heat loss. For stable, tight rooms 10–15% can work; for busy spaces 20–30% is often safer.
5) What if my heater is oversized?
Oversizing can cause short cycling and uneven humidity. Use staged heating, a thermostat with wider differential, or a fan to mix air. Slight oversize is usually acceptable if controls are good.
6) Does heater placement matter?
Yes. Place heaters to avoid cold corners and keep airflow moving across plants. Use circulation fans to prevent stratification, and keep combustion heaters properly vented to manage moisture and safety.