Calculator
Formula used
The calculator estimates heating load (BTU/hr) using envelope conduction and air exchange.
- Q_conduction = (Σ(Uᵢ × Aᵢ) × ΔT) × weather_factor
- CFM = (Volume × ACH) ÷ 60
- Q_infiltration = 1.08 × CFM × ΔT
- Q_total = Q_conduction + Q_infiltration − gains
- Q_with_margin = Q_total × (1 + safety_margin)
- Heater input = Q_with_margin ÷ efficiency
How to use this calculator
- Pick a structure type that matches your garden space.
- Enter dimensions and your target inside temperature.
- Set a realistic outside temperature for your coldest nights.
- Adjust ACH to reflect leaks and any ventilation you operate.
- Add a safety margin if you want faster recovery.
- Press Calculate, then download CSV or PDF.
Example data
| Scenario | Size | Inside / Outside | ACH | Cover | Estimated BTU/hr (delivered) |
|---|---|---|---|---|---|
| Small double-cover greenhouse | 12×10×8 ft | 65°F / 30°F | 1.0 | Greenhouse (double cover) | ≈ 20,000–28,000 |
| Grow tent with moderate leaks | 4×4×7 ft | 75°F / 40°F | 2.5 | Grow tent (fabric) | ≈ 3,000–6,000 |
| Insulated shed grow room | 10×12×8 ft | 70°F / 25°F | 0.7 | Shed / room (basic walls) | ≈ 9,000–15,000 |
Key drivers of heating load
BTU per hour is mainly driven by temperature difference (ΔT), exposed surface area, and how quickly warm air is replaced. As a practical check, raising your target temperature by 10°F increases both conduction and infiltration loads by roughly that same 10°F step. This calculator combines conduction (U×A×ΔT) and ventilation loss (1.08×CFM×ΔT) to estimate demand, then applies weather and safety factors.
Typical ACH ranges for garden spaces
Air changes per hour (ACH) can be high in lightweight structures. A sealed grow room may run 0.3–0.8 ACH, a typical hobby greenhouse about 0.8–2.0 ACH, and leaky tents or active exhaust setups can exceed 3–6 ACH. Because infiltration scales with airflow, tightening doors and seams often cuts load more than small insulation upgrades.
Cover and insulation impact
Cover R-value changes heat loss linearly through U = 1/R. Moving from a single cover near R≈1.2 to a double cover near R≈1.8 reduces glazing conduction by roughly 33% at the same area and ΔT. Insulating opaque walls from about R5 to R10 halves opaque conduction, which matters most when glazing percentage is lower and walls dominate area.
Sizing margin and recovery planning
A 10–25% safety margin is common to handle cold snaps and to recover after humidity venting. If you need rapid recovery—such as returning from 55°F to 70°F—use a higher margin and reduce ACH where possible. For windy sites, set the weather factor near 1.2–1.3. Heater efficiency affects input rate; a 90% heater needs about 11% more input.
Reading cost outputs with confidence
Cost estimates use runtime hours and fuel conversions (1 kWh ≈ 3,412 BTU; 1 therm ≈ 100,000 BTU). For budgeting, test two cases: a typical night and a colder design night, then average them. If your thermostat cycles often, actual runtime may be lower than the assumption.
FAQs
1) What outside temperature should I enter?
Use a realistic cold-night design value for your area, not the daily average. If you only heat at night, pick a typical nighttime low and a colder “worst-case” low to compare.
2) How do I estimate ACH if I don’t know it?
Start with 1.0 ACH for many hobby greenhouses, 0.5 ACH for sealed rooms, and 2.0+ ACH for leaky tents. If you run fans or open vents, increase it accordingly.
3) Should I use delivered BTU/hr or heater input BTU/hr?
Use delivered BTU/hr to compare heat demand. Use heater input BTU/hr to match fuel consumption and to select equipment when efficiency is below 100%.
4) What does glazing percentage change?
It splits the envelope between “skin/glazing” and “opaque” surfaces. Higher glazing percentage increases dependence on the cover R-value, often raising BTU/hr in greenhouses.
5) Why is my cost estimate high?
Common causes are high ACH, a large ΔT, high glazing area, or long runtime assumptions. Lower leaks, add thermal curtains, reduce nighttime ventilation, or adjust runtime to actual thermostat cycling.
6) Can lights and equipment reduce heater size?
Yes. High-watt grow lights and pumps add internal heat. Enter an internal gain estimate to reduce BTU/hr demand, but be conservative because equipment may not run during the coldest hours.
Notes and limitations
- This is an estimating tool, not a substitute for professional design in high-stakes builds.
- Radiant loss to clear night skies and soil thermal storage are simplified.
- Solar gain is highly site-specific; use a conservative value if unsure.
- For very humid spaces, consider dehumidification loads separately.