| Scenario | Power | Env | Insulation | Wind | Ambient → Desired | Estimated Radius | Coverage |
|---|---|---|---|---|---|---|---|
| Seedling bench (enclosed) | 2000 W | Greenhouse | Average | 1 m/s | 0°C → 10°C | ~2.3 m | ~16.6 m² |
| Open bed (breezy) | 3000 W | Open garden | Poor | 4 m/s | 2°C → 8°C | ~1.2 m | ~4.5 m² |
| Covered patio pots | 12,000 BTU/h | Covered patio | Average | 2 m/s | 5°C → 12°C | ~1.6 m | ~8.0 m² |
Examples are illustrative; adjust for your setup and plant sensitivity.
The calculator estimates radius by balancing usable heater power against the heat flux required to maintain your chosen temperature rise:
- Ubase reflects insulation and sealing quality.
- hwind increases with wind speed, reduced in enclosed spaces.
- Type/Reflector/Height adjust how much heat reaches the plant zone.
- Enter heater power and choose the correct unit.
- Set ambient and desired canopy temperatures for the coldest hour.
- Select your environment and insulation level to match reality.
- Add wind speed if the area is exposed or drafty.
- Use a 10–20% safety margin for real-world losses.
- Optionally enter a target area to estimate heater count.
- Press Calculate, then download CSV or PDF if needed.
Safety note: Always follow the heater manufacturer’s clearance and ventilation rules, especially in enclosed spaces.
Heat Output and Plant Zone Efficiency
Heater rating is only the starting point because losses occur before warmth reaches plants. Radiant and infrared units often deliver 10–20% more usable heat to foliage than fan heaters when aimed downward. Use efficiency near 90–95% for electric heaters and 70–85% for gas models. A hood can lift delivery by about 8–15%.
Temperature Rise Drives Required Heat Flux
Coverage shrinks as ΔT increases because required heat flux scales with temperature rise. The model uses a base loss term tied to sealing and insulation: good ≈ 4 W/m²·K, average ≈ 7, poor ≈ 10. A greenhouse holding ΔT = 10°C may need roughly 60–120 W/m², while an exposed bed can exceed 200 W/m² in drafts.
Wind, Drafts, and Real Coverage Losses
Wind speed is a common reason field coverage is smaller than indoor tests. In open areas, wind above 4 m/s can cut practical radius by 25–40%. Enclosures reduce wind impact, so the calculator dampens wind effects for greenhouses and cold frames. Add a 10–20% safety margin and use windbreaks or sealing to reduce losses.
Mounting Height, Reflectors, and Hotspot Control
Raising a heater spreads heat but reduces intensity at canopy level, especially above 1.5 m. Lower mounting concentrates heat and can create hotspots, so monitor canopy temperature and keep safe clearances. Reflectors help shape the heat cone and reduce wasted upward radiation.
Sizing Multiple Heaters for Long Beds
For beds longer than one radius, overlap coverage circles by 20–30% to avoid cold lanes. Use the optional target area field to estimate heater count and place units evenly. Example data: 2000 W in a greenhouse, wind 1 m/s, 0°C to 10°C gives ~2.3 m radius (~16.6 m²). An open bed with 3000 W, wind 4 m/s, 2°C to 8°C gives ~1.2 m radius (~4.5 m²).
1) Which heater type usually gives the largest radius?
Radiant and infrared units often perform best because more heat reaches the plant zone. Fan heaters can warm air quickly, but drafts and wind remove that heat faster in open beds.
2) Why does wind reduce the radius so much?
Moving air increases convective heat loss from leaves and soil. As wind rises, the heat required per square meter grows, so the same usable heater power covers a smaller circle.
3) What safety margin should I enter?
Start with 10% in a sealed greenhouse and 15–25% outdoors. Increase it when wind is variable, plants are sensitive, or you cannot position the heater close to the canopy.
4) Can I work in Fahrenheit?
Yes. Convert to Celsius before entering values: °C = (°F − 32) ÷ 1.8. The calculator relies on the temperature difference, so accurate ΔT is more important than the unit.
5) How do I heat a long row without cold gaps?
Use multiple heaters and overlap their coverage by 20–30%. Position units at equal spacing, then validate with a canopy thermometer at the row ends and in the middle.
6) Why do I get a near-zero radius?
This happens when desired temperature is not above ambient, or when required heat flux is very high due to large ΔT, poor insulation, and wind. Increase power, improve sealing, reduce wind, or lower the target ΔT.
7) What should I save in the CSV or PDF report?
Save the scenario inputs, the calculated radius and area, and any notes. Keeping records for different nights helps you refine insulation settings and safety margin for your specific garden layout.