Calculator inputs
Formula used
This calculator uses the standard convection model: Q = h × A × (Ts − Ta)
- Q is convection heat rate (W).
- h is the convection coefficient (W/m²·K).
- A is exposed area (m²).
- Ts is surface temperature, Ta is air temperature.
In estimate mode, a simple wind-based approximation is used: h ≈ 5.7 + 3.8v with v in m/s (clamped to a practical range).
How to use
- Measure the exposed surface area of the cover, tank, or bench.
- Enter the surface temperature and the surrounding air temperature.
- Enter wind speed near the surface (or inside the structure).
- Pick “Estimate from wind” for quick planning, or input h manually.
- If windbreaks exist, enable “Sheltered” and choose a factor.
- Press Calculate to see the convection rate and export files.
Example data table
| Scenario | Area (m²) | Surface (°C) | Air (°C) | Wind (m/s) | Mode | Estimated Q (W) |
|---|---|---|---|---|---|---|
| Small greenhouse panel | 8 | 16 | 4 | 2 | Estimate | ~1,032 |
| Water tank (insulation check) | 3 | 12 | 2 | 1 | Estimate | ~267 |
| Bench under row cover (sheltered) | 5 | 14 | 6 | 1.5 | Estimate + shelter | ~434 |
| Custom coefficient from spec sheet | 10 | 18 | 5 | — | Manual (h=9) | ~1,170 |
Examples are approximate and assume steady conditions. Real gardens also lose heat by radiation and evaporation.
Professional field notes
Convection as a planning load
Convection loss is the moving-air part of sensible heat transfer. In gardens, it dominates when wind strips away the warm boundary layer around row covers, greenhouse film, or exposed water surfaces. The calculator reports Q as watts, which can be treated as a continuous heating requirement under steady conditions. Use it to compare scenarios by changing wind, area, and temperature.
Heat-transfer coefficient ranges
The convection coefficient h changes with wind, geometry, and surface texture. Calm outdoor conditions can sit near 2–8 W/m²·K, while breezy exposure can exceed 15–25 W/m²·K. Estimate mode uses a wind-based correlation and clamps values to a practical horticulture range for quick decisions. If your surface is inside a greenhouse with circulation, expect lower values than outdoor exposure.
Interpreting temperature difference
The driving force is (Ts − Ta). A warm soil bed at 18°C with 6°C air yields a 12°C difference; doubling that difference doubles convection. If air is warmer than the surface, the sign flips and the result becomes heat gain, useful for daytime warming checks. For frost protection, focus on nighttime minimum air temperature, because it sets the worst-case delta.
Windbreaks, shelter, and placement
Windbreak fencing, hedges, and low tunnels reduce air velocity at the surface. Use the “Sheltered” option as a multiplier on h to represent this reduction. Even a modest 0.75 factor can meaningfully lower required heat, especially for large exposed areas. Place sensors near crop height, because local wind can differ from station reports.
Using results for equipment sizing
Convert the magnitude to energy by multiplying by time: watts × hours = watt-hours. For example, 800 W sustained for 6 hours is 4.8 kWh of heat delivered, before heater efficiency losses. Combine this convection estimate with radiation and evaporation allowances to set reliable heater capacity margins. When comparing cover materials, keep A constant, then adjust surface temperature targets to see how much heat reduction you gain per degree of warming.
FAQs
1) What does a positive convection value mean?
Positive Q means heat is leaving the surface and warming the surrounding air. In cold nights, that is heat you may need to replace with covers, insulation, or heaters.
2) When should I use manual h instead of estimate mode?
Use manual mode when you have a measured coefficient, a supplier specification, or a validated model for your geometry. Estimate mode is best for quick comparisons across wind speeds and temperatures.
3) Why is wind speed so important?
Wind increases turbulence and thins the warm boundary layer, raising h. That increases heat transfer even if temperatures stay the same, which is why windy frost events are often more damaging.
4) How do I estimate exposed area for a bed or tunnel?
Approximate the outside surface that touches moving air: top and sides of covers, or the wetted perimeter for tanks. For quick planning, slight overestimation is safer than underestimating.
5) Does this include radiation or evaporation losses?
No. This tool isolates convection. Nighttime radiation to the sky and evaporation from wet soil can add major losses. For conservative sizing, add extra capacity or use a combined heat-loss worksheet.
6) Can I use this for indoor growing rooms?
Yes, if you input the local air speed near surfaces. Fans and circulation can raise h significantly. Manual h is recommended when airflow patterns are known.
Notes for accurate planning
- Convection is only one part of total heat loss. Add radiation and evaporation for full estimates.
- Wind and surface roughness can change h significantly.
- If air is warmer than the surface, results show heat gain (negative Q).
- For long, thin objects or pipes, consider using measured h.