Garden Preheat Time Calculator

Dial in preheating for seedlings and tools. Enter target temperature, heater output, and volume now. Get reliable minutes before work begins in comfort always.

Inputs

All calculations use °C internally.
Preset fills density and heat capacity.
Mass helps for tool bins or water tanks.
Typical dawn greenhouse air: 8–14.
Seedlings often prefer 18–24.
Only used when mass is not provided.
Used when “Use known mass” is Yes.
Air ~1.2, water 1000, soil 1400–1700.
Air ~1.0, water 4.186, soil ~0.8.
Use the continuous rated output.
Accounts for exhaust and distribution losses.
If cycling, average heat is lower.
Wind, leaks, cold glass, open vents.
Adds cushion for unknown conditions.
Staged mode adds extra time buffer.

Example Data Table

Scenario Medium Start → Target Volume / Mass Heater Loss Estimated Time
Small greenhouse morning warm-up Greenhouse Air 10°C → 22°C 30 m³ 3.0 kW @ 85% 0.4 kW ~33 minutes
Water tank for frost buffering Water 8°C → 18°C 200 kg 2.0 kW @ 90% 0.2 kW ~246 minutes
Tool sterilizer heat-up Metal Tools 15°C → 70°C 25 kg 1.5 kW @ 80% 0.1 kW ~38 minutes
Examples are illustrative. Real times vary with insulation, airflow, and equipment behavior.

Formula Used

The calculator estimates the heating energy needed, then divides by the net heating power. It assumes the medium warms uniformly.

  • Energy (kWh) = Mass (kg) × Specific Heat (kJ/kg·K) × ΔT (K) ÷ 3600
  • Effective Heater Power (kW) = Heater Rating × Efficiency × Duty Cycle
  • Net Power (kW) = Effective Heater Power − Heat Losses
  • Time (hours) = Energy ÷ Net Power, then apply warmup mode and margin
If ΔT is negative, the calculator returns zero minutes. When losses exceed effective power, net power is floored to avoid divide-by-zero.

How to Use This Calculator

  1. Select the temperature unit that matches your thermometer.
  2. Pick the heating medium, or use custom values.
  3. Enter start and target temperatures for your task.
  4. Use volume for greenhouse air, or mass for tanks/tools.
  5. Enter heater rating, efficiency, and expected duty cycle.
  6. Add heat losses if vents leak or wind is strong.
  7. Set a safety margin, then press Calculate.
After calculating, download a CSV for logs or a PDF for job sheets.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.