Formulas Used
BTU/h = Total Watts × 3.412CFM = BTU/h ÷ (1.08 × ΔT °F)m³/h = CFM × 1.699Frequently Asked Questions
How do I calculate BTU for a grow room?
Multiply your total wattage (lights + equipment) by 3.412 to get BTU/hour. A 400W LED grow room produces approximately 1,365 BTU/h. This heat must be removed by your extraction system to maintain target temperatures.
How do I size my extraction fan?
Use the formula: CFM = BTU/h ÷ (1.08 × ΔT). ΔT is the desired temperature difference (typically 10°F). For a 400W setup generating 1,365 BTU/h with a 10°F ΔT: CFM = 1365 ÷ 10.8 = 126 CFM. Always upsize by 20–30% and account for carbon filter static pressure restriction.
Do LEDs produce less heat than HPS?
LEDs convert more electricity to light and less to heat, but they still generate significant heat — approximately 60–70% of their input power as heat in the grow space. A 400W LED may produce 240–280W of heat (vs 400W for HPS). The BTU calculation is based on total input watts, not lamp type.
What is ΔT (delta T) in ventilation?
ΔT is the maximum acceptable temperature rise from fresh air intake to exhaust. A ΔT of 10°F (5.6°C) means your grow room temperature stays within 10°F of your intake air temperature. Smaller ΔT requires more airflow; larger ΔT reduces fan requirements but may cause hot spots.
Should I account for carbon filter resistance?
Yes. Carbon filters add 20–40% static pressure restriction to your fan's actual output. If your calculation requires 200 m³/h, choose a fan rated at 250–280 m³/h to compensate for the filter restriction. Always buy rated airflow from manufacturers with carbon filter attached.