Heat Load Calculator

What it does

A heat load calculator for grow rooms that adds up sensible and latent loads from lighting, the building envelope, ventilation, infiltration, solar gain, equipment, occupants and crop transpiration. It turns the total into cooling tons, dehumidification and facility energy use.

When to use it

Use it for early HVAC sizing of an indoor grow room. The report says to confirm with a mechanical engineer before selecting equipment.

How it calculates

• Lighting heat = W × [LED efficiency × (1 − photosynthesis efficiency) + (1 − LED efficiency)] × 3.412. Envelope heat = U × A × ΔT. Solar gain = glazing area × SHGC × peak irradiance.
• Ventilation and infiltration: sensible = 1.08 × CFM × ΔT, and latent = 4840 × CFM × Δ humidity ratio. Humidity ratio uses Magnus saturation vapor pressure.
• Transpiration averages a crop light-based coefficient with FAO-56 Penman-Monteith and scales the result by LAI ÷ 3. Latent heat uses 1060 BTU/lb.
• Tons = total BTU/hr ÷ 12,000. Recommended unit size adds a 20% safety factor and rounds to 0.5 ton. HVAC electricity = cooling load ÷ COP.

What you enter

Length, Width, Height(ft)

Total Power(W)

LED Efficiency(%)

Photosynthesis Efficiency(%)

Outdoor Temp and Indoor Temp(°F)

Outdoor RH and Indoor RH(%)

Crop Type and Growth Stage

Leaf Area Index(LAI)

Air Changes/Hour

Wall, Ceiling, Floor and Glazing U-Values(BTU/hr·ft²·°F)

What you get

Total Cooling Required(tons)

Recommended Unit Size(tons)

Sensible Heat Sources(BTU/hr)

Latent Heat Sources(BTU/hr)

VPD Status, Current vs Target(kPa)

Transpiration Rate(g/m²/hr)

Dehumidification(pints/day)

Facility Energy(kWh/day)

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