Greenhouse Heating Calculator

Heating Load estimates the BTU/hr a heater must supply to hold the greenhouse at Inside Temperature when outdoor conditions hit Outside Design Temp, using the classic heat-loss formula from NGMA's own "Heat Loss" standard -- Q = UA(Ti − To) -- surface area times a glazing U-value times the temperature difference, scaled by a wind exposure factor. Total Surface Area is built from Length, Width, and Sidewall Height as a gable shape at a nominal 4:12 pitch — walls, a sloped roof with the rise set to one sixth of the width, and the two triangular gable ends, not a true user-specified roof pitch. Glazing Type sets the U-value (heat loss per square foot per degree): single glass loses heat fastest, polycarbonate 16mm loses it slowest, and double poly sits in the middle as the common cost-effective choice. The U-values are NGMA's own published "suggested heat transmission coefficients" (1.13 single glass, 0.65 double glass, 0.7 double poly, 0.65 twin-wall polycarbonate, 0.6 triple-wall polycarbonate — NGMA's table gives single glass at exactly 1.13 and double poly at exactly 0.70) and already carry an average infiltration allowance, which is why the Wind Exposure Factor should sit at 1.0 for a normally sheltered house rather than being treated as a free safety margin. Inside Temperature and Outside Design Temp enter the formula only through their difference, so a one-degree change to either moves Heating Load by exactly the same amount -- about 3,700 BTU/hr per degree at the default 96 x 30 ft double-poly house.

Raising the inside setpoint by five degrees and dropping the design temperature by five degrees cost identically; the practical difference is that your design temperature is dictated by your climate while the setpoint is a crop decision you control. If the outside design temperature you enter is at or above the inside setpoint there is no heating load at all and every figure reads zero — see the Greenhouse Cooling calculator for that case. The calculator converts the raw BTU/day figure into three fuel estimates (natural gas therms, propane gallons, electric kWh) assuming 100% conversion efficiency, using standard heat-content conversions, but it does not account for solar heat gain during the day, floor/soil heat loss, infiltration through vents and doors beyond the single wind factor, or a heater's own combustion or distribution efficiency — all of which change the real equipment size needed. A combustion heater at a typical 80% AFUE burns about 25% more gas or propane than shown; electric resistance heat converts at essentially 100%, so the kWh figure needs no adjustment.