Size a planning heater BTU/hr from greenhouse volume (or area × height), an insulation/heat-loss factor, and the design temperature rise.
Use this for a first-pass heater size before you buy a unit heater or propane stove for a hoop house or freestanding greenhouse.
Heat load ≈ volume × ΔT × a loss factor that reflects glazing and tightness. Single-layer poly loses more heat than twin-wall polycarbonate.
Rule-of-thumb horticulture sizing: BTU/hr ≈ volume_ft³ × ΔT_°F × loss_factor. ΔT = inside − outside design temperatures.
volume = length × width × avg_height
ΔT = T_inside − T_outside
BTU/hr = volume × ΔT × loss_factor
Pick the coldest night you plan to hold temperature against — not an average winter day.
A 40×20×10 ft house = 8,000 ft³. Hold 55°F when it is 10°F outside (ΔT=45) with double poly (1.4): 8,000 × 45 × 1.4 = 504,000 BTU/hr design load — then choose equipment with margin and proper venting.
Assumptions: steady-state loss; factor presets are planning guides, not ASHRAE enclosure models.
Limits: ignores solar gain, thermal mass, wind, and infiltration detail. Not a stamped heating design.
Responsibility: follow heater clearances, CO alarms, and fuel codes; consult a greenhouse HVAC pro for large structures.
Cold design nights and thin glazing drive large loads. Thermal curtains and double layers cut the factor.
1 kW ≈ 3,412 BTU/hr. Divide BTU/hr by 3,412 for approximate electric kW (rarely practical at full greenhouse load).
Use a higher loss factor for single poly and leaky end walls; lower for insulated twin-wall.