Greenhouse Heater Size & Heat Loss Calculator
Enter the greenhouse dimensions, covering, design temperatures, leakage rate, and heater efficiency. The calculator estimates exposed area, heat loss, required BTU/h, a rounded heater size, and fuel use at the design load.
Recommended nominal heater size23,000
- Minimum heater input
- 22,295
- Required delivered heat
- 17,836
- Covering transmission loss
- 15,269
- Air-infiltration heat loss
- 945
- Exposed glazing area
- 623.24
- Enclosed air volume
- 1,500
- Fuel use at design load
- 0.24 gal/h propane
Heat-loss balance
| Component | Formula | Result |
|---|---|---|
| Transmission | area * U * temperature difference | 15,269 |
| Infiltration | 0.018 * volume * air changes * temperature difference | 945 |
| Delivered with margin | (transmission + infiltration) * margin | 17,836 |
| Input after efficiency | delivered heat / efficiency | 22,295 |
Exposed area excludes the greenhouse floor.
The load is zero when the outdoor design temperature is equal to or warmer than the desired inside temperature.
How to use this calculator
- Choose whether the greenhouse is a gable-roof house or a semicircular hoop house.
- Enter length, width, sidewall height, and roof rise where it applies.
- Select the covering material and the inside and outdoor design temperatures.
- Set the air-change rate, design margin, heater efficiency, and fuel type.
- Read the recommended heater size and compare the transmission and infiltration losses.
How greenhouse heater size is calculated
This calculator uses a steady nighttime heat balance. It first estimates the exposed surface area and enclosed air volume from the greenhouse shape. For a gable house, the exposed area excludes the floor and includes sidewalls, end walls, triangular gable ends, and both roof planes:
A = 2LH + 2WH + WR + 2L * sqrt((W / 2)^2 + R^2)
V = LW * (H + R / 2)
For a hoop house, the curved roof is treated as a semicylinder with radius r = W / 2:
A = 2LH + pi*r*L + 2WH + pi*r^2
V = LWH + pi*r^2*L / 2
The temperature difference is max(inside temperature - outdoor design temperature, 0). If the outdoor design temperature is already at or above the target indoor temperature, the heating load is zero.
Transmission and infiltration losses
Covering loss is calculated as Qt = A * U * deltaT, where U is the selected covering heat-transfer coefficient in BTU/h per square foot per F. Lower U-factors reduce the load directly. A double polyethylene or multiwall polycarbonate covering can therefore change the required heater size as much as changing the floor area.
Air leakage is estimated as Qi = 0.018 * V * ACH * deltaT. The constant converts the volume of exchanged air and temperature difference into BTU/h. A tight new structure may be near 0.5 to 1 air change per hour, while a loose or windy greenhouse can be several times higher. Zero air changes is allowed for comparison, but real greenhouses leak.
The delivered heat requirement is Qd = (Qt + Qi) * (1 + margin). Heater input is Qh = Qd / efficiency. The recommended nominal size rounds Qh upward to the next 1,000 BTU/h so a heater is not undersized by ordinary rounding.
What moves the answer most
The largest drivers are exposed area, the temperature difference, the U-factor, air leakage, and efficiency. Width matters twice for gable houses because it affects end-wall area and roof slope. For hoop houses, width controls the radius, so it affects both curved area and volume.
What this calculator leaves out
The estimate does not model daytime solar gain, thermal mass, thermostat cycling, ground heat flow, perimeter losses, doors, framing, crop benches, or hour-by-hour weather. Wind and construction quality are represented only through the air-change rate and design margin. Combustion heaters also need correct fuel installation, ventilation, clearances, and carbon-monoxide protection beyond this sizing calculation.
Worked example
For the default 20 ft by 10 ft gable greenhouse with 6 ft sidewalls, a 3 ft roof rise, inflated double polyethylene, a 55 F target temperature, and a 20 F outdoor design temperature, the temperature difference is 35 F.
The exposed area is 623.2 sq ft and the enclosed volume is 1,500 cu ft. Transmission loss is 623.2 * 0.70 * 35 = 15,269 BTU/h. Infiltration loss at 1 air change per hour is 0.018 * 1,500 * 1 * 35 = 945 BTU/h.
After a 10% design margin, the required delivered heat is 17,836 BTU/h. At 80% heater efficiency, the minimum heater input is 22,295 BTU/h, so the recommended nominal heater size rounds up to 23,000 BTU/h. Propane use at that design load is about 0.24 gal/h.
Common questions
What size heater do I need for a 10-by-20-foot greenhouse?
With the default assumptions in this calculator, a 10 ft by 20 ft gable greenhouse needs about 22,300 BTU/h of heater input and a nominal 23,000 BTU/h heater. The answer changes with covering U-factor, sidewall height, roof rise, outdoor design temperature, leakage, and heater efficiency.
Which outdoor temperature should I use for heater sizing?
Use a cold design temperature for your site, not the seasonal average. The heater has to carry the greenhouse through the coldest nights you choose to design for, so a warmer average temperature will usually undersize the equipment.
What U-factor should I use for polyethylene or polycarbonate?
Use the manufacturer value for the actual covering assembly when you have it. If you do not, this calculator provides common planning values, such as 0.70 for inflated double polyethylene and 0.60 for 8 mm twin-wall polycarbonate.
How many air changes per hour should I assume?
A tight newer greenhouse may be near 0.5 to 1.0 air changes per hour. Older, drafty, or wind-exposed houses may need 1.5 to 3.0 or more. Changing ACH is a practical way to see the cost of leaks and the value of sealing obvious gaps.
Why is floor area excluded from greenhouse glazing area?
The exposed glazing area in this method counts the cover surfaces losing heat to outdoor air. The floor and soil exchange heat differently, so this simplified calculator does not include them as glazing transmission area or model ground loss separately.
How much propane does a greenhouse heater use per hour?
Propane use at design load is the required heater input divided by 91,500 BTU per gallon. A 22,300 BTU/h input load uses about 0.24 gallons per hour while the heater is firing at that load, before any thermostat cycling.