Greenhouse heater calculator.
BTU sizing and estimated monthly fuel cost in one place. Enter your structure and climate; the numbers update as you go, from hobby houses to commercial high tunnels.
| Model | Output BTU | Number needed | Est. fuel $/month |
|---|
Heating a small house?
Loads well under 120,000 BTU per hour sit below the smallest unit in our gas heater line, and a single heater will carry them with room to spare. A thermostat keeps it from short-cycling, and the rest of the heating & cooling line covers controls and air movement. If you're between sizes, send the numbers through the form and we'll point you at the right setup.
Commercial-scale load.
This load calls for multiple units and some layout planning (heater placement, gas supply sizing, and controls). That is quoting work we do every week. Send the build through the form below and an owner will spec it with you.
Greenhouse heater BTU chart
Design heat loss for common structure sizes, computed with this page's calculator: inside 60°F, design low 0°F (a 60° difference), new construction, 15 mph wind. Your covering changes the number as much as the size does.
| Structure | Double poly, inflated | 6 mil single poly | 8 mm polycarbonate |
|---|
BTU per hour of heat loss. Sized against heater output: divide by a heater's output BTU for the unit count.
How the sizing works.
This calculator follows the NGMA greenhouse heat-loss method, the same math our sales team sizes with. Each exposed surface (roof, sidewalls, end walls, and the end area above the sidewall) loses heat in proportion to its area, the covering's U value, and the temperature difference you design for. On top of that, infiltration accounts for the air the house exchanges with the outside, scaled by house volume, construction tightness, and wind.
Two practical rules ride along. First, size against heater output, not nameplate input: a direct-fired heater like the Therma Grow delivers essentially all of its input as heat to the house, while a vented unit heater like the Modine PTP delivers 80 percent, with the rest going out the flue. Second, leave headroom for the coldest snaps rather than sizing to the average night; the design-low input is where that margin comes from.
The monthly fuel estimate runs the same heat-loss math at your average winter temperature instead of the design low, burns that load around the clock for 30 days, and divides by the fuel's heat content (91,500 BTU per gallon of propane, 100,000 BTU per therm of natural gas) and the heater's efficiency. It is a planning figure, not a fuel quote: real usage follows your weather, thermostat, and crop.
About heater sizing.
Size on heat loss, not floor area. Add up the exposed surface area (roof, walls, and ends, not the floor), multiply by the covering's heat-loss value and by the difference between your growing temperature and your design low, then add an infiltration allowance for air leakage. The calculator above runs that math and shows how many of each heater model cover the load.
Your structure
Send us your sizing.
We'll confirm the load, match the heater and thermostat, and walk through mounting, gas supply, and delivery. Your numbers come along with the form.
(860) 341-8707 · info@ctgreenhouse.com
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