Greenhouse and high tunnel heating calculator
Peak heat loss, heater size, and what it costs to run on propane, natural gas or electricity. Follows the ANSI/ASAE EP406.4 engineering practice, endwalls and air leakage included. Everything runs in your browser.
1. Structure
Envelope
2. Glazing, leakage and temperatures
Peak heat loss
3. Heater and fuel
Heater sizing
Cost per million BTU delivered
The honest way to compare fuels, since it accounts for both price and how much of the energy reaches the crop. Uses your system efficiency.
| Fuel | Energy content | Your price | $ / million BTU delivered | Seasonal fuel | Seasonal cost |
|---|---|---|---|---|---|
| Propane | 91,500 BTU/gal | ||||
| Natural gas | 100,000 BTU/therm | ||||
| Electric resistance | 3,412 BTU/kWh |
How it works
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Area means the envelope, not the floor
Conduction (BTU/hr) = Σ (U × area) × temperature difference
The load depends on the surface the heat escapes through. A 30 by 96 foot hoop has about 5,230 square feet of cover over 2,880 square feet of floor — using floor area understates the heat loss by 38%, which is the most common and most expensive mistake in a home-made spreadsheet.
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Endwalls are not a rounding error
They are 13.5% of the envelope on a 96 foot tunnel and 23.8% on a 48 foot one, so the shorter the house the more it matters. They are also usually a different material from the roof, which is why this calculator asks about them separately — single-layer endwalls on a double-poly house add around 7% to the load.
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Air leakage is a separate term
Infiltration (BTU/hr) = 0.02 × volume × air changes per hour × temperature difference
On a tight new double-poly house that is about 16% of the total. The constant is the heat capacity of air. Watch for double counting: some quick methods inflate the glazing U-value to stand in for leakage, and adding both an inflated U and an explicit infiltration term overstates the load by around 12%.
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Size on the coldest night, not the average
Use the ASHRAE 99% winter design temperature for your location — the temperature it is colder than for only about 88 hours a year. And use the night setpoint the crop needs, because peak load happens after dark; daytime setpoint does not enter into it, since solar gain covers the difference. Setpoint is the single most leveraged input here: 45 °F for hardy greens instead of 60 °F cuts the load by a quarter.
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Match the load to heater output, not input
Heater input rating = heat loss ÷ burner efficiency
Unit heaters are catalogued by both. A heater badged with your BTU number as its input rating delivers only about 80% of that as heat, leaving you 20% short on the coldest night. Size on output, then check the input rating you need when shopping.
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A thermal curtain saves fuel but should not shrink the heater
Closing a blanket over double poly takes the effective U-value from 0.70 to about 0.44, roughly a third off the load, and seasonal savings of 25–50% are reported. But the design case is the night the curtain fails or is left open. Claim the saving on fuel, not on heater capacity.
Ground and perimeter losses are not included — the EP406.4 practice has no term for them, which is standard for poly-covered structures. Solar gain is deliberately excluded from peak sizing, because peak load happens at night.
Planning a build? Size the grow space and yields with the substrate and yield calculator, or work out supplemental lighting with the grow light and DLI calculator.