Indirect Heat Setup Calculator

• Temperature difference: dT = T_inside - T_outside
• Envelope area: walls + roof, with a roof multiplier for shape.
• Transmission loss: Q_trans = U * A_envelope * dT
• Infiltration airflow: CFM = (ACH * Volume) / 60
• Infiltration loss: Q_inf = 1.08 * CFM * dT
• Total loss: Q_total = Q_trans + Q_inf
• Heater output: Q_out = Q_total * (1 + Safety)
• Heater input: Q_in = Q_out / Efficiency
• Airflow target: CFM_supply = Q_out / (1.08 * Rise)
• Power conversion: kW = BTU/hr / 3412.142

U-values and multipliers are typical planning values. If you have measured U-values, choose "Custom".

1. Select your measurement system and enter greenhouse dimensions.
2. Enter your target inside temperature and cold outside temperature.
3. Pick a cover option, then choose an ACH value.
4. Set efficiency and a safety margin based on risk.
5. Choose fuel type and price, then set budget hours.
6. Press Calculate to view results above.
7. Use the download buttons to save outputs.

Examples are illustrative. Your results depend on structure and wind exposure.

Heating plans should use the coldest expected hour, not the daily average. A typical greenhouse thermostat holds within 1–2 degrees, so heat loss is driven by the design temperature difference. When you enter inside and outside targets, this calculator converts that gap into BTU/hr and recommends the next practical heater size. For planning, document the design outside temperature you used .

Transmission loss depends on U-value and envelope area. Common planning U-values are 1.20 for single film, 0.70 for double film, 0.60 for polycarbonate, and 0.35 for insulated panels. If you know your measured U, choose Custom to reduce uncertainty. Small improvements in U can cut required capacity noticeably during windy freezes, especially on large roof areas.

Infiltration is modeled with air changes per hour (ACH) and building volume. Tight structures may run near 0.25–0.75 ACH, while older tunnels can exceed 1.5 ACH. Because infiltration scales with both volume and temperature difference, sealing end walls, closing gaps, and managing door traffic often saves more fuel than upgrading equipment. Track repairs and re-run the calculator to quantify improvements.

Indirect setups move warm air from a heat source through ducting without exposing plants to combustion byproducts. The calculator uses a temperature rise target to estimate required airflow in CFM. Lower rise values increase airflow but improve uniformity. A practical range is 25–45 degrees for gentle mixing across benches and along long rows. Use circulation fans to reduce stratification when ceilings are high.

Operating cost comes from heater input energy and fuel price. Reference energy contents include 3,412 BTU per kWh, 100,000 BTU per therm, about 91,500 BTU per gallon of propane, and about 138,500 BTU per gallon of diesel. Add a 10–20% safety margin to cover wind gusts, vent events, and aging covers. If you expect frequent opening cycles, choose the higher end of that margin.

1) What does “recommended heater output” mean?

It is the delivered heat needed in the growing space, after adding your safety margin. It is not the fuel input. Input is higher when efficiency is below 100%.

2) Which U-value option should I choose?

Pick the option that matches your covering. If you have lab or manufacturer data for your exact assembly, use Custom. Better U-values reduce transmission loss and required heater size.

3) How do I estimate ACH if I have no test data?

Start with 0.75–1.0 for newer builds and 1.5 for older tunnels. Then adjust after observing how quickly temperature drops when the heater cycles off on cold nights.

4) Why does a lower temperature rise increase airflow?

Airflow is inversely related to the rise across the heater. A smaller rise means each cubic foot carries less heat, so more CFM is required, often improving distribution and reducing hot spots.

5) Is the monthly cost an exact bill estimate?

No. It is a planning estimate using your hours per day and fuel price. Real costs depend on weather swings, thermostat cycling, ventilation use, and how well the structure is sealed.

6) What should I check before installing an indirect heater?

Confirm safe exhaust routing, adequate combustion air, and correct duct materials and clearances. Verify sizing with local codes and a qualified installer, especially for gas or liquid fuel systems.