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Calculation Machine

Greenhouse Heater BTU Calculator

Heating load for a greenhouse.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Heater size
22,226 BTU/h
In kW
6.51
Surface area
907 sq ft
Temperature difference
35 °F
With 15% margin
25,560 BTU/h

Quick answer

Greenhouse heat loss is surface area times the U-value of the glazing times the temperature difference.

The formula

BTU/h = area x U x ΔT

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Length24 ft
  • Width12 ft
  • Average height8 ft
  • Inside target55 °F
  • Coldest outside20 °F
  • GlazingDouble poly (0.70)

Result

  • Heater size22,226 BTU/h
  • In kW6.51
  • Surface area907 sq ft
  • Temperature difference35 °F
  • With 15% margin25,560 BTU/h

How to use the Greenhouse Heater BTU Calculator

  1. 1Enter the greenhouse dimensions and average height.
  2. 2Choose the glazing and enter your coldest expected night.
  3. 3Add 10-20% margin, and use a thermal blanket to cut the load sharply.

Frequently asked questions

Vented or unvented heater?

Always vent combustion heaters — unvented units dump moisture and CO2 into the house.

Cheapest saving?

An internal thermal screen at night can cut heat loss by 30-50%.

About this calculator