(877) 746-6544

How to Size a Heating System

Warming your greenhouse in the most efficient manner

Sizing a heating system for a greenhouse is not an enormously difficult task requiring complex calculations. True, there are as many formulas as there are growers, but the basic principle remains the same – to warm the greenhouse in the most efficient manner.


Determine the total exterior surface area.

For example, let’s use a 22' wide, 96' long peaked free-standing house.
  • Start by calculating end wall surface area:
     22' (width) x 8' (average wall height) = 176' (one end) x 2 = 352 sq. ft. (both ends)
  • Then, calculate roof surface area:
    In our example, the roof uses 36’ wide poly, so 36’ x 96’ = 3,456 sq. ft.

Multiply the square feet by the "U" Factor (see chart)

In this example, the house is covered with 8 mm polycarbonate on the ends, which has a U Factor of .53, so 176 x .53 = 93. The roof is covered with double layer poly, which has a U Factor of .7, so 3,456 x .7 = 2,419.


Single Layer Glass


Single Layer Poly


Double Layer Poly


Corrugated Polycarb


8 mm Polycarbonate (3-wall)


8” Concrete


1” Thick Insulation



Note: U Factor is inversely related to R-value. The lower the number, the better the insulating value!

Add up the numbers.

93 + 2,419 = 2,512.

Multiply by “Delta T.”

Delta T is an expression of heat loss over the length of the house. We use a Delta T of 70, the highest, to ensure plenty of heat. So, 2,512 x 70 = 175,840.

Determine the amount of BTUH’s.

Since most heaters are 80% efficient, 175,840 divided by .8 = 219,800, which is the amount of BTUH’s needed to heat the house with a heater.

Decide what size heater you need.

We need a heater with an input of 219,800 BTUH’s. Oil and gas heaters come in set sizes and we always size up when selecting heaters, so we would select the 245,000 BTUH model for oil heat, and 225,000 BTUH for gas heat.