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How to Size a Heating System

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.

  1. Start by calculating end wall surface area:
    22' (width) × 8' (average wall height) = 176' (one end) × 2 = 352ft2 (both ends)
  2. Then, calculate roof surface area:
    In our example, the roof uses 36' wide poly, so 36' × 96' = 3,456ft2

Multiply the square feet by the "U" Factor

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 × .7 = 2,419.

Roof Covering Material "U" Factor
Single Layer Glass 1.13
Single Layer Poly 1.15
Double Layer Poly 0.70
Corrugated Polycarbonate 1.00
8mm Polycarbonate (3-wall) 0.53
8″ Concrete 0.51
1″ Thick Insulation 0.14

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 × 70 = 175,840.

Determine the amount of BTUHs

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 BTUHs.

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.