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How To Size A Greenhouse Heating System

The quickest way to figure what your greenhouse needs for a heating system is explained below. If you are not familiar with heating systems or appliances in general, have no fear -- this process is self explanatory! After all, our mission is to help you figure out how to warm your greenhouse as efficiently as possible.

The first step involves calculating the entire exterior surface area of your greenhouse. The example we’ll use is also found on this “How-To” page, which focuses on the popular free-standing Rimol greenhouse, the Northpoint. This particular example concerns a 22’ wide and 96’ long greenhouse, so keep that in mind.

1. Calculate the entire exterior surface area:

A. Start by calculating end wall surface area: 22' (width) x 8' (average wall height) = 176' (one end) x 2 = 352 sq. ft. (both ends)

B. Then, calculate roof surface area: In our example, the roof uses 36’ wide poly, so 36’ x 96’ = 3,456 sq. ft.

2. Multiply greenhouse square footage by the U-factor (explained below)

Next is taking the square footage and multiplying it by the U-factor (the number for this example is 3,456 sq. ft, but please double check your own greenhouse numbers because they will be different!). If you’re not familiar as to what the “U-factor” refers to, you are not alone; it is the rate of heat loss within a window assembly (such as the Northpoint Greenhouse). If your greenhouse is properly insulated, that is a result of positive window resistance to outdoor temperatures.

You must identify the greenhouse coverings before multiplying your square footage by the U-factor. In our Northpoint example, the greenhouse is covered with 8mm polycarbonate on the ends, which has a U-factor of .53.

A. 352 x .53 = 186.56.

B. The roof is covered with double layer poly, which has a U-factor of .7, so 3,456 x .7 = 2,419.

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 352 x .53 = 186.56. 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

1.13

Single Layer Poly

1.15

Double Layer Poly

.7

Corrugated Polycarb

1.00

8 mm Polycarbonate (3-wall)

.53

8” Concrete

.51

1” Thick Insulation

.14

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

3. Add together the two U-factors!

     A. 186.56 + 2,419 = 2,605.56

4. Multiply the U-factor sum by “Delta T.”

You can never have too much heat potential within a greenhouse, which is why we apply a Delta T of 70 (the highest number possible). Delta T is an expression of heat loss over the length of the house.

     A. 2,605.56 x 70 = 182,389.2

5. Determine the amount of BTUH’s!

     A. Since most heaters are 80% efficient, 182,389.2 divided by .8 = 227,986.5, which is the amount of BTUH’s needed to heat the house with a heater.

6. The math is over -- it’s decision time!

This free-standing Norpoint Greenhouse will require a heater with an input of 227,986.5 BTUH’s. Greenhouse owners have some flexibility as both oil and gas heaters come in set sizes, which makes rounding up the preferred route. In this case, we would select the 245,000 BTUH model for oil heat, and 260,000 BTUH for gas heat.

There are many other formulas available for sizing the correct heating system but we feel this is the most straightforward and proven way for greenhouse growers such as yourself. If you have any additional questions about utilizing this formula, give our greenhouse specialists a call at (877) 746-6544 or email .