Tip #8: Sizing Heating and Cooling Equipment for a Greenhouse

Maintaining consistent temperatures is one of the most important aspects of greenhouse gardening. During the hot, summer months, most greenhouses will require some sort of cooling to ensure the temperatures don’t get too hot. During the cold winter months, most greenhouses will require some sort of heating to ensure temperatures do not get too cold. Before purchasing any heating or cooling equipment, a grower should take a close look at his or her space and make a few sizing calculations. With these calculations and a few considerations, a horticulturist can be sure to get the properly sized heating and cooling devices needed to keep the greenhouses’ temperature in check.

Cooling/Ventilation
The most effective way to cool most greenhouses is with a powered fan. A powered fan will actively draw fresh air through the greenhouse and exhaust it outside of the greenhouse. The cooler air from outside along with a natural evaporative effect help keep the greenhouse cooler. In fact, a powered ventilation system will typically make a greenhouse run 10 degrees cooler than if the greenhouse is passively cooled (vents only). To make sure there is enough cooling power, a gardener must “size” the fan that will be needed for the given garden space. Fans are rated by their CFM or cubic feet per minute. Ideally, a greenhouse should have all of its air exchanged in 1-2 minutes. A simple and straightforward way to determine the needed CFM is to multiply the length by the width by the wall height of the greenhouse as shown below:

CFM Calculation
Length x Width x Wall Height = Recommended CFM (Cubic Feet of air volume per Minute)

Granted, this is not an exact calculation of the greenhouse’s cubic feet because it does not take into consideration the roof pitch, etc. However, this measurement is accurate enough to properly size a powered fan for a greenhouse. Once a grower has calculated the recommended CFM, he or she can set out to find a fan that meets that criteria. For example, a greenhouse that is 20 feet long, 10 feet wide, and has a wall height of 10 feet will have a recommended CFM of 2,000 (20 x 10 x 10 = 2,000). The owner of this greenhouse should purchase a fan with a minimum of a 2,000 CFM rating.

Heating
A gardener planning to heat his or her greenhouse with gas or electric must first determine the size of the heater. The best way to determine this is to figure out how many BTUs will be required to heat the space. This is done by first calculating the total square footage of exposed surface area. In other words, the exposed wall and roof surface area. Remember that the roof area, due to the pitch, will not be equivalent to the area of the floor. In fact, the exposed surface area of the roof will be larger than the floor area footage. After determining the total square footage exposed, the grower must determine the maximum desired temperature in the winter (the temperature at which the gardener wants the greenhouse to operate during the winter months) and the minimum temperature outside of the greenhouse. Remember to be realistic when making the temperature determinations. In other words, base your calculations on average temperatures rather than the extremes. The BTU calculation is completed when you multiply the total exposed surface area by the difference between the desired temperature and the outside minimum temperature, and then divide that number by the R-value of the greenhouse material. It’s important to remember when calculating the required BTUs that gas heaters operate at 80% efficiency compared to electric heaters which operate at 100% efficiency. In other words, a grower must take the lower efficiency into consideration in his or her calculations or he or she must find the gas heater’s output BTU rating (which already takes the 80% efficiency into consideration). The following is the formula that should be used to calculate the needed BTUs:

BTU Calculation

Sq. Ft. Exposed Surface Area x ( Tmax – Tmin ) ÷ R

Square Feet of Glass or Poly Surface Area x (Desired Temperature Inside – Minimum Temperature Outside) ÷ R-value

Here are the R-values of some commonly used greenhouse glazing materials:

Although all of these R-values may seem low when compared to a home or commercial building, there is a significant difference when comparing these materials to each other. For example, a greenhouse with single pane glass will require twice the BTUs (and cost twice as much to heat) as a greenhouse with a triple-wall polycarbonate. In other words, when compared to each other, the R-values of these materials are quite significant, especially when considering how the required heating load is affected.

With a few simple calculations, any greenhouse grower can determine the appropriate size cooling fan and heater for his or her greenhouse. Greenhouses that have properly sized heating and cooling equipment will not only allow a gardener to extend the growing season but will also efficiently control temperatures in the greenhouse.