How to Size a Greenhouse Heating System

March 20, 2018   

Properly sizing a greenhouse heating system is crucial for maintaining the perfect temperatures year-round.  An efficient heater delivers consistently uniform heat to the plant canopy, promoting strong, rapid plant growth. Sizing a heating system for a greenhouse only requires taking some measurements and making a few calculations. Once the numbers are crunched, growers can rest assured they are well on their way to efficiently heating their operation.

Efficiency ratings are of utmost importance when outfitting a greenhouse with a heater. Heating manufacturers have even put special emphasis on heating efficiency in recent years, so that growers can more easily find a high-efficiency unit. GrowSpan offers the Modine Effinity 93 Condensing Unit Heater, which is 93 percent efficient, making it one of the most energy-efficient heaters available. High-efficiency models like this enable growers to save considerably on heating costs and reduce their carbon footprint.

Heaters are measured by their output in BTUH (British Thermal Units per Hour) as opposed to their physical size. A heater that is larger in size does not automatically mean more heat output. BTUH reflects the size of an area that a heater can keep warm each hour.

GrowSpan’s Greenhouse and Cannabis Specialists are experts at designing a greenhouse heating system. To find out more about how they can help your project, visit out page on greenhouse heating or Request a Quote 

Sizing a Greenhouse Heating System

To find the right size heater, follow this step by step guide to calculate the amount of BTUH the heater needs to generate for the greenhouse. As an example, let’s use a GrowSpan Series 2000 Commercial Greenhouse that measures 35 ft wide by 96 ft long.

Calculate the total surface area

First measure the end wall: 35 ft width x 14 ft (average wall height) = 490 (one end)
490 x 2 = 980 sq ft (both ends)

Next measure the roof. Each side of the roof measures: 18’ x 96’ = 1,728

1,728 x 2 = 3,456 sq ft total roof surface area

Multiply the square feet by the U-factor

 GrowSpan’s S2000 Greenhouse is covered with 8 mm polycarbonate, which has a U-factor of .62  

490 (one end wall) x .62 = 304

3,456 (roof surface area) x .62 = 2,143

Add the numbers

304 + 2,142 = 2,447

Multiply by Delta T (the amount of heat loss over the length of the house.) We will use the highest Delta T number of 70 to ensure maximum heat capability.

2,447 x 70 = 171,290

Calculate the amount of BTUH

With GrowSpan’s 93% efficiency heater: 171,290 divided by .93 = 184,183. This is how many BTUH are needed to heat the GrowSpan S2000 Greenhouse.

For more detailed information on heat-retaining ability, look over the chart that compares the R-value and U-value of different types of polycarbonate covering. R-Value is the measurement of insulating ability of a given material. A higher R-Value means better insulation. U-Value is the measurement of heat loss. As such, the lower the U-Value, the less heat is escaping.

 R-value (insulation)

U-factor (heat loss)

Poly Film


Poly Film


Double Layer Poly Film


Double Layer Poly Film


8mm Twin Wall Polycarb


8mm Twin Wall Polycarb



As is evidenced by the numbers, a high-efficiency heater means a high return on investment. Many growers have been able to make their money back within the first two years of operating with a new heating system. Moreover, some operations may qualify for utility rebates depending on the region they live in.

GrowSpan’s Greenhouse Specialists can help to accurately design effective heating systems for any growing operation. They can also help sync heating systems with greenhouse climate controllers for optimal performance and automation. To ask a Greenhouse Specialist today about designing a heating system Request a Quote