In many regions, growers don’t have to worry about the burden of snow or having to use a greenhouse heater to cultivate year-round. Not everyone is afforded this luxury, though, and for cold-climate growers, a winter greenhouse offers a lifeline to stay competitive by enabling them to grow uninterrupted. Still, success can fluctuate greatly from operation to operation.
There are a number of factors that influence performance in a winter greenhouse, and growers often have to consider the smallest details when they prepare their operation for cold weather. To keep production at the highest level, without breaking the bank on utility costs, growers must lay out an effective winter growing strategy.
Primarily, growers should first understand the basics of optimizing heating in their winter greenhouse. This includes knowledge of the various factors that influence heating, different types of greenhouse heaters, as well as how a structure’s design impacts winter crop production.
UNDERSTANDING BTUS AND R-VALUE
As it is, there are a number of calculations commercial growers have to make to maximize production. The looming threat of cold weather only adds to the complications, forcing growers with a winter greenhouse to do some extra problem solving.
The first equation to understand in optimizing heating is solving how many BTUs are required to sufficiently heat a winter greenhouse. A BTU, or British Thermal Unit, is a unit of heat, and is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Calculating the needed BTUs will determine the appropriate size for an operation’s greenhouse heater.
To solve how many BTUs are needed for a specific winter greenhouse to be heated, growers can follow this formula:
(Desired Temperature Change) x (Cubic Feet of Space) x .133 = BTUs needed per hour
Every operation’s winter greenhouse will vary in size and require different temperature changes to function properly, so the numbers to fit this equation will almost always vary. Still, it is the starting point for growers in sizing an effective greenhouse heater system.
Determining the size of a greenhouse heater system is an important first step for growers, but any efforts to heat properly will be nullified without also understanding R-Value. R-Value represents a cladding’s thermal resistance, which impacts how effective a winter greenhouse is at retaining heat. It’s important for growers to understand how to calculate heat loss, so they can choose the best cladding for their greenhouse and maintain optimal heating efficiency.
Heat loss can be calculated by using the following formula:
In this equation, A is the area of the winter greenhouse, while Ti and To represent the indoor and outdoor temperatures. The R symbolizes a cladding’s R-value, and with a higher number in R, Q results in a lower number, meaning fewer BTUs lost per hour.
While a greenhouse heater works to supply BTUs in a winter greenhouse, the cladding keeps that heat in. Understanding the relationship between these two factors helps growers make informed decisions about their winter greenhouse, which can aid them in keeping utility costs down and ensuring valuable heat doesn’t escape their structure.
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CHOOSING THE RIGHT GREENHOUSE HEATER
Sizing out a greenhouse heater is critical to growers’ budgets, as well as the health of their plants. To truly optimize heating, though, growers also need to understand the fundamental differences between various greenhouse heaters and choose a high-efficiency option that can save them money, while still being effective.
The two most common types of greenhouse heaters are gas and electric greenhouse heaters. Gas greenhouse heaters typically employ propane or natural gas and are commonly used because of their relatively lower cost and high productivity. They are also convenient for growers who do not have access to electricity, although this is uncommon in larger commercial operations.
Gas heaters, however, emit fumes into the air, which can be an issue in a winter greenhouse that is sealed up to prevent heat loss. Without proper ventilation and air circulation, these fumes can cause issues for both plant and worker health.
An electric heater generally offers a better solution for a winter greenhouse, as it does not emit the same fumes, can have greater safety features and doesn’t require a natural gas line or consistent fuel supply. Electric greenhouse heaters work well with programmable thermostats or environmental controllers, meaning they can be more easily automated.
With both types of greenhouse heaters, a high-efficiency model can keep production at the highest level possible. By adding an efficient heat source, winter greenhouses can maintain their interior temperature and keep even the warmest-climate crops healthy, all while keeping utility costs to a minimum.
One of the best high-efficiency greenhouse heaters available is the Modine Effinity 93 Condensing Unit Heater. Although this is a gas greenhouse heater, it is 93% efficient, so growers can benefit from substantial energy savings during the cold season.
To further enhance their heating efficiency, growers can also take advantage of the principle of thermal mass, which relates to an object or structure’s ability to store and re-release heat. One common way of doing this is by utilizing large water barrels, as water has a high heat capacity and can absorb a significant amount of solar energy throughout the day, then release it into the greenhouse at night when temperatures drop. This is a proven method in smaller, unheated greenhouses where growers want to spend as little as possible, but it can also be an effective way to supplement heating efforts inside a commercial structure.
WINTER GREENHOUSE DESIGN
When it comes to retaining heat inside a winter greenhouse, it’s critical to focus on the structure’s design and increase its R-value. Design optimization will help growers get the most out of their greenhouse heater, which will be important to their budget.
Although engineering shouldn’t be overlooked, the most significant design component for a winter greenhouse is far and away its cladding. The right cladding will provide enough insulation to help growers thrive during the colder months and keep plant growth optimal.
Air can be an excellent insulator in a winter greenhouse and presents a number of possibilities for growers. From higher-end cladding to more budget options, many types of cladding use air to provide structures with added thermal resistance.
For growers with the capability to retrofit their winter greenhouse with new cladding, an excellent year-round option is twin-wall polycarbonate. In 8mm thickness, twin-wall polycarbonate possesses an R-value of 1.72, one of the highest for greenhouse cladding.
The spaces between the two layers of polycarbonate trap air, creating air pockets that help keep heat from escaping. Its thermal resistance makes it a popular choice amongst growers in northern climates, and it far outweighs other cladding options, like single-layered film, which provides an approximate R-value of just 0.83.
Since this may not be a viable option for all growers, there are other effective solutions to increasing heat retention, without upgrading to a higher-end cladding.
One solution is double poly film, which uses air to create a well-insulated structure, at a fraction of the cost of other cladding options. This cladding utilizes a small compressor to inflate the space between two layers of film, where it captures air to create a buffer between the structure’s interior and the outdoors. As a more cost-effective choice, double poly film can provide an R-value of up to 1.5, creating valuable insulation amidst winter temperatures.
Another easy way to bolster insulation is with a material like TekFoil Reflective Foil, which works to reduce radiant heat transfer across open spaces, a primary contributor to winter heat loss. This helps growers ensure heat isn’t leaking from their winter greenhouse, without costing a fortune.
Finding the right set up in a winter greenhouse can be time consuming, but proper preparation is necessary to thrive through the cold season. With the help of a GrowSpan Greenhouse Specialist, growers can make the right choices for their operation, sizing out their greenhouse heater system and choosing a cladding that will help them stay competitive year-round.
To effectively grow winter vegetables and other plants, while also reducing energy costs, call or REQUEST A QUOTE today.
This post was originally published in January, 2022 and has been updated for freshness, accuracy and comprehensiveness.
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