Greenhouse Heating Basics From The National Greenhouse Manufacturers Association

NGMAHow did your heating systems perform this winter? Could they use a little boost? Here’s a Q&A from the National Greenhouse Manufacturers Association that might help you improve your greenhouse. For more, including the differences between benchtop heating and infrared radiant heating, visit the NGMA Helpful Hints site on heating.

How does heat loss occur?

Heat loss occurs from a greenhouse structure whenever the interior temperature exceeds the exterior temperature. The rate at which it occurs is affected primarily by the efficiency of the covering materials (glazings) installed on each surface (roof, side walls and end walls). The most commonly used covering materials all have published heat transfer factors called “U” factors that provide a means of calculating their impact on heat loss in different scenarios.

What is the “U” factor?

“U” factors are the inverse of the commonly used “R” factors, where “U” = 1 / “R”. The lower the “U” factor, the less ability your glazing material has to transfer heat, therefore, the lower the heat loss.

What is a British Thermal Unit?

In North America, heat loss is typically expressed in terms of total British Thermal Units per Hour (BTUH) loss. British Thermal Unit (BTU) is the amount of energy it takes to warm one pound of water 1°F.

Are there any special considerations for calculating heat loss?

Yes, there are unique crop considerations. Many growers do not use the total volume of their structures to grow crops. Consequently, they may not need to have the total conductive heat load available in their heating system. While the standard means of calculating heat loss are well accepted and documented, considerations should be made as to whether your crop requirements justify installing the total number of BTUH indicated by this calculation.

What is zone heating?

A zone heating system is one that places the BTUs in a specific portion of the structure.

How is air circulation affected by cold weather?

During cold weather, when greenhouses are virtually closed in, there is often insufficient air circulation to maintain desired conditions. The appropriate type of air circulation equipment will help obtain a more uniform relative humidity and provide the proper air movement. Continuous circulation produces gentle air movement and has been reported to maintain better leaf surface microclimates and prevent pockets of disease‐producing high humidity. This gentle air circulation may result in slightly higher heating demand, yet many regard it as advantageous from a plant production and quality standpoint.

Does a thermal blanket save on heating bills?

Installation of a thermal blanket system can impact the total heating requirement of your greenhouse by reducing the heat loss. These systems are typically designed to automatically retract in the daytime and close in the nighttime to trap energy.

What is convection heat?

Convection heat utilizes the forces of natural air circulation currents to transfer heat. Convection involves two basic principles: First, cold air displaces warm air and second, warm air rises in the presence of cold air. With convection, heat is transferred by air currents, which transport energy throughout the structure. When these air currents pass by plant material, energy is transferred to the plant. Because of this, it is very important that some means of air circulation is used (HAF fans, perforated polyethylene duct tubes or ceiling fans) to assure the maximum amount of warm air is transferred to the plant environment to evenly distribute heat throughout the structure.

What is conduction heat?

Conduction heat utilizes direct application to transfer heat energy to the plant. Physically touching any warm object demonstrates the principle of conductive heating. In greenhouses, this type of heating is most commonly distributed with hot water tubes, and occasionally electric resistance strips, which are placed directly on the growing surface or in the growing media. The soil, containers and growing surface in direct contact with the warm tubes or strips are heated and subsequently transfer that heat energy to adjacent material.

What is radiant heat?

Radiant heat utilizes electro‐magnetic infrared waves to transfer heat energy. Since this is a little
understood form of heating, it bears detailed explanation. Anyone who has warmed themselves by a hot wood stove or warmed their hands at a campfire has experienced radiant heat. It is also demonstrated by standing in the sun on a winter’s day or walking near a brick wall that has been exposed to the sun during the day. In both examples, although the air may not be warm, you are able to feel the heat energy radiating from these surfaces.

For more, visit the NGMA Helpful Hints site on heating.

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