You’ve got questions, and the experts at the National Greenhouse Manufacturers Association have the answers. Here are some frequently asked questions related to heating systems and heat loss for greenhouse structures.
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.
“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.
In North America, heat loss is typically expressed in terms of total British Thermal Units per Hour (BTUH) loss. British Thermal Unit (BTU) = the amount of energy it takes to warm one pound of water 1° F.
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.
Ventilation essentially provides the same benefit regardless of the season. Ventilation, in addition to removing excess heat in the summer, replenishes carbon dioxide and assists in the control of humidity levels.
During cold weather, when greenhouses are virtually closed in, there is often insufficient air circulation to maintain desired conditions. Appropriate air circulation 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.
A substantial amount of heat energy can be lost out of the perimeter of a greenhouse through the ground below the perimeter walls and ends. This can be minimized by installation of perimeter insulation below the frost line.
All forms of heating fall into one or more of three basic principles: convection, conduction or radiation. Since all three methods of heat transfer are common and effective in greenhouse heating, it is important to understand the fundamentals of each.
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.
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.
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 has experienced radiant heat. Although the air may not be warm, you are able to feel the heat energy radiating from the stove.