1. 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.
2. 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.
3. 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) = The amount of energy it takes to warm one pound of water 1° (degree) Fahrenheit (F).
4. 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.
5. What is zone heating?
A zone heating system is one that places the BTUs in a specific portion of the structure.
6. How important is good ventilation?
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. Recommended summer ventilation rates vary but a common accepted rate is 8 cubic feet per minute (CFM) per square footage of floor space. Winter recommended ventilation rates are 1.5 CFM. Winter ventilation needs to be introduced without producing cold drafts on the plants. Winter ventilation requires a thorough mixing of the ventilation air and the warm inside air. Mixing is readily achieved by admitting the air in small high velocity openings.
7. How is air circulation effected 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.
8. When do I need perimeter insulation?
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 is conductive heat loss that can be minimized by installation of perimeter installation of insulating boards below the frost line.
9. 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.
10. What is combustion efficiency?
This term is the most basic description of efficiency. It denotes the percentage of fuel burned and turned into heating energy.
11. What is thermal efficiency?
This term is a measurement of the actual amount of available energy that transfers into the heating medium. It is derived by operating a piece of equipment at a steady state and measuring how much fuel is used vs. how much useable heat comes out. It is most typically used in reference to boilers.
12. What is distribution efficiency?
This is the measure of efficiency of how well the heating equipment actually delivers the BTUs to your plants and structure. This expression addresses how energy is distributed and transferred to the objects requiring heat.
13. What are the different heating types?
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.
14. 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.
15. 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.
16. What is radiation heat?
Radiation heat utilizes electro-magnetic infra-red 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 camp fire 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.
17. What are the main functions to heating a greenhouse?
The conversion of fuel to heat energy is typically accomplished through combustion with a burner installed in a boiler or heater combustion chamber. That heat energy is then distributed through the greenhouse through pipes, ducts, tubes, or air. Once the energy is distributed through the house, it must then be transferred to the plants and soil by convection, conduction, or radiation. Finally, once transferred to the plants and soil, they must in turn absorb its energy and convert it to usable heat. How each of these functions is accomplished has a significant effect on both the efficiency and effectiveness of the heating system.
18. What is a unit heater?
The definition of a unit heater is a fan equipped device with a means to heat the air being provided by the fan. They are the most commonly used greenhouse heating equipment. Unit heaters are valuable in that they provide warm air temperatures which are imperative for leaf transpiration and snow load concerns. Unit heaters gently circulate warm air to prevent temperature stratification, reduce mold and fungal disease. Unit heaters are available in oil fired, electric, hot water or steam, and gas fired. The most popular being the gas fired unit. Unit heaters are typically suspended from the greenhouse framing. Floor mounted units are also available.
19. Are there any cost considerations?
Comparing initial unit costs using comparable BTU capacities, oil fired units are the most expensive followed by gas, then electric, then hot water unit heaters. When installation expenses are considered, oil fired again are the most expensive followed by hot water units. Gas fired unit heaters with one way gas piping and venting can be installed for one-half the cost of hydronic units.
20. Should I consider a steam system?
In the past century, there was a time when steam was very common as a means of heating greenhouses. It is a manageable, transportable method of heating that is easy to provide bottom, top, and perimeter heating with. Steam can be plumbed through pipe coils in the greenhouse with either bare or finned pipes, or using hydronic forced air unit heaters. The initial cost of equipment, excessive maintenance, the lack of qualified technicians and installers, and the many advances in other systems have all conspired to make steam systems most probably a heating method of the past. There is however, renewed interest in the ability to generate steam as a means of sterilizing soil media as chemicals such as methyl bromide are pulled from the market over environmental concerns.
21. What are the benefits of a hot water system?
Hot water or "hydronics" systems are available for providing heating to a greenhouse space. Because of their initial cost, many growers have felt that they cannot justify the investment required to install hot water. However, many advancements in technology over the past decades have made these systems more affordable. Hot water systems do require specific engineering for each and every application, and installation is complex. However, the energy efficiency advantages they offer and the potential to enhance evenness and growth in a greenhouse facility make them attractive enough to warrant researching if hot water is a logical alternative for you.
22. How many ways can hot water be used to heat?
Perhaps the simplest of means of transferring heat from hot water is through bare pipes of steel, black iron, copper, or aluminum. Pipes can be located around the perimeter of a structure and installed under benches, in the rows of cut flowers or vegetable crops, and at gutter height as well. The addition of finned elements to the surface of a pipe enhances it's ability to transfer heat by expanding the pipe's surface area. This reduces water volume to heat and less actual footage of piping is required to do the same work. Concrete floors are becoming quite common in greenhouses. In part because of environmental legislation that requires growers to capture drainage and run-off, and also to accommodate mechanization equipment and general cleanliness.
23. How important is insulation?
Insulate! It cannot be stressed enough that a heated concrete slab floor should be insulated around the perimeter and as deep as the frost line in your area. Some reports indicate that up to 50% of your heat can be lost out the perimeter of a heated slab if no below grade insulation is installed. Some growers have even installed insulation below the slab. The best type of insulation to use is one of the extruded styrene boards, usually 1.5" - 2" inches thick.
24. What is floor heating without a slab?
Some systems can be installed on top of weed barrier or down in the sand or gravel of the floor of your greenhouse. These systems are quick and simple to install, but they do not provide the run-off protection of a slab.
25. How does bench top heating work?
Bench top heating systems use conductive heat transfer to deliver heat to plants placed directly on top of a multiplicity of parallel tubes. Small synthetic rubber tubes with high UV, heat, and chemical resistance are placed on the growing surface of benches and contain warm water which is circulated to and from a hot water supply, typically a boiler. These systems offer growers the ability to control the soil temperature of their plants and they also contribute a portion of the heating energy required to meet the total conduction heat loss of a greenhouse structure. Usually gas fired unit heaters are used to augment these systems. Again, it is vital to have an engineered design to ensure proper performance of a bench top system.
26. What is a hot water (hydronic) unit heater?
Hydronic unit heaters are enclosures equipped with a finned-tube heat exchanger and a fan. They utilize hot water from a hot water source like a boiler and extract heat from the hot water by moving air from the fan over the heat exchanger. Hydronic unit heaters are often used as a way to supplement another hydronic heating system, like a heated slab floor system or bench top heating system. They are quick and very responsive to control inputs.
27. What type of boiler should I purchase?
They all rely on the availability of hot water and typically the hot water comes from a boiler system. Boilers come in many shapes and sizes. They also come in many styles and construction methods and materials. Because the boiler is where the action of converting your fuel into heat energy takes place, it's important that you consider purchasing the most efficient boiler for your particular needs.
28. What are the most common boiler designs available for greenhouse heating?
The cast iron sectional boiler is the oldest design and is still quite popular. It is manufactured by casting "sections" of iron The steel water tube boiler design circulates water through steel tubes. The flames and products of combustion pass over these tubes, transferring heating energy and then exit through the vent stack. The copper water tube boilers are similar to the steel fire tube design except that they use copper instead of fire tubes. Copper has a higher capacity for transferring energy and can withstand quick temperature changes that might adversely affect cast iron or steel boilers.
29. How does infrared radiant heating work?
Any object that is warmer than absolute 0°, radiates energy. Approximately 50% of the sun's energy is infrared radiation or energy in the "far-end" spectrum. This spectrum of energy is not visible so it does not interfere with photosynthesis or photo-period sensitive plant material. Infrared energy travels at the speed of light, reaching the 93 million mile distance from sun to Earth in about 8 1/3 minutes. This energy is then absorbed by the Earth and objects on the Earth, and then converted to heat. That's why it is warmer close to the ground than it is at 35,000 feet in the air, even though that is closer to the sun.
30. When should I use a small electrical resistance system?
Because electricity is typically substantially more expensive to heat with than other energy sources, these types of systems are not common in commercial environments. In most locations in the United States, electricity costs 3-6 times what natural gas costs per BTU delivered. For small applications of bench top heating, there are several choices of electrical resistance heating systems. These systems are typically installed for "spot heat", usually in spaces of under one hundred square feet.
31. Should I install more than one heating system?
Many growers are installing multiple heating systems in an attempt to "get the best of all worlds". For example, on temperate nights, they can rely on a bench top system only, and on the cold nights unit heaters are utilized to carry the worst case load and melt snow. Others are installing Infrared systems in conjunction with bench top or floor heating systems.
32. So, which heating type is best for me?
Every type of heating equipment has its advantages and disadvantages. Depending on the type of production you are planning, you'll need to choose the style of system that integrates best with your production scheme. For example, growers that install mobile tray systems for benching cannot install a bench top heating system because the bench tops need to be free to move around. There are almost endless variations and combinations that can be utilized to create the best environment for your particular production. It's simply a matter of determining your needs and researching the best options with expert industry professionals to develop the best strategy for you.
33. How do I find out more about heating?
Contact the NGMA for your free copy of the GREENHOUSE HEATING EFFICIENCY DESIGN CONSIDERATIONS.
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