Chapter 5

Greenhouse Heating

Greenhouse Central Heating Systems

Central heating systems are typically used for large greenhouse operations. Central heat systems for greenhouses consist of a central boiler. Boilers are pressure vessels designed to heat water or produce steam, which can then be used to provide space heating and/or service water heating to a building. Hot water systems offer the advantage of forward compatibility. Water is a great medium for transporting and delivering heating energy. Per volume, water can carry 3,500 times more energy than air can. The reason water is the heating medium of choice for so many growers is because of its inherent ability to be managed. Hot water can transport tremendous heating energy over great distances with very little loss of efficiency. Once at its destination, it can be routed to myriad heating projects, mixed down to any temperature, exchanged and separated in a heat exchanger without losing much efficiency, and used in floor heating, bench heating, top heating, perimeter heating, snow and ice removal, office heating, irrigation water warming, etc.

Advantages and Disadvantages of Heating Plant Boilers

There are a number of advantages to the central heating system: (1) a central plant offers greater flexibility to use alternative energy sources; (2) it uses less greenhouse space since the central plant is located in a separate building; (3) partial load performance might be much more efficient; (4) maintenance and control is easier and cheaper; and (5) since combustion is done outside the greenhouse, improper combustion does not increase the probability of damaging the crop due to toxic flue gases (e.g., ethylene).

Boiler Components

Both gas and oil-fired boilers use controlled combustion of the fuel to heat water. The key boiler components involved in this process are the burner, combustion chamber, heat exchanger, and controls. Hot water produced by a boiler is pumped through pipes and delivered to the greenhouse. Steam boilers produce steam that flows through pipes from areas of high pressure to areas of low pressure, unaided by an external energy source such as a pump. Insulated pipes and manifolds deliver the water or steam to the zone being heated, normally located under the benches and along the side and end walls along the greenhouse perimeter.

Types of Heating Plant Boilers

Boilers are classified into different types based on their working pressure and temperature, fuel type, draft method, size and capacity, and whether they condense the water vapor in the combustion gases. Boilers are also sometimes described by their key components, such as heat exchanger materials or tube design. There are two primary types of boilers types that you can choose from for your specific application; fire-tube and water-tube boilers (Figure 5.3).

Fire-Tube Boilers

In a fire-tube boiler, the flue gas travels through the tubes, which are surrounded by hot water. As the heated air moves through the cold water by way of the tubes, the water is heated. Fire-tube boilers are often characterized by their number of passes, referring to the number of times the combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water.

Wate-Tube Boilers

In a water-tube boiler, water travels through tubes that are surrounded by the by-products of combustion, or flue gas. Water-tube boilers are safer by design and generally can operate many years longer than fire-tube boilers. Water-tube boilers are available in larger capacities and recover faster than fire-tube boilers.

Copper-Fin Tube Boilers. Copper-fin tube boilers is a newer low-mass boiler, which generally contains copper fin tubes. It is also known as a compact boiler. The compact boiler burns only natural gas or manufactured gas since soot from oil or coal could plug the narrow spaces between the fins of the heat-exchanging copper tubes. The compact boiler is typically cheaper to purchase and occupies considerably less space.

Condensing Boilers

Condensing boilers are the most common boilers used for greenhouses with central heating systems (Figure 5.4). There are two types of condensing boilers which are fire-tube boilers and water-tube boilers. A condensing boiler works in the same way a conventional non-condensing fire-tube or water-tube boiler works but with extra functions added. This is in contrast with the conventional non-condensing boiler, which allows water vapor, a byproduct of the combustion of gas or oil, to escape up the stack along with the other byproducts to the atmosphere.

Heat Distribution from a Boiler System

Hot water or steam leaves the boiler and is pumped into the greenhouse via a pipe network or unit heaters, or both. The pipes are typically located overhead and above the crop, or alternatively, they may be located beneath the benches and on the perimeter walls or in a combination of each. Floor heating is another method to heat the greenhouse with hot water. Hot water unit heaters are another option for delivering heat into the greenhouse.

Perimeter Heating Systems

Perimeter piping systems can be used in cold weather climates to protect plants growing near side and end walls to provide supplemental heat and contribute to a uniform thermal environment throughout the greenhouse (Figure 5.5). Perimeter heating can be integrated into existing zone controls or can be controlled as a stand-alone system if extra perimeter heat is desired at night when a heat retention curtain is in use.

Bench Heating Systems

Bench heating systems can either be under-the-bench or bench-top heating. An under-bench hydronic system uses a low-output finned tube (such as Delta FinTM TF or DuoFin) or a standard pipe to provide heat under the plants. Placing a skirt around the bench will help keep the heat under the bench and allow cooler air temperatures in the rest of the greenhouse (Figure 5.6). Bench-top water heating systems distribute the heat via a mat or tubing placed on top of the bench.

Floor Heating Systems

Many greenhouses today are constructed with a heated floor (Figure 5.7). As mentioned, different heating systems are available to heat greenhouses, from hot air to hot water and radiant systems. The challenge of all these systems is to provide heat in the right amount at the right location and as uniformly as possible at a reasonable cost. Generally, hot-water floor heating systems can provide the most uniform heat throughout the crop canopy.

Overhead Heating Systems

Overhead heating is important in areas with cold nighttime temperatures. While providing additional heat the overhead heating system can also be integrated for use with hanging basket systems and monorail and bi-rail cart setups. An overhead system with snow melt protection is the perfect solution for northern climates that get accumulated snowfall.

Hot Water (Hydronic) Unit Heaters

This style of unit heater is used in conjunction with a boiler system or other hot water or steam sources. They extract heat from the water or steam and deliver the heated air to the greenhouse. Hot water unit heaters are very reliable and are inexpensive. Hydroponic unit heaters are often used to supplement other hydroponic heating systems, like a heated floor system or benchtop heating system. They are quick and very responsive to control inputs.

Click on the following topics for more information on greenhouse heating.