Chapter 2

Greenhouse Heating

Greenhouse Unit Heaters

Unit heaters are often referred to as forced-air heaters (See Figure 2.1). The unit heater has capacity to heat one bay or zone of the greenhouse. These heaters consist of three functional parts. Fuel is combusted in a firebox to provide heat. The heat is initially contained in the exhaust, which rises through the inside of a set of thin walled metal tubes on it way to the exhaust stack. The warm exhaust transfers heat to the cooler metal walls of the tubes. Much of the heat is removed from the exhaust by the time it reaches the stack through which it leaves the greenhouse. A fan in the back of the unit heater draws in greenhouse air, passing it over the exterior side of the tubes and then out from the heater to the greenhouse environment again.

Types of Unit Heaters

There are two main types of unit heaters that are used for space heating in greenhouses: vented and unvented. The traditional vented, gas-fired unit heater transfers heat from the combustion of gases to the air through a heat exchanger, and exhausts the combustion gases outside the greenhouse through a flue pipe. An unvented unit heater burns the gas and exhausts all combustion gases directly into the greenhouse, so virtually all the heat from the fuel is used to heat the air.

Vented Unit Heaters

There are four types of vented unit heaters: gravity-vented, power-vented, separated-combustion, and high-efficiency condensing heaters. Gravity-vented, power-vented, and separated-combustion heaters have thermal efficiencies of 80 percent.

Unvented Unit Heaters

Direct unvented gas fired heaters do not vent hot flue products out of the greenhouse; instead, a direct-fired heater expels all heated combustion products into the greenhouse. Some unvented heaters are designed with a fresh-air intake duct so that air inside the greenhouse is not used for combustion. This increases the heater efficiency to about 90 percent. Unvented heaters without air intake ducts must have the heater interlocked with an exhaust fan and inlet louver so that fresh air enters the greenhouse when the heater is firing.

Fresh-air Vents

The unit heater must have a sufficient supply of fresh air to prevent oxygen starvation and maximize heater performance. Combustion problems caused by the lack of oxygen can be solved by providing a fresh-air vent from the outside to the vicinity of the burner. The smell of flue gases inside a greenhouse, which typically happens on cold nights when many heaters are running at once, is a sign of insufficient air inlets to replace the air combusted by the heaters.

Sizing Unit Heating Systems

The heating system must be able to supply enough heat to offset the heat lost to the outside. Heater size will depend on the surface area of the greenhouse, the type of covering material, wind speed and the difference between outside temperature and the inside growing temperature. Greenhouse heaters are normally rated on their input capacity in BTUs per hour, not the number of BTUs the unit is capable of putting out.

Distribution of Heat

Unit heaters are often placed at the gable end of the greenhouse with the heated air directed toward the middle of the house through one or more short ducts. Heat distribution with these heaters tends to be highly non-uniform which generally leads to higher energy requirements. Louvers on the unit heater can be positioned to direct some of the heated air downward to try to reduce immediate heat stratification.

Polyethylene Tubing

The polyethylene tube is connected to the air outlet of the unit heater to improve the distribution of heat (See Figure 2.2). The polyethylene tube is installed along the length of the greenhouse above plant height and is sealed at the distant end. The tubing can also be installed under the benches. The tubing is from 18 to 24 inches (45 to 60 cm) in diameter and has small holes, 1.5 to 2 inches (4 to 6 cm) in diameter, spaced every 2 feet (50 cm) along the length. Warm air from the heater moves through the tube and out the side holes.

Horizontal Airflow Fans (HAF)

In recent years, the use of perforated polyethylene tubing for improving air circulation has decreased. Polyethylene tubing installed above the crops collects dirt and debris and casts shadows. An alternative solution to polyethylene tubing is the use of horizontal airflow (HAF) fans (See Figure 2.3).

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