Chapter 5

Greenhouse Heating

Greenhouse Unit Heaters

Unit heaters are often referred to as forced-air heaters (Figure 5.1) and are the most commonly used greenhouse heating equipment. The low initial investment for the unit heater system is suitable for greenhouse operations that start small and expand steadily, purchasing heaters as needed. The unit heater has capacity to heat one bay or zone of the greenhouse. 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. Heater output capacity will vary according to the efficiency of the unit. Input rating and heater efficiency must be taken into consideration when selecting the proper heater size.

Types of Unit Heaters

Unit heaters are either mounted overhead from the greenhouse structure at roof level or on the floor, depending on the type of unit preferred. If mounted overhead they are available in either horizontal or vertical configurations.

Vented versus Unvented 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

Vented gas heaters are preferred for greenhouses because they provide a cleaner, less humid and safer environment for both the growing plants and humans working in the greenhouse. They are usually located inside the greenhouse and the vent pipe, or exhaust stack, carries the products of combustion to the outside. Some heat will escape through the vent pipe and be lost to the outside too. The exhaust stack on unit heaters must be sufficiently tall to develop an updraft to draw fumes out of the heater and must be high enough above the greenhouse roof to permit dissipation of the smoke without reentry into the greenhouse.

Unvented Unit Heaters

Unvented gas heaters, such as those that burn LP gas, are located inside the greenhouse and have no vent pipe to carry the products of combustion to the outside. Since there is no heat loss through the stack, unvented heaters are often advertised as nearly 100 percent efficient. Although they are more efficient than vented gas heaters, growers must consider the additional buildup of condensation and the potential for air pollution problems inside the greenhouse. All greenhouses experience some condensation; however, greenhouses with unvented gas heaters experience especially high levels of condensation.

Unit Heater Performance

The performance of unit heaters is affected by fresh air, the heat exchanger material, and the distribution of heat in the greenhouse.

Fresh-Air Vents

Each 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.

Heat Exchanger Material

The standard heat exchanger material for unit heaters is aluminized steel, while the optional material is stainless-steel. In high-humidity environments such as greenhouses, the aluminized steel heat exchangers have reduced life spans. Stainless-steel heat exchangers have longer life spans.

Heated Air Distribution

The heat distribution location in a greenhouse can decrease total energy usage and increase growth and yields at the same time. Forced air under-bench heating is an option by connecting a unit heater with a blower (instead of a propeller fan) to a duct or poly tube positioned under the growing benches. Installing skirts around the sides of the bench will help keep the heat under the bench. If floor or under-bench heating is not a viable option, using poly tube overhead and horizontal airflow fans can help provide more uniform air temperature distribution.

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