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Greenhouse Unit Heaters: Types, Placement and Efficiency

By Scott Sanford

The Tomato Magazine
June 2006

Unit Heater Types
Unit heaters for greenhouses typically use propane or natural gas fuel but may also be fueled with heating oil. They are popular because of their low capital and installation cost, high reliability, and the ability to stage operation of multiple 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 gases to the air through a heat exchanger, and the combustion gases are exhausted through a flue pipe outside the greenhouse. 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. Let’s take a look at each of these technologies.

Vented Unit Heaters
There are four types of vented unit heaters available: gravity-vented, power-vented, separated combustion and high efficiency condensing heaters. Gravity-vented, power-vented and separated combustion heaters have thermal efficiencies of 80 to 82 percent (energy efficiency at a snapshot in time); however their seasonal efficiencies (efficiencies that account for heated air being lost out the vent pipe or used as combustion air over the heating season) range from 65 to 80 percent due to various losses as discussed later. High efficiency condensing heaters have thermal efficiencies greater than 90 percent.

Gravity-vented Unit Heaters
Gravity-vented unit heaters rely on thermal buoyancy and draw from wind blowing past the vent pipe to exhaust the flue gases. These heaters are typically rated at 80 percent thermal efficiency; however the seasonal or overall efficiency drops to 65 percent because heated air from inside the greenhouse is continuously being lost through the exhaust pipe to the outside. These heaters use inside air for combustion, which will account for about two percent of the loss, and some heaters use continuous pilot lights that will consume a small amount of additional energy.

Power-vented Unit Heaters
A power-vented unit heater has a small fan that operates only when the heater unit is firing to meter the correct amount of air for combustion and exhaust the flue gases. This type of heater uses a smaller exhaust pipe that can be run horizontally through the wall of the greenhouse which reduces installation costs and acts like a vent damper to minimize thermal buoyancy losses. Gas-fired, power-vented heaters often use an intermittent or electronic pilot which reduces flameouts and pilot gas use. The seasonal efficiency of a power-vented unit heater is typically about 78 percent compared to its thermal efficiency of 80 percent.

Separated Combustion Unit Heaters
The separated combustion heater is designed for heating areas that may contain flammable materials, high humidity or corrosive environments. These heaters use a power-vented exhaust and a separate air intake duct for combustion air. Modern plastic greenhouses have low infiltration rates, so it is possible during times of peak heating to use up enough oxygen in the greenhouse to cause poor combustion or have flue gases being drawn into the greenhouse through the flue pipes. Back drafts of flue gases can be a problem if a greenhouse is located in a windy area or if exhaust fans and heaters are inadvertently used at the same time. Flue gases can affect plant and human health as will be discussed later. Separated combustion units eliminate these problems. The thermal efficiency of a standard separated combustion heater is 82 percent with a seasonal efficiency of 80 percent.

High Efficiency Condensing Unit Heaters
Several manufacturers make high efficiency unit heaters that have thermal efficiencies exceeding 90 percent to get every Btu possible out of the fuel with a vented heater. These heaters condense some of the moisture out of the flue gas to squeeze out more energy. This technology has been around for many years and is used in most residential furnaces sold today. These heaters use a power-vented exhaust and a separate air intake, so heated greenhouse air is not used for combustion. These unit heaters also need a drain or a way to dispose of the acidic condensate. Thermal efficiencies are typically between 90 and 95 percent with seasonal efficiencies of about two percent less. These heaters are more expensive but provide more heated air per unit of fuel.

Unvented Heaters
Small, unvented heaters have been used for carbon dioxide (CO2) enrichment in greenhouses, but high capacity direct fire heaters are also available for heating. Because these units are unvented, 99 percent of the energy is transferred into the greenhouse for heating (92 percent seasonal efficiency), which is appealing in a climate of increasing fuel costs. Unvented heaters are designed with a fresh-air intake duct so air inside the greenhouse is not used for combustion, or the heater interlocks with an exhaust fan and inlet louver so fresh air enters the greenhouse when the heater is firing. Along with high energy efficiency and the addition of CO2 come some disadvantages. One of the byproducts of burning propane or natural gas is water vapor, which may increase the potential for fungal disease among certain plant varieties and block sunlight if the water vapor condenses on the glazing. For every gallon of LP gas that is burned, approximately 11/2 pounds of water are added to the air. For a 250,000 BTU/hr heater, almost four pounds of water would be added to the air per hour of heater operation.

Other by-products of combustion (e.g. ethylene, sulfur dioxide, nitrous oxide and carbon monoxide) can be harmful to plants and humans depending on concentration. Kerosene and fuel oil generally produce these gases in higher quantities than propane and natural gas. Colorless and odorless ethylene gas can seriously affect plants in very low concentrations, and the subtle effects may not show until long after the ethylene was released. Tomatoes, cucumbers, lettuce, melons, peppers, tobacco, flowers and bedding plants are all susceptible to ethylene gas. Sulfur dioxide exposure can cause leaf curling and/or necrotic spots on leaves. Malfunctioning vented heaters can also cause problems with ethylene and sulfur dioxide due to flue gases entering the greenhouse. If unvented heaters are installed for high energy efficiency and CO2 enrichment, the plants will likely not get full utilization of the CO2 because 80 percent of the heating in greenhouses occurs at night when the plants are not using CO2. In a greenhouse with multiple heaters, it may be effective to have one unvented heater for CO2 enrichment and heating with the other heaters being vented. The unvented unit would be used as the primary heater during daytime hours when plants are using CO2, while the vented heaters would be the primary heaters at night.

Portable Unit Heaters
Portable units are often used for temporary or emergency heating and operate on kerosene, heating oil or LP gas. These unvented units are not designed to be operated in enclosed structures because they do not have intake air venting. If using portable units for emergency or temporary greenhouse heating, use only LP gas-fired units and open a vent or prop open a door to replace the oxygen burned by the heater.

Performance of Unit Heaters

Combustion Air Inlet Size for Heaters
As pointed out earlier, modern plastic greenhouses are tightly constructed with fewer seams than glass greenhouses and therefore have lower infiltration losses (cold air that leaks into the greenhouse and must be heated and heated air leaking out). The smell of flue gases inside the greenhouse, which typically happens on a cold night when many heaters are running at once, is a sign of insufficient air inlets to replace air combusted by the heaters. A solution is to provide a fresh-air duct to each heater to insure ample air and complete combustion. The general recommendation is one inch of inlet opening per 2,000 to 2,500 BTU/hr of heating capacity but always follow manufacturer’s recommendations. The fresh-air vent should ideally be positioned within 12 inches of the combustion unit and should be outfitted with a power damper so it is open only when the heater is operating. Based on this recommendation, a 250,000 BTU/hr rated heater would require a vent opening of 100 to 125 inches or equivalent to approximately a 12-inch diameter pipe.

Unit Heater Heat Exchanger Material
Because of the high humidity environment of a greenhouse, the heat exchanger material can affect the unit’s life span and warranty. The standard heat exchanger material for unit heaters is aluminized steel, while the optional material is stainless steel. In high humidity environments like greenhouses, the aluminized steel heat exchangers have reduced life spans. For greenhouses, stainless steel heat exchangers are recommended, and the manufacturer’s warranties reflect that. The standard warranty from a leading manufacturer is 10 years for an aluminized steel heat exchanger, but if used in a high humidity environment like a greenhouse, the warranty is reduced to one year. Under the same conditions, the manufacturer provides a 10-year warranty on a stainless steel heat exchanger. The cost of a stainless steel heat exchanger for a 250,000 BTU/hr unit heater is about $450 more than an aluminized steel unit, but purchasing a stainless steel heat exchanger extends the unit’s life span by nine years.

Heated Air Distribution
The heat distribution system location in a greenhouse can decrease total energy usage and increase growth and yields at the same time. Using floor heating and growing plants on the floor can save 20 to 30 percent in heating costs. If a hydronic floor heating system is used, unit heaters will still be needed; on very cold nights, a heated floor system will not be able to keep up with peak greenhouse heat losses. Using under-bench heating with forced air can provide similar savings. Heat rises, so heat supplied into the greenhouse above the plants will not drop to the plant zone until the air above the plants is completely heated. When the heat is discharged overhead, more air volume must be heated in order to warm the plants than if the heat is distributed under the plants. When plants are grown on a heated floor or bench, a micro-environment is formed, warming the plants and the immediate surroundings—but not the entire greenhouse—to the desired growing temperature. A unit heater with a blower connected to a duct or poly-tube positioned under the growing benches can be used to provide an effect similar to floor heating. With floor or under-bench heating, the greenhouse air temperature at six feet can be reduced by 5°F or more and still maintain proper growing temperature at the plant level. Studies of floor heating have reported increased production, faster root growth, and reduced disease pressure, along with energy savings. A study of greenhouse tomatoes at Louisiana State University showed a 28 percent savings in fuel and a seven percent increase in yields with heated air distributed with poly-tubes at the floor level between the rows of tomatoes. If bottom heating is not a viable option, the use of poly-tube overhead and horizontal air flow fans can aid in providing more uniform air temperature distribution. Horizontal stirring fans are typically used to push heat down to the plant level, mix the air to reduce temperature stratification, eliminate dead air spots, and reduce condensation formation on the glazing.

Case study:
Vented Heater Type Versus Heating Cost

For this example, a 30’ x 96’ greenhouse with a double polyethylene infrared inhibited cover and double wall polycarbonate end walls was used. The greenhouse has average infiltration losses of .75 air changes per hour and is heated with two 250,000 BTU/hr rated LP gas unit heaters with propeller fans. The growing season lasts from Feb. 1 through June 1 for Madison, Wis. All other costs and conditions are assumed the same except for heater costs and efficiencies. Table 1 shows the costs and efficiencies of four different types of vented unit heaters and the amount of LP gas required to heat the greenhouse based on the 30-year average weather data for Madison, Wis. The capital costs listed are the “manufacturer’s suggested retail price” (MSRP); in most cases, the units can be purchased for less than the MSRP. The installation costs are assumed to be the same for all heaters. Based on the cost of LP gas at $1.50/gallon (average price–July 2005), Table 2 lists the fuel consumption for each heater type, fuel savings and the simple payback, assuming that the fuel cost savings would pay for the incremental purchase cost of the higher efficiency heater.

Based on Table 2, one should always purchase a power-vented unit heater versus a gravity-vented unit heater due to the short payback from energy savings. The case study greenhouse can be heated with one heater on all but a few cold days when two heaters would be needed to meet the temperature specifications. A high-efficiency condensing combustion has a long payback if two heaters are used. But if one high-efficiency condensing combustion is used for the primary heater and a power-vented unit heater for the secondary heater for the few cold days when a single heater can’t keep up, the overall payback would be reduced to under four years.

Growers should evaluate the annual cost of ownership and not solely the purchase price when purchasing energy consuming equipment such as heaters. Based on the energy and capital costs presented, growers should always purchase power-vented heaters instead of gravity-vented heaters, because the payback is about one heating month based on the incremental cost difference and current energy prices. Stainless steel heat exchangers are recommended because of their longer expected life span (10 to 15 years versus just a few years for an aluminized steel heat exchanger). Moving the heat distribution under-bench or using floor heating could save an additional 20 percent or more in heating costs. Properly sized air inlets are important to provide heaters with enough oxygen for complete combustion. Energy prices are expected to continue to escalate due to increasing world energy consumption, so purchasing a 90 percent plus efficiency heater today, despite the longer payback period, may be one of the best long-term investments you can make for your business.

Thermal efficiency— Ratio of a heater’s heat output versus the heat content of the fuel input at an instant in time. A 250,000 BTU/hr rated heater with an 80 percent thermal efficiency will have a heat output of 200,000 BTU/hr (250,000 x 80 percent = 200,000).
Seasonal efficiency—The overall efficiency when all losses incurred by the heater (such as combusting already heated air, losses of heated air out of the flue/exhaust pipe when the heater is not firing, and losses from continuously lit pilots) are taken into account.
MSRP—Manufacturer’s Suggested Retail Price
LP gas—Liquid petroleum gas, a byproduct of the petroleum distillation industry. Its cost follows the world market price for oil.

Editor’s note: Scott Sanford is a senior outreach specialist with the Department of Biological Systems Engineering at the University of Wisconsin-Madison. The full text of his study, along with references, is available in bulletin form (A3784-15) from Cooperative Extension Publishing, 103 Extension Building, 432 N. Lake St., Madison, WI 53706 or call 608-262-2655. This study was funded in part by the Wisconsin Focus on Energy program. © 2006 by the Board of Regents of the University of Wisconsin System. Used with permission. To order multiple copies, call toll free: 1-877-WIS-PUBS (947-7827). To see more Cooperative Extension publications, visit: cecommerce.uwex.edu.

© 2006 Columbia Publishing

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