Greenhouses. Planning A Home Greenhouse

A piece of plastic pipe about the size of the flue

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Unformatted text preview: he flue pipe. A piece of plastic pipe about the size of the flue pipe through the outside cover to a point near the burner combustion air intake would be adequate. This ensures adequate air for combustion in an airtight greenhouse. Unvented heaters (no chimney) using propane gas or kerosene are not recommended. Example. If a rigid-frame or post and rafter freestanding greenhouse 16 feet wide by 24 feet long, 12 feet high at the ridge, with 6 feet sidewalls, is covered with single-layer glass from the ground to the ridge, what size gas heater would be needed to maintain 60 °F on the coldest winter night (0 °F)? Calculate the total outside area (Figure 4): two long sides two ends roof gable ends A straight-eave lean-to greenhouse can fit under the roof of a single-story house. 6 ' 2' 4 1. A is the total exposed (outside) area of the greenhouse sides, ends, and roof in square feet (ft2). On a quonset, the sides and roof are one unit; measure the length of the curved rafter (ground to ground) and multiply by the length of the house. The curved end area is 2 (ends) × 2/3 × height × width. Add the sum of the first calculation with that of the second. 2. u is the heat loss factor that quantifies the rate at which heat energy flows out of the greenhouse. For example, a single cover of plastic or glass has a value of 1.2 Btu/h × ft2 × °F (heat loss in Btu’s per hour per each square foot of area per degree in Fahrenheit); a double-layer cover has a value of 0.8 Btu/h × ft2 × °F. A table of u values is provided in Extension Bulletin 351 Greenhouse Heating, Circulation, and Ventilation Systems. The values allow for some air infiltration but are based on the assumption that the greenhouse is fairly airtight. 3. (Ti – To) is the maximum temperature difference between the lowest outside temperature (To) in your region and the temperature to be maintained in the greenhouse (Ti). For example, the maximum difference will usually occur in the early morning with the occurrence of a 0 °F to –5 °F outside temperature while a 60 °F inside temperature is maintained. Plan for a temperature differential of 60 to 65 °F. The following equation summarizes this description: Q = A × u × (Ti – To). 2 × 6 ft × 24 ft = 288 ft2 2 × 6 ft × 16 ft = 192 ft2 2 × 10 ft × 24 ft = 480 ft2 2 × 6 ft × 8 ft = 96 ft2 A = 1,056 ft2 Select the proper heat loss factor, u = 1.2 Btu/h × ft2 × ° F. The temperature differential is 60 °F – 0 ºF = 60 °F. Q = 1,056 × 1.2 × 60 = 76,032 Btu/h (furnace output). An even-span attached to a garage allows a larger greenhouse in a limited space. Free-standing greenhouses allow more location choices and can be larger than attached greenhouses. Air Circulation Installing circulating fans in your greenhouse is a good investment. During the winter when the greenhouse is heated, you need to maintain air circulation so that temperatures remain uniform throughout the greenhouse. Without air-mixing fans, the warm air rises to the top and cool air settles around the plants on the floor. Although this is a relatively small greenhouse, the furnace output is equivalent to that in a small residence such as a townhouse. The actual furnace rated capacity takes into account the efficiency of the furnace and is called the furnace input fuel rating. 6 A window-mounted unit extends a house’s growing space. Figure 2. Different types of greenhouses allow many options. 3 and a two-stage thermostat are needed to control the operation. A two-speed motor on low speed delivers about 70 percent of its full capacity. If the two fans have the same capacity rating, then the low-speed fan supplies about 35 percent of the combined total. This rate of ventilation is reasonable for the winter. In spring, the fan operates on high speed. In summer, both fans operate on high speed. Refer to the earlier example of a small greenhouse. A 16-foot wide by 24-foot long house would need an estimated ft3 per minute (cubic feet per minute; CFM) total capacity; that is, 16 × 24 × 12 ft3 per minute. For use all year, select two fans to deliver 2,300 ft3 per minute each, one fan to have two speeds so that the low-speed rating is about 1,600 ft3 per minute and the high speed is 2,300 ft3 per minute. Adding the second fan, the third ventilation rate is the sum of both fans on high speed, or 4,600 ft3 per minute. Some glass greenhouses are sold with a manual ridge vent, even when a mechanical system is specified. The manual system can be a backup system, but it does not take the place of a motorized louver. Do not take shortcuts in developing an automatic control system. Small fans with a cubic-foot-per-minute (ft3/min) air-moving capacity equal to one quarter of the air volume of the greenhouse are sufficient. For small greenhouses (less than 60 feet long), place the fans in diagonally opposite corners but out from the ends and sides. The goal is to develop a circular (oval) pattern of air movement. Operate the fans continuously during the winter. T...
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This note was uploaded on 03/27/2013 for the course BOT 101 taught by Professor Drake,d during the Spring '08 term at University of Hawaii, Manoa.

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