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 rigidframe 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 singlelayer 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 straighteave leanto greenhouse can fit
under the roof of a singlestory 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
doublelayer 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 evenspan attached to a garage allows a larger greenhouse in a limited space. Freestanding 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 airmixing 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 windowmounted unit extends a house’s growing space. Figure 2. Different types of greenhouses allow many options. 3 and a twostage thermostat are needed to control the
operation.
A twospeed motor on low speed delivers about 70
percent of its full capacity. If the two fans have the same
capacity rating, then the lowspeed 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
16foot wide by 24foot 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
lowspeed 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 cubicfootperminute (ft3/min)
airmoving 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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 Spring '08
 Drake,D
 Botany, Thermodynamics, Heat, Central heating

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