Heat Loss Factors--Typical Small Greenhouse

Ventilation system for greenhouses, mechanical.

Small fans can be used in the greenhouse to circulate the air continuously to maintain a more uniform temperature during the heating period.

The ventilation rate is calculated by multiplying the greenhouse floor area, in square feet, times 12 to give the fan capacity, in cubic feet per minute (CFM). Since there are pressure losses in moving air through the greenhouse, the fan should have the required capacity at a pressure rating of 1/8 inch of water static pressure (s.p.). Check your catalog for the pressure rating or ask the salesperson for assistance.

The number 12 is cubic feet per minute per square foot of floor area. The number is 1.5 to 2.0 times the average greenhouse height so the fan capacity is, in reality, 1.5 to 2.0 times the volume of the greenhouse. This volume is necessary to remove the summer heat load.

Vents at the ridge, side walls, and door can be used for natural convective air exchange. Using the principle that warm air rises, the vents are opened and closed manually or by motorized unit in response to the inside temperature. Cooler outside air enters to replace escaping warm inside air.

Shading devices over the greenhouse can reduce the light intensity and heat load in summer. Evaporative coolers can be used effectively where the temperature exceeds 90 F for more than 10 to 15 days a year. In dry areas of the country the inside temperature can be reduced 20 or more. In humid areas the temperature reduction is less. The evaporative cooler fan pulls air through a wet pad. Heat is removed from the air to evaporate the water, thus reducing the air temperature.

Big Investment

The greenhouse requires the greatest commitment of money. Shortcuts in equipping the greenhouse with proper heating and ventilating equipment can lead to disappointments. Consult local greenhouse owners before making your investment. A good manual for hotbeds, coldframes, and greenhouses is available from the Northeast Regional Agricultural Engineering Service for $2.00. Ask for Hobby Greenhouses and Other Structures, NRAES-2, from NRAES, Riley-Robb Hall, Cornell University, Ithaca, N.Y. 14853.

A propagation unit is used to start plants from seeds or cuttings by providing the environmental factors which encourage good growth. The unit typically has lights and may be equipped with a water misting system and heating cable or other means of providing bottom heat to the soil. A ventilation fan is often needed to remove lamp heat.

Good controls are needed to do the job right. Time clocks can control the lighting and misting cycles. Thermostats can be used to control the air and soil temperatures.

Typically, people start vegetable seed in their homes under fluorescent lamps. The plant containers may be wooden or plastic boxes. To maintain a high humidity, the propagating containers are enclosed by a plastic cover. Heat may be supplied by the room air temperature or by locating the propagating unit over a radiator or other heat source. This simple arrangement works well for the once-a year spring project if you exercise Care.

Young vegetable plants grow best with a lot of light, and a minimum of 1,000 foot-candles is desirable. Good light can be supplied by 40-watt cool white fluorescent lamps spaced 6 inches apart and hung a few inches above the plants. A height equal to the spacing between lamps will give fairly uniform lighting.