Heating and Cooling

Heating

Maintaining a constant temperature and protecting against cold snaps and frost is crucial for successful gardening.  In cold, clear or windy weather your nursery may need more heat than is provided by solar radiation; especially at night.  Heat can be supplied using electricity, natural gas or propane, fuel oil, solar energy or kerosene.  Your greenhouse can also be connected to your home heating system. Natural gas or propane is one of the most cost-effective ways of maintaining a constant temperature in your nursery. If natural gas is available in the home, plumbing into the existing line saves considerable cost over a new meter and gas line. If natural gas is not available, check with a local propane gas company for cost and availability. Determine if the company provides a storage tank free or if it can be rented or purchased. Consult the company to determine the tank size appropriate for the greenhouse.

A variety of gas unit heaters are available to heat hobby greenhouses. Unit heaters burn gas in a firebox, the heated air rises through the inside of a thin-walled heat exchanger on the way to the exhaust chimney. A fan draws air in from outside, across the heat exchanger, and out into the greenhouse. Thus, most of the heat is removed from the exhaust before it exits the structure. The exhaust chimney must be sufficiently tall to maintain an upward draft and extend above the greenhouse roof. An 8- to 12-foot chimney is usually sufficient. All open-flame heaters must be vented to the outside and given a fresh air supply for complete combustion. Be sure to provide fresh air circulation to prevent carbon-dioxide build up.

In larger greenhouses, a plastic tube or ducting system may be needed to distribute the heat evenly within the house. The system is made of a perforated polyethylene tube or plastic and wire ducting, suspended overhead in the ridge and extending the length of the greenhouse. A fan connected to the tube blows warm air from the heater into the tube for greenhouse distribution. This system can also be used for circulating internal air when heating or cooling is not required.

A variety of electric heaters can also be used. Those available in department stores and home centers are usually only adequate in the smallest greenhouse for starting seedlings in the spring. Larger units can be purchased, but operating them can be costly.

Heater size for a given greenhouse and geographic location depends on the surface area of the greenhouse and the temperature difference between the inside and outside of the greenhouse. To determine the size heating system you need, calculate the total surface area of the greenhouse covering. Then determine the difference between the minimum expected outside temperature during winter in your area and the maximum night temperature you wish to maintain. Multiply the greenhouse surface area by the temperature difference by the heat loss conversion factor in Table 1 for the appropriate covering. The answer will be in BTUs per hour. Most heaters are rated in this unit. Many greenhouse supply or construction companies can help you determine the proper size heater for your situation.

 Cooling

One of the best ways to cool a greenhouse in the summer is to reduce the amount of light in the nursery. The reduction depends on the heat load in the greenhouse and the vegetation’s light requirements.  Greenhouse whitewash, shade cloth, screens of wood or aluminum, or Venetian blinds can be used for shading. Greenhouse whitewash is a special kind of latex paint that is diluted in water and sprayed on the covering surface. It is designed to be applied in the spring and gradually degrade by the action of rain and sun so that little remains by fall. Shade cloth is a black, green, or white woven fabric of polypropylene, cloth or aluminum weave that is laid over the outside of the covering. Shade cloth can be purchased with various weave densities that result in 20 to 80 percent light reduction. For most hobby greenhouse applications, 30 to 50 percent light reduction should be sufficient.

Another method for cooling a greenhouse is evaporative cooling, which relies on air passing through a porous pad saturated with water. The evaporating water removes heat from the greenhouse. There are two types of evaporative coolers: fan-and-pad systems and misting systems. Fan-and-pad systems consist of a cellulose pad at least 2 feet tall and extending the length of one wall, with water supplied from the top to keep the pad wet during operation. Fans are installed in the opposite wall to draw outside air through the pads.  Misting systems can serve two functions, the help cool the greenhouse by extruding clouds of mist that evaporates and cools the greenhouse space.  At the same time they keep humidity levels up for vegetation that likes a wetter climate.  They usually consist of irrigation hose with mister emitters that are controlled by an actuated valve controlled by a thermostat.