Section 3: Winter Cooling Systems
In some locations, high light levels (high solar input) or fluctuating temperatures might necessitate cooling even during winter days. Additionally, during spring and fall months, heating may be required at night and early mornings while cooling may be required during the day when solar loads are high. Passive venting as discussed above is one method that may be used for this type of cooling need. However, if the solar load is too high, an active cooling system may be required to increase the rate at which warm inside air is replaced with cool air from outside of the greenhouse. In this situation, the roof vent may be closed and fans in the greenhouse walls activated. Louvered vents in the opposite walls open to allow air to move into the greenhouse. The exhaust fans may be multi-speed fans so that just enough air is exhausted from the greenhouse (and replaced with cooler outside air) to maintain the desired temperature. If temperatures continue to increase, the fans' speed can be increased.
Another method used for winter cooling utilizes fans placed in the gable of the greenhouse and combined with a polyethylene tube extended the length of the greenhouse in the gable. The inlet vent is louvered and opens only when the fans turn on. There is an additional set of louvered vents at the opposite end of the greenhouse that allows warm greenhouse air to escape while cooler outside air is forced into the structure. The cool air is forced through the polyethylene tube to allow for a more even distribution of the cool air. In some cases, a unit heater may be used in conjunction with this system. The unit heater is placed in front of the louvered inlet. If heat is needed, the louver and vents are closed. The heater draws in air from within the greenhouse, heats it and forces the warm air through the polyethylene distribution tube. If cooling is needed, the louvered vent and exhaust vent open, and the unit heater fan turns (without ignition of the flame) on to force cool air from outside into the polyethylene tube and into the greenhouse.
