This scheme (above) is similar to general venting except the blower is larger, sized to vent the heat from the HID light. A commercial grade blower pulls hot, moist air from the top of the space and exhausts it outside. Air enters from a large opening in the bottom of the space. Note that no air can be exhausted from an airtight space, and too small an air inlet restricts the amount of air exhausted by the blower. Since the blower is larger than what is needed to provide fresh air for the plants, it is actuated by a controller. This controller is a combination humidistat and thermostat. If the temperature or humidity rises above set levels, the blower is activated, rapidly venting air from the space. Although the blower may operate quite often, set the temperature / humidity so the blower operates at least every 30-60 minutes. This is to provide fresh air for the plants. An oscillating fan keeps the air among the plants fresh. A surge strip is plugged into the light timer, protecting the light from voltage surges and allows one timer to control the light and oscillating fan. The ballast is set off the floor in case water is spilled.
If you find the blower does not operate often enough to provide fresh air for the plants, a better method would be to combine general venting with this blower. A small blower runs whenever the light is on, exchanging the air every five minutes. A more powerful blower sized to vent the HID heat is controlled by the thermostat / humidistat. Thus fresh air is continually provided for the plants and if the temperature or humidity rises too high, the powerful blower kicks on, rapidly venting the space.
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