Ventilation is a key consideration especially when growing indoors under artificial lighting. The combination of lighting and plants generates excessive amounts of heat and humidity which will harm plants if not properly controlled.
Equipment
Exhaust fan
An inline exhaust fan should be capable of replacing the volume of air in the room in well under 5 minutes. Consult your growshop for the best fan size and type. You will need to account for the room size, configuration of ducting (diameter, length, bends), inlet and outlet filters, the maximum temperature of incoming air and total wattage of all electrical devices in the grow room. Inline fans are generally more effective at pushing air through ducting, as opposed to pulling air. Therefore, especially for long sections of ducting, position the fan at the inlet end, not the outlet (Fig 4.1).
Inlet fan
This is used for pushing air into the room and helps to maximize the effectiveness and lifespan of the exhaust fan. To ensure the exhaust fan’s output is not wasted, use an inlet fan of equivalent airflow capacity. With the aid of ducting, you can choose where the incoming air comes from. 
For example, in hot or cold weather it would be better to draw air from an air-conditioned room instead of from outdoors. However, either way, be careful that the inlet air is fresh and not sourced from the same area where the exhaust air is dumped. Where the incoming air enters the room, do not have it blowing directly onto plants, especially if its temperature is extreme.
Ducting
For maximum fan efficiency keep ducting as short and straight as possible. Where junctions are needed employ ‘Y’ junctions instead of ‘T’s.
Oscillating fan
Usually operates 24 hours a day to ensure air is always distributed evenly throughout the room (Fig 4.1). This eliminates ‘hot spots’ i.e. zones prone to CO2 depletion or excessive humidity and temperature that are most likely to occur nearest lights and dense foliage. Ensure the fan is blowing air across the top of the canopy that is located immediately below the lamp.
Thermometers and hygrometers
Position the probe in the place of highest temperature or humidity. This is typically directly beneath the lights and amongst the foliage. Where multiple lamps are being used it is best to have a dedicated meter for each lamp.
Growroom height
It is beneficial to have a minimum gap of 1 meter (3 ft) between the lamp shade and the ceiling. Because hot air rises, this space helps shift the maximum temperature further from the plants.
Location of inlet, exhaust and oscillating fans
As a general rule, try to keep the air moving in one direction as this creates and maintains air momentum. Locate the ‘inlet’ and ‘exhaust’ at opposite ends of the room, with an oscillating fan maintaining the general airflow direction provided by the inlet (Fig 4.1). For wider rooms especially, employ multiple inlets and exhausts, and space them evenly across the width of the room. This will help ensure all air is replaced. For best airflow, keep objects at least 1 meter (3 ft) away from the fan inlet.
Switching fans “on” and “off”
“Automated” fan switch
Thermostats and humidistats are useful for activating both the inlet and the exhaust fan (Fig 4.1). Position the thermostat’s sensor at the hottest point in the foliage, and the humidistat’s sensor at the most humid point. Since the optimum night-time temperature is different to that of day, the controller will need to handle two lots of settings.
“Manual” fan switch
A modest degree of control can be gained using a timer – a surprisingly effective aid especially if the weather is consistent or predictable. Control gear such as thermostats, are often not essential in extremely hot or humid climates because it is usually necessary to have fans running constantly anyway.
A fan speed controller is a useful addition where the fan’s flow rate is too high during the colder or less humid months.
Hot climates
If heat is too difficult to control:
1. Shift the ‘lights on’ period to night.
2. Air-condition the incoming air.
3. Employ air cooled shades.
4. Where the “hot days” are infrequent and the ventilation system is otherwise satisfactory, turn some (or all) lights off during those hot days.
© Andrew M Taylor (FloraMax)