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Controlling and optimizing ventilation systems

Switching fans on & off

“Automated” fan switch:  Thermostats and humidistats are useful for activating both the inlet and the exhaust fan (Fig 5.7). Position the thermostat’s sensor at the hottest point in the foliage, and the humidistat’s sensor at the most humid point. Determine these points using a thermometer and hygrometer. For “day time” (lights-on) they should be set to activate the fans when either the temperature exceeds ~27oC (80oF), or the humidity exceeds 70% RH.

At “night time”, the temperature should be allowed to drop below 20oC (68oF), but no further than 15oC (59oF). To achieve this, you will need a controller that allows separate settings for both day and night. Alternatively, install a separate, dedicated set of fans for both day and night. An ‘axial’ fan in the ceiling (controlled with or without a timer) may be sufficient to produce the correct night time conditions.

“Manual” fan switch:  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. However, the above mentioned temperature and humidity targets still apply. A modest degree of control can be gained using a timer - a surprisingly effective aid especially if the weather is consistent or predictable.

We strongly suggest speaking to your local growshop when designing your ventilation system. A system which is perfect for one particular climate may be useless in another.

Ventilation for extreme climates

Hot climates

Consider the following options if the grow room temperature is too high:

1. Shift the ‘lights on’ period to night.

2. Air-condition the incoming air.

3. Reduce the amount of lighting and/or plants.

4. Employ lights or shades that are air cooled (Fig 4.5).

5. Where the “hot days” are infrequent and the ventilation system is otherwise satisfactory, turn some (or all) lights off during those hot days.

6. Relocate the grow room to a cooler area. For example, a shaded part of the building or that part most exposed to the prevailing wind.

7. Use insulation in the surrounding rooms so they do not amplify the temperature of the grow room.

Cold climates

If the temperature is too low, consider pre-heating the incoming air. If air is coming directly from outside, bring it from a warmer room. If incoming air is not heated you may be restricted as to how frequently fans are operated, especially at night when lights are off.

A fan speed controller is a useful addition where the fan's flow rate is too high during the colder or less humid months (Fig 5.8).

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Related topics


Ventilation: Equipment overview | Growroom layout | Controlling & optimizing ventilation systems

Lighting (Indoors): Lamp selection | Optimizing light intensity | Hints for setup & maintenance

Background information: Humidity | Air temperature