Garden Fan on a Timer

by Jerry Morse
(vicksburg, MS USA)

I have an oscillating fan to blow on my indoor tomato plant, it is under a hydrofarm combo HPS & MH light system. 600 & 400 watt respectively. I have a timer for the fan. How much time out of each hour should the fan be on? What is a light mover? What are the advantages of a light mover? How much does a light mover cost?

Answer: An oscillating fan should be kept on 24 hours a day in my opinion. There are several reasons for this, which I get into in more detail about on my troubleshooting humidity page. In short, it helps your plants to breathe properly.

While your grow lights are on, the exhaust fan is used mainly to keep the temperature from getting too high (above 80*F). How long you have to run your exhaust fan for each hour will depend completely on the temperature inside the grow room. It is not unusual to have an exhaust fan on constantly whenever the lights are on. In fact, this may be your best strategy.

If you are pulling cold air into the grow room from outside (in December, for example), you may find that running the exhaust fan constantly makes the garden area too cold (65*F or less). I have run into this problem before myself. My solution was to re-circulate the air inside the grow room, adding only enough fresh air from outside to keep the temperature under control.

Keep in mind your plants are constantly using CO2 to grow. In a closed grow room with re-circulated air, the fresh air you introduce also becomes an important source of fresh CO2. If CO2 levels become too low all plant growth will stop, no matter how bright your lights are or how well you are feeding them.

Another way to control the temperature would be to time how long it takes for your garden to warm up to 80*F once the lights kick on. At that point, turn your exhaust fan on and measure how long it takes to bring the garden area back down to the 68*F-72*F range. This will give you a good idea how often you need to turn your exhaust fan on, and how long you need to leave it on for each time.

The third way to control temperature would be to plug your exhaust fan into a thermostat that will automatically turn your fan on when it gets to a certain temperature, and automatically turn it off when the room has cooled to a certain temperature. I admit, these devices are more expensive than a digital timer (about $60.00), but they take some of the legwork out of the exhaust fan setup.

When your grow lights go off for the night, the humidity will go up in the grow room. Humidity can cause all kinds of problems if it is too high, mainly with plant killing fungus and mildew. To prevent these problems, you will want to turn your exhaust fan on and completely exhaust the air volume inside the garden area once an hour. Directions on how to calculate this exhaust period can be found on my exhaust fan calculator page.

A light mover is a motorized device that moves your grow lights back and forth a few feet over your garden. A stationary light will always cast a shadow on the same leaves of your plants. By moving the light back and forth, you actually activate more leaf surfaces to photosynthesize (and so get a little more growth). Also, a 1000 watt grow light is capable of covering a garden area of 4'x 8' with good results (on a light mover).

Without a light mover, however, the light must be hung in one spot. As you pack plants into a more square 5'x 6' area, the plants on the perimeter of the garden will always get considerably less light than the ones in the center. If you are gardening plants in individual containers, you could switch them around every day to even things out. When all of your plants are plugged into a hydroponic system, however, this is not usually an option. One thing you can do is make sure your shortest plants are in the center of the system, with your tallest plants at the edges of the garden.

A light mover costs about $200.00. If this is your first garden, I would suggest making the setup as simple as possible at first. Definitely use reflective material around the perimeter of your garden to make the best use of the light. Once you have the garden environment dialed in (especially temperature) and you are producing excellent results, than go ahead and add a light mover if you want to try to maximize your yields.