We use greenhouse fans to do two things in the greenhouse.
- The first is to reduce the air temperature so the plants will grow and
- the second is to move fresh air into the greenhouse to reduce the relative humidity of the air surrounding the plants.
Both are important functions of greenhouse ventilation. No, let me restate this – both are critical to success in the home greenhouse and these are mostly ignored by enthusiastic beginners. (Don’t be one of those people who build first and figure out the details second) 🙂
Let’s look at something many home greenhouse growers don’t take into account – the relative humidity around the leaf surfaces.
When relative humidity increases past 80% on many plants (tomatoes or other soft leaf plants in particular) then diseases start and run rampant. Over 90% (typical of many home greenhouses) and you are almost guaranteed to have fungal problems. Under 70% relative humidity and these problems almost disappear.
Two considerations are important here. Number one problem with this of course is that it costs money to heat the air you’re blowing out of the greenhouse. Secondly, you have to heat the incoming air before it hits the leaf surfaces and chills them.
Winter greenhouse operation then becomes a balancing act of ventilation and heating to keep the plants healthy and growing well. We have to ventilate to drop the humidity but we want to conserve energy. This is something for which there are no guidelines on the home scale of things.
You can install humidistats to turn on the greenhouse fans when the humidity gets too high or you can run by guess and by golly. But keeping it below 80% humidity is going to be your goal.
Heating the incoming air can partially be done by running greenhouse fans to blow air around within the greenhouse. Good air circulation and air mixing is important.
In a small greenhouse, a house fan with 18-24 inch blades will mix the air if it is suspended at roof level and blown down the greenhouse. In larger home greenhouses – say over 20-feet long – I’d use an old furnace fan unit connected to commercially available plastic ventilation tube. The tube carries the air down the greenhouse and holes in the plastic force air downwards, mixing the warmer upper air with the colder incoming air.
Determining Your cubic footage
The math to determine your greenhouse fan need is to take the square footage of one end wall and multiply it by the length of the greenhouse.
This will give you the cubic feet of your greenhouse.
Figuring this Out: Rule of Thumb
Winter air changes should be 2-3 total air changes an hour. So if your greenhouse is 15 feet long, 10 feet wide and 8 feet tall, your total cubic area is 15x10x8 or 1200 cubic feet.
You will need to move 2400 to 3600 cubic feet of air per hour and your fan sizing should be able to handle this.
Greenhouse Fan Sizing Measurement
Fans are measured in cubic feet per minute capacity. So moving 3600 cubic feet per hour requires a fan of 60 cubic feet per minute. (3600 cubic feet divided by 60 minutes) But check out the summer ventilation rates to understand why you’re going to be using two greenhouse fans.
Reducing Air Temperatures
Again, we need to reduce air temperatures for the plants to grow properly. Once leaf temperatures start to climb into the 80’s F, then the plant reduces its growth. It is too hot. We use greenhouse fans to ventilate and blow this warm air out of the home greenhouse.
A single visit to a greenhouse during the summer will quickly convince you of the power of solar energy and the resulting heat (if not ventilated) can quickly cook plants and soil.
In the winter, we need to move 3 air changes an hour. In the summer we need to move one air change a minute.
So the greenhouse example above of 3600 cubic feet of space means that our fan now has to be rated for 3600 cubic feet per minute.