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Greenhouses & Garden Structures

ventilation question (roof vents AND fans)

11 years ago

If you have roof vents that open at, say, 75 degrees, and fans that kick on at, maybe 85 (for example), is there a risk that, since both the vents and the exhaust fan are at the upper part of the GH, that the airflow will be from vent to fan, all the while the heat stays stuck down at the bottom around your plants? I guess I'm thinking in my head that, even though heat rises, you'll create an airflow once the exhaust fan kicks on that basically sucks air out the roof vents, straight across to the exhaust fan, leaving the hot air down near the plants in place.

Does that happen? If so, do ventilation shutters down at ground level that open with the fan kicking on solve the problem?

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Comments (3)

karin_mt
11 years ago
I think the answers are yes and yes.
Ventilation shutters down low will ensure airflow throughout your greenhouse. If you are designing from scratch, I'd definitely go that route.
fruitnut Z7 4500ft SW TX
11 years ago
If you have exhaust fans forget the roof vents. If you have both neither will work properly unless you have a complicated system to close the roof vents when the fans come on.
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fruitnut Z7 4500ft SW TX
11 years ago
If you go the exhaust fans route, put the fans as high as you want cooling. Hot air rises so the upper part of the greenhouse will always be hotter than the lower. You want the exhaust fans to push out the hottest air and replace it with the coolest air possible.

Natural ventillation, roof vents, roll up sides, etc, will cool to about 10-30F warmer than outside. To do that you need vents that are 40% of the area of the greenhouse. Most are much smaller. So you have a very hot greenhouse except right at ground level where it can be reasonable.

As discussed earlier exhaust fans with one air exchange per minute can cool to about 10-15F warmer than outside. Two exchanges per minute will cut the temperature rise in half.

In my area with moderate dew points an evaporative cooling system will keep the inside as cool or cooler than outside with one air change per minute.

The best evaporative system is exhaust fans and a wet wall opposite. A house evap cooler pushing in cool air is not as good because the cooler air pushed in mixes with hot air inside and a mix is pushed out. You want to exhaust the hottest air and replace with the coolest no matter the system.

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