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using both roof vents and fans/shutters for ventilation

Posted by hairmetal4ever Z7 MD (My Page) on
Fri, Jul 18, 14 at 9:25

I've seen this topic discussed here, but not quite this way.

Fans are great for cooling, but can put too much cold air in the GH too quickly if it's 24 degrees outside...conversely, roof vents don't really do the trick if it's even moderately warm outside.

Can a GH be designed in such a way, that it can be ventilated by roof vents, but, in an automated fashion, if temps still get too hot, a fan and shutter system also kicks on?

Almost a "two-stage" thing. Roof vent opens at 67, fans at 82 or whatever...

In winter, I'd envision that the roof vents would suffice on most days unless you're keeping the GH very cool (like 50s or cooler). In summer, the fans only (plus shade cloth or whatever else).

It's the spring/fall time where it might get dicey. I've heard that if you have an open vent on the roof *and* a fan running on the sidewall, you can end up basically creating an "air loop" (I can't think of what else you'd call that) between the two, meaning air moves between the vent and fan but doesn't circulate - leaving the air in the rest of the GH unventilated - meaning no temperature regulation either.

Is there a way to design this so that you could have a fan kick on and shutters open only if temps still get too high with the roof vent, but to ensure the above "air loop" situation doesn't happen?

Or would you have to do this manually - watching the forecast and, on warmer days, just keeping the vent closed and using the fan only, and vice versa on cold days?

Or is there a thermostat that would open the vent at XX temp, and CLOSE it again at the same temp at which the fan kicks on so they're not operating together?

Follow-Up Postings:

RE: using both roof vents and fans/shutters for ventilation

What about putting a fan IN the vent? It won't be "perfect" circulation but far better, I'd think.

RE: using both roof vents and fans/shutters for ventilation

  • Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
    Fri, Jul 18, 14 at 19:55

It could be automated. All it takes is the right equipment. Obviously you have to avoid the loop. I've seen that in person, open doors right next to the exhaust fans. That assures that nether the doors nor the fans do much good.

The trouble with a fan in the roof vent are at least a couple. Fans will be much smaller than what is needed for effective natural ventillation. My 1700 sq ft greenhouse has three 36 inch exhaust fans. Mostly I run only two. That area of roof vents would be totally inadequate. You'd need 10-20 times that area of passive roof venting. Also a fan in the vent area would impede air movement when the fan wasn't operating.

To me roof vents are too expensive for what they do. With roll up sides I'd rather have a means to uncover the ends all the way to the peak. This would allow air into the sides and out the peak at both ends. Neither roof vents nor open ends sound cheap or easy. But I'd rather jury rig the ends than the top for venting. Roll up sides by themselves will work during cool weather. Then during the freeze free period, uncover the ends and replace with bird netting if critters might be an issue.

RE: using both roof vents and fans/shutters for ventilation

The main thing is not having TOO much cold air too fast in winter if I'm using fans. Even if they run at 1/4 exchange per minute, if it's 23 degrees outside and 80 inside, that fan kicks on and it seems too much too fast...that's why I was thinking of the "hybrid" type system.

I also thought - what about a fan and shutter system, but have the fan's shutter open along with the shutters on the opposing wall, but no fan. However for the reasons you state, there wouldn't be nearly enough airflow since it would be a much smaller area than an actual roof vent system would provide.l

However, what about just ventilating enough so CO2 doesn't plummet on sunny winter days...even if it doesn't provide much cooling, would just opening the fan SHUTTERS without running the fan do anything in that regard?

RE: using both roof vents and fans/shutters for ventilation

  • Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
    Sat, Jul 19, 14 at 0:04

Any vent up high will cool if there's a place for cold air to come in down low. We don't have many of those cold sunny days where I'm concerned about a cold draft. But when needed my greenhouse has an inlet shutter up high opposite the exhaust fans. This allows more mixing before cold air hits the plants.

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