using both roof vents and fans/shutters for ventilation

9 years ago

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?

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Comments (4)

hairmetal4ever
Original Author
9 years ago
What about putting a fan IN the vent? It won't be "perfect" circulation but far better, I'd think.
fruitnut Z7 4500ft SW TX
9 years ago
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.
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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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I got my louvered fan at ventilationdirect.com. I got a 10", but they sell larger ones. I think my fan was under $90. I found a great deal for my automatic louver on ebay. I got my thermostat from kkontrols.com, but Farmtek is cheaper. I have both my fan and louver hooked up the the same thermostat. The louver draws very little power and can easily be hooked up to open with the fan. Stuart
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Kate: As Stress noted, we have debated ceiling fans, but with a pair, mounted at each end of a space, turning in opposite directions, the air flow IS horizontal. Actually, within the closed space of a heated, winter GH, it's circular. In summer, with both set to pull, they force the cooler air from the floor up, and out of typical GH roof vents. But if your ceiling is too low to allow any, the issue is moot for you in any case. Powered roof fans are designed to survive more humidity than you might think, although eventually the motors do give out. The one on our small poultry house ran pretty continually for over 14 summers before it finally seized up, but was probably as affected by feather dust as moisture and the ducks made plenty of it, splashing about. My DH found it both cheap and easiest to just buy a new unit at Menards and take the motor from it. The old frame was still fine, so he just switched the innards. Stress did make a vital point about the location of exhaust fans and there we do concur. If it's facing prevailing winds, they will reduce the effectivenes of a wall mounted exhaust fan, so coupled with the fact of hot air rising, logic would dictate the roof to be the best location of all.
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Happy to help. I've had a bit of this work done myself, in two different houses. In one, I had a Fantech BFRK 100 installed. There was an existing bathroom exhaust fan that worked poorly but I didn't want to put in a new grill. With the BFRK, you take out the guts of the old fan, keep the housing in place, and then connect ducting that runs first to the Fantech unit located a few feet away up in the attic and then from it to the roof passthrough. I had that done by an HVAC contractor who was on site replacing HVAC equipment. I'm not sure they would have done the work otherwise, it was like a favor to me. The second one was the replacement of a 20 year old exhaust fan with light in a WC closet that was noisy and ineffective. I had that done by an electrician who also was doing other work. He needed to go into the attic to remove the old unit, connect the new one to the framing, put in a new duct and connect the power. It made a huge difference. That one also vents through a roof pass-through, important because we use it to vent the bathroom of moisture from showers and baths. Good luck and keep your BS detector handy. If other exhaust vents terminate in the attic, it's time to fix those too. Each duct run needs its own roof passthrough unless you're using a Y setup with one fan at the base of the Y for two locations. Be careful if so, those can be tricky if the duct runs from the rooms to the base of the Y aren't of equal length. As far as duct run distances go, the unit will specify the maximum distance. For my most recent job, the ceiling unit with light that was 110 CFM, I want to say the spec for the duct run was something like 60 feet. I wrestled with putting a wall vent on a gable end but decided to stick with the existing through-roof vent.
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hairmetal4ever
Original Author
9 years ago
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?
fruitnut Z7 4500ft SW TX
9 years ago
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.

using both roof vents and fans/shutters for ventilation

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hairmetal4ever
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fruitnut Z7 4500ft SW TX

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using both roof vents and fans/shutters for ventilation

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hairmetal4everOriginal Author

fruitnut Z7 4500ft SW TX

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