Ah. Well, I used an Attic Exhaust Fan and wired it to a switch, so yes, it's a good idea.

I have my shutters on a 3 minute delay after the 2 exhaust fans come on. It's supposed to help prevent the cold blast of air. And since I have 3 fans and 3 vents I have the 2 ends set so they don't open until it gets over 90 or so in the winter. In the summer I set them to open first(around 80) and the middle one comes on when it get hotter.(around 85). I asked my DH to re-wire so only one vent opens in the winter so I can seal the other two up. I tried covering all 3 in the winter and just let the one exhaust fan pull what air it could through the air leaks. But it draws so much I can't hardly open the door.
It's still on his Honey Do list.

I just cover on of the two shutters with plastic film. The shutters open out, so it is easy to cover them on the inside without interfering with the mechanism. The covered shutter is then free to open and shut normally, but no air enters.

Thanks all.

Cactusfreak, I've read about using a thermostat or controller with two stages to open the shutters independentsly of exhuast fan but I understood the idea was to open the shutters *before* the fan so that when the fan came on it has access to free air makeup.

You are right, Clavero, normally a controller with this feature would open the shutters first, then the fan. That's the way the feature is set up on my controller.

I'm not sure why you would want to disable the shutter rather than the fan. I have my shutters set to open at a lower temp than the fan. Those days when it is warm, but not hot enough for the exhaust fan to come on, the shutters open and provide a little passive ventilation on their own or through the ridge vent. It doesn't move enough air through to shock any plants, but enough to cool a bit. By the time it is hot enough to require the fan, it's warm enough outside that the outside air temp isn't too cold for the plants.

My controller powers an outlet near the exhaust fan. The fan plugs in like any other appliance. To disable, I just unplug the fan.

Thanks stressbaby....the idea was, that since I will have two 16" shuttered vents, disabling one of them for the winter (and reducing the speed of the exhaust fan) would still allow venting of the GH but at lower volume....reduced cold air shock to plants.

Maybe just reducing the fan cfm at the speed control would accomplish the same end?

The GH will not have ridge vents...seems to me that with a controlled opening of shutters and fan the ridge vents work at cross purposes...allowing air to be drawn *in* through the vents high in the GH and blown out the fan , which also sits high on the end wall.

I have fretted a bit over power failure and failed vents and toasted plants, but seems that system failure is possible with ridge vents also.

Jkirk-what is the advantage of using an attic fan? Is it more economical?

By closing one of the shutters, you will not be reducing the amount of cold air that comes into the greenhouse. The fan will just bring in twice as much from the open shutter increasing the likelyhood of cold air shock to the plants around the open shutter.

The best thing to do is turn the thermostat to a higher temperature, and cut the speed down on the fan with your speed control. You should be fine with these modifications, but if you continue to have problems, upgrade to a 2 stage thermostat. This would open the shutters first. Then if the temperature continues to rise another few degrees, it will cut the fan on.

I can't remember the reference for doing my system the way it is. Everything I've read since says it's backwards. I think the guy that installed the greenhouse and fans told me to do it this way.
It could be because I also have full ridge vents on both sides and they open first. Except I had them turned off in the winter since I had the solar pool cover over the greenhouse.
But most people that sell and install greenhouses usually don't know how they should work. If I had listened to the salesman I would only have one shutter and fan. They told me I was using way too much ventilation. I had ordered 30 inch shutters and they made me order 24 because it would interfer with the framing.
I though they would just adjust the framework like they did for the door.
I don't seem to have any problems with my plants. And I don't have too much ventilation. The fans and shutters are higher for the most part than the plants.
I can always reverse the relay and have it on the fans so they come on after the shutters open. Or put in another thermostat.
I'll get my DH to that and see how it works out.

Thanks Tim

I do plan both a speed control AND a two stage thermostat from the beginning. I can certainly see how disabling one shutter would only route the volume of cold air in through the other. Your observation to set speed control lower will, I think, accomplish what is needed.

I have my motorized shutter on a thermostat and set to open a few degrees before my exhaust fan kicks on.

Marv-Montezuma, IA
http://community.webshots.com/user/vmwood

Here is a link that might be useful: Photo Albums

Who here has a fan that operates in winter but just at lower capacity?

If I have my fan and shutter thermostat set at, say, 80 degrees in January, and it's 18 degrees and sunny outside, what I fear is that when the shutter and fan kick on that even at lower speed, by the time the thermostat "realizes" the temperature has dropped, the GH will be down to 45 degrees!

Does it really make that much of a difference to have a 2-stage thermostat? What would I do, set the shutters at maybe 75 and the fan at 80? In summer I can't see it really making a difference and might as well set both to the same temperature (I was thinking 90-does that sound right?).

i have my thermostat wired to the electric loovers,then to the fan.even when it's 20 degrees outside,it doesn't overcool the greenhouse.it may only come on for a minute or two.it's amazing,when it's a clear day,even when it's 20f outside,the greenhouse easily maintains 82f.the vent will come on every 10 minutes for about 2 minutes-but the temp stays very constant.

Hairmetal, my experience is that in zone 5b/6a, my fan, set at 80F, won't ever kick on from the end of October or so until March.

Recently I've been keeping track of the inside/outside temp differentials in my GH on sunny days, comparing the max temp inside to the daily high temps. Here in zone 5b/6a, it will get maybe 30F warmer in the GH. Yesterday the GH was 76F and the outside temp was 45F. I'm convinced that I'll never see 80F when it is 20F outside.

From the middle of November until the early spring, I unplug my fan, cover it and insulate it, and turn off the fan controller. I still today have one of the two intake louvers uncovered and operable. But with the forecast low of 18F tomorrow night, I'll cover the second intake and try to keep that heat in. The rare occasions I need to increase ventilation are predictable based on the weather forecast. On those days I'll open up the screen on the door just a little more.

A multiple stage controller makes things easy. Here is a link.

Here is a link that might be useful: Bartlett controller

Can see why the concern over plant shock if you have a small GH. Even a 20mph freezing wind (like today in the NE) with an open louver will shock plants in close proximty. A baffle on the intake louver helps(I just slant a piece of plywood on the inside so the air force is blocked); some outfits even have a plastic tube with holes running the length of the GH from the intake louver to allow makeup air to filter in more uniformly.

I don't grow anything in the winter months but if I did I would probably have multiple smaller intake louvers located as high as possible equiped with baffles to avoid drafts onto plants.

Thermostat placement is also going to affect fan and louver operation. I have the thermostat 2/3 of the distance from intake on a 40' GH and approx. 1/3 from an intake louver on two 96' GHs. I don't think you want the thermostat too close to the makeup air or it will be constantly opening and closing, although I'm not sure what I'm doing is correct.