First thought is, if you install a exhaust fan with shutters on one side, why not install shutters on the opposite side. Fan blows open one set and draws open the other.

I'm use a squirrel cage blower out of a forced air furnace system. It's connected to a in-line thermostat. These blower are easy to come by and cheap.

Quick thoughts

Eric

Our GH already has two roof vents and two front wall vents. The roof vents also have fans already BUT everything is under the solar cover. I've considered cutting a hole in the solar cover on the roof so one of them open so it can operate, but that seems to be a huge opening for heat loss all winter. I guess I'm trying to keep the holes in the solar cover to a minimum so I'd rather not cut a fan hole AND an intake hole.

I wonder what would happen if I just had the fan opening? I assume it would pull air in through the cracks in the greenhouse/solar cover from the outside? But would it even work enough to get the heat out of the GH?

I guess I lean toward a 16-inch or so louvered fan in the uppwe vertical wall with an opening cut in the solar cover. Even then I can cover the opening on the inside until we approach the warm season to save heat.

I can't help too much as this is our 1st full year with a greenhouse, and hubby built it not me. Anywho, we installed the automatic fan & had the same problem. He put in a vent on the other side, and it wasn't enough to open it. So they electrified both and now the 2nd vent opens when the fan goes on at 80*. So far so good, and we're having the same types of temperature swings as you.

Brendasue

If you have the space you could build a raised bed for heat storage, route the 80F-100F air through tubing buried in the soil bed. You won`t get any return on the electrical power exhausting valuable excess heat out into the yard :)
Dumping the heat into the soil of a raised bed makes more sense especially at this time of year. It could even work out be easier/cheaper than fitting an exhaust fan with electrical shutter too.

You have to ventilate this time of year. I uncovered my exhaust and intakes last weekend.

There are vents that open and shut automatically, using no power - usually used for foundations.

Lazy....I have 4 vents that are automatic already....UNDER the sealed pool cover and disabled.

Thanks Stress....ventilating and insulating while I'm out of town is the challenge.

hex...wow...wait...what?

Thanks brendasue...I think that's my question. Are you saying your exhaust fan wouldn't open the shutters without an intake?? What size if your fan, intake and greenhouse??

After much debate we concluded that cutting an exhaust fan in without any air intake opening would probably not be the most effective use of $$ and may not have enough air to even open the vents. In the end we decided to cut the solar pool cover around one of our existing roof vents and re-activate that vent. So far this seems to be moderating the heat quite well, especiall with the vent fans running. We added some wood screwed to the new flap in our solar pool cover to get it to lay down better when the vent is closed. I suspect we'll lose a littl heat in the winter through this opening, but likely no more than we would have with a fan cut in anyway. I guess time and winter will tell, but for now it appears to have affordably solved out crazy temps. By the way we had 22F last Thu night and 85F Friday afternoon. The GH never dipped below 60F at night and only hit 104F in the day...and all plants are happy.

We are suppose to have a couple more chilly nights (normal lows are 42) so I've been reluctant to cut some plastic and install a screen but so far, I've been able to manage the heat well by just opening the door and running an oscillating fan. It's on the same side as the door but in the front (the door is at the back at the highest point) and it blows air toward the other wall. We've been in the lower 80s the past couple of days but with bright sun and I've been able to keep it in double digits so far. But my seedlings need watering a lot more frequent!

Mike

In our greenhouse the key word is "automatic." We frequently travel this time of year so whatever our greenhouse does, it needs to do it on it's own. Even on manual control our door to the outside is covered by the pool cover, so we can't open it. We hesitate to open the door to our house since it introduces a ton of moisture to the house, which isn't good. We currently have a thermostatically controlled oscillating fan (18"), two 20-inch box fans blowing in opposite directions below the vent that now opens (one pulls air in and one exhausting air), a double 12-inch fan on the ceiling to move warm air away from the celing and back down to the plants in winter, and an additional fan above our propane heater to circulate heat...all thermostatically controlled.

With the vent cut out we've been able to turn off the misting for now, and just keep the irrigation on, mostly to keep our strawberry towers happy. I think we found a happy balance and we'll be pulling the solar pool cover in a week or three anyway. My only concern now is how it will behave next winter with the opening in the solar pool cover for the vent. Despite my efforts to weigh down the flap, it's not sealed completely so some heat will leak...just not sure how much. I guess I could try to mitigate the leak with some additional plastic through winter but I'd rather not have to climb up there in the winter if I can avoid it.

Our greenhouse is in use heavily all 4 seasons...but the transition seasons can be crazy still. Hopefully we've figured it out for now.