I don't think you can expect a fan to have a three fold range in air movement. At least not the bigger ones like mine, 36 inch.

In my greenhouse one 36 inch fan and 1/4 hp motor is at your lower exchange rate, 8,000 cfm, ~40%/min. I run that 24/7 during the freeze free period to hold down humidity and to exclude insects. They can't fly in against the fan where all the nice smells are coming from. This has excluded leaf rollers and peach twig borer, both serious issues when I was shutting off the fans at night.

Then I can boost up to two or three identical fans as needed during real heat. Mostly I run just two fans. The third only lowers temperature about 2F.

The professionally engineered design I was given 10 yrs ago was two 3/4 hp fans and one 1/2 hp. A 1/4 hp moves 2/3 the air of a 3/4 hp. So I'm saving huge on electricity.

My point is that you'd likely be better off with two efficient fans than one doing double duty. One that moves 33% another 66 or 100%. Run one or both as needed. I do understand the small fans aren't very efficient so this might not pertain to a small greenhouse.

So multiple fans makes more sense...I can see that.

Schaefer fans makes a variable speed controller, works great I have 3 and do what you are looking for

Carl

Here is a link that might be useful: Schaefer Controller

I have a couple two-stage fans in each g'house, but for the weather when it's frigid outside, and yet the sun is shining, I prefer manual drop vents for several reasons. First off, I do not want ANY frigid air, regardless of its velocity pouring over seedlings or sensitive plants. When it's sunny and the g'house gets very hot, dropping a manual vent gives it very gentle passive heat control. The hot air expands, causing a positive pressure and pushes the heat out without pulling lots of cool air in. It sets up a very gentle convection. You can buy automated louvers, and hook them up so that they do not run in tandem with fans if you are not around to manually override fans in winter and run them off a stat just like a fan, to give you some ventilation but not with moving air.

I want as little manual as possible, since I have a job and am never home during the day during the week.

Would having shutters with motors, that open at a lower temp than the fan kicks on accomplish the same goal?

Maybe in addition to the shutters that typicalyl are placed near ground level on the wall opposite the exhaust fan, place a motorized shutter within the roof of the gh, would that work?

Say something like this:

Winter - heat is set at 40. The shutter on the roof is motorized and hooked to a thermostat. if the temp reaches 70, it opens. Circulation fans inside the GH move the cool air that sinks down through the shutter in with the air inside.

I didn't design the system the same in all of my g'houses, because I grew a variety of stuff with different cultural needs. In one of them I didn't even have auto louvers, but manual drop vents, but did install a gravity louver system. If an exhaust fan kicked on, the vacuum would pull the louvers open just enough to equalize the air and not cause the fan motor to overwork. I put it at the roof peak for a reason, so that if frigid air were drawn in, it would enter above the benches and not scrub the plant material. This was not part of the system, but there as an emergency vent in case I did not have the manuals open on very bright days, especially in winter. I don't see why your idea wouldnt work, but you need to keep the temperature differentials on your stats far enough apart that the heaters and the exhausts don't start dancing together each one setting the other off and your gas dials spinning to heat up the great outdoors.