In my veg room I have an exhaust fan on a TMP-DNe temp sensor so it kicks on when it gets hot in the room and turns off when the temp drops to a set level. Outside temps have been low recently so it's not turning on as often as it was when I set it up. I'm concerned that my babies may not be getting enough CO2. Is there any way to turn it on every hour for 15 minutes and also have it respond to the temp sensor?

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I would just switch to a standard timer instead.

But if it's 90 degrees outside you don't want to pull in a lot of hot air and burn up the plants, so you have to adjust the timer based on the day to day temperature? Or vice versa, if it 40 degrees outside you don't want to pull in a lot of really cold air and slow down growth either, right? You have to pull in some air for CO2 but depending on the temp of intake you might want to limit the amount.

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A seperate intake fan (motorized shutter) tee`d off your main intake duct could be an option.
Add a cycle timer and set it to run the fan once every X hours for 30 seconds or whatever the minimum duration would be for a single airchange of your room. The outside air temperature wont matter as its only enough to refresh the co2.

I had this exact idea the other day, but then I thought whenever the fan shuts off it wont be exchanging the air anymore as it has 2 functions to exhaust heat AND spent air. please tell me how you sorted this problem? 8P

True. I'm trying to avoid the cost of another fan. i would like to solve this for both my flower and my veg room and if there was a more efficient way rather than purchasing two more fans that would be great.

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you should wire a motor speed controller up with your thermostat, when the temp gets down to where you want it, it runs at a slow speed, when it gets above it runs full speed.

Now that sounds like a good idea. Can you give a bit more detail for the electrically challenged? Any idea where I can get an appropriate motor speed controller? Specs for the controller? Is there any way to check if my fan is variable speed capable? (It's a Dayton 463 blower.) My thermostat is the TMP-Dne.

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Lol, talk about ironic - just hooked up my TMP-Dne to my fans in my Sun Hut today and was having the exact same thoughts as to when it got cold.

Doubly problematic for me is that because my intake and exhaust fans are so powerful, I'm not using an oscillating fan in there.

I'm subscribed for this answer, whatever it may be.

Or you could just do nothing.