75 - 80 is a fine temp to grow pot at unless you are aiming to use CO2 then you want a sealed room to do it right.
My grow room takes up a 7x9' area of the 10x30' basement which is really a 7' high concrete bunker under my house that stays nice and cool even in the hot summer we just had. Never got over 67F down there and the air that goes into the grow room comes from that larger space thru filtered vents.
What you want is a temp/RH and speed controller on your exhaust fan so it only runs when it's told to so you can control the grow conditions within close parameters. I also got a baseboard line voltage thermostat and wired that into a simple light switch box with cords to control my electric heater to keep the grow room from getting too cold during lights out times. I have a 1500W heater with two 750W switches and just have one on and leave it's thermostat on high. My thermostat turns it on and off as needed to raise the temp a bit when it cools down.
During hot weather your exhaust may need to run all the time but the controller will keep it on if that's the case but shut it off when it's cool so your heater isn't running to keep it warm enough.
I'm looking to get a 600W or better LED to replace a 600W HPS and won't even have to change my settings to keep the room just as I want it.
My analog temp/rh controller and speed controller I made from a ceiling fan controller. Do not use a light dimmer to try and make one of those. Electronic versions can be had on Amazon etc for under $50. The thermostat for the heater is just like the speed controller but uses a 120/240V thermostat and not ones used for your home furnace that use 24V. About 20 - $30Can here.
The speed controller is plugged into the temp/rh unit then the fan is plugged into the black plug. I set the speed around half.
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