Temp Question?

Hello there, I have a question regarding the temps in my room. I WAS using Floros and CFLs up until yesterday and I had to run a heater to maintain 75deg.
I went out and purchased a 400w HPS and took the heater out of the room because everyone kept telling me to keep a close eye on the temps. My room is 5'x5'x7' tall. My temps with the 400w HPS are 74deg during lights on and 64 degrees with the lights out and these temps are without the exhaust fan running.
My concern would be that if I turn the fan on the temps would drop even more. But correct me if i am wrong but I need the fresh air coming into my room correct? I have a passive vent down low, but not sure how much fresh air will come in without the exhaust pulling it in. Can I get by without running the exhaust fan?
What is the lowest temp I can run in my room before It will negatively impact my plants?
Thanks.

Dont let you room drop any lower then 65F with the lights on or off. With thoguhs temps you would get by very nicely but yes try to keep a fan circulating in your room makes plants much stronger you get less stretch as well

Thanks for the reply, I have a fan blowing in the room just not exhausting out of the room. Do you think I need to bring in fresh air or will the passive vent allow enough fresh air??

fresh air is recommended for co2/ oxygen exchange. I'd say try a small fan in the day time to see what the temp goes to, and you can always put the heater back in the room but you shouldn't have to