Yeah I live in Chicago and it is not hard to keep it at 450ppm. Right now it’s 493ppm in here.
I mean that’s really low. If it’s cold outside you’re probable just wasting energy keeping your spaces warm with so much ventilation, making the problem worse in the process.
What are you using to measure this?
(I'm sure I could do my own research, but was more curious since it seems like you've already got a setup.)