I found there was a period of some weeks (2-5, depending on the
season) where unheated hoop house protected plants into the fall, and
gave me at least 4 weeks jump in the spring.
If you have an actual greenhouse, ie, glass/plexi, I suspect there
might be a leak somewhere. I routinely had air temps in the 40s in
the dead of winter (z5) when the sun was out, and I KNOW my doors were
not airlocked/air tight.
This is NOT to imply you should be able to grow warm season crops with
no heat a-toll.. But cool weather crops should be no problem for you.
If you go that route, you want to make sure you do not overheat the
GH.. keep the highs around 39-40 max.