You got it easy. If it were my situation I would try to get to learn what happens while closed. That is, the chemistry and algae issues. As long as you can circulate water at any time, I'd try circulating for a couple hours every few days. Monitor the chemistry. If, after a few weeks you see nothing happening algae wise, cut back on the frequency of the runs. If you get algae or anything else messed up, fix it and tweak the procedure. After a season of this you should have a pretty good handle on the issue for next Winter. If you have solar that may be another issue. As I stated in my sticky about closing, solar heaters are good heat absorbers and therefore good heat radiators and can freeze quickly on a clear cold night.
Al