Radiant heat in the fall

[Lakegeezer]

Its our first fall with radiant heat (water system, tacked up under subfloor). So far, we haven't turned it on. As the seasons change, we'd like to open windows when the day is warm, but that seems like throwing away stored heat. Yet, if we don't start up the system, the first nite it gets into the 40's, we're going to wish it had been going for days to warm everything up. What is the right approach to warm days and chilly nights with radiant heat