Is there a simple, effective way to store heat from a hot air collector for 12-36 hours? I live in Montana in a 30-year-old, passive/active solar residence. The very effective, 600 SF, active, air collector was linked to a large rock storage bin which never stored heat at a temperature high enough for use 12-36 hours later. On sunny days in Montana the passive features heat the house adequately. If I run the active collector heat into the house at the same time, it becomes unbearably over-heated. I am looking for a means of storing the solar heated air in my generously-sized crawl space.