DOE - Active Solar Heating discontinued
The helpful information once on the EnergySavers.gov website designed to help American residents save energy in their homes, workplaces, and vehicles is no longer available. This resource was removed in February 2019. However the information was rather dated. The above link now goes to an article about the history of EnergySavers.gov by a commercial organization. Linked for info only, not as a recommendation.
Space Heating for Commercial Buildings
From the Florida Solar Energy Center at the University of Central Florida.
Space Heating with Active Solar Energy Systems
From North Carolina State University.