posted 8 years ago
Yes, small tubes, about 1-1.5 inches or less plus insulation generally carry the heat. Stainless flex line seems expensive on its face, but especially for diy and some retrofit installs i say go for flex.
Your apartment size and building envelope efficiency, comfort level versus outdoor temps-desired, and mechanical efficiency and type of existing heating all paint a picture of how much solar thermal capacity you will require for collection and storage, next is space and budget.