In the collectors? Because water holds a lot of heat for it's volume. It's cheap. I am planning a drainback system so freezing isn't a concern. By using water I can get away with a single wall heat exchanger and not need to worry about poisoning the family should a leak develop( the inspector has yet to approve my system so this may have to change...)
Where you thinking of going with some form of antifreeze/water combination or something more exotic?
I suppose you could use hot air to heat water but I've yet to see a air system that would compete with water/water antifreeze systems.
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