Take a look at this link:
This link is broken, either the page no longer exists or there is some other issue like a typo.
. It will answer many of your questions about heating air with a hot water in a duct system. A well designed system will allow you to heat, cool, humidify, and de-humidify your home as the season requires. It's a common design for the expensive McMansions being built today that demand zoned complete comfort control year round.