I've been looking around at solar heating options, and it seems to me that a really great system (given that the panels are relatively inexpensive) would be to have a large solar collector, or array of collectors that would tie into a home's existing heat vents and blow hot air throughout the house during the daytime.

I'm not terribly familiar with how forced air gas furnaces work, but would it be possible to tie the collector's output directly to the furnace's input and either preheat the air for the furnace, or use the existing "fan only" to draw the air from the collectors into the home? I can imagine there might be issues with how much air it might pull (either making it ineffective because the air would have to flow through the collectors before having time to heat, or even damaging something because the fan is unable to pull through the amount of air it's designed to pull).

Alternatively, could the existing furnace be circumvented, and the air routed directly from the collector to the existing air ducts so that it heats the air, reducing the need for the gas or electric unit to come on, but not interfering with it when it does?

I'm curious if anyone knows of anyone selling such units, or if there are any plans on the web somewhere for building such units, or if it's even practical/possible to do it that way. It seems there would be a substantial market for something that used the home's existing ducts and helped heat the entire house, unless the price would have to be ridiculous or something.

Any thoughts/links?