I'm looking for a way to supplement/reduce the natural gas heating in my home. It is an old rowhouse with a flat roof and the furnace/AC mounted on the roof. There are two ducts going into the furnace, one for the intake and one for the heat/AC return into the house.

I'm thinking I can just tap into these ducts and mount something like this next to the furnace: http://home.att.net/~cleardomesolar/lowprofilehtr.html

The back of the house faces south, so it would seem to be a very easy setup with very little visual impact. But it seems a little too easy...

Will this work, or am I missing something?