If you have space then solar can be easy and cheap for a homebrew setup. Attic heat exchangers always run the risk of a leak where you don't want one. You may also have to drain condensate. It was discussed here years ago and have not heard of anyone doing it. For solar, one of my heaters is nothing more than 1200' of 1/2" black poly pipe layed out on rolled roofing in my back field.
Al