I'm starting to realize that heating 3 separate structures with 4 different wood burning appliances is not sustainable long term ;-). One of the structures is suitable for boiler with storage and lots of wood storage. First thought is a Garn JR because of simplicity and I am with in 100miles of their manufacturing facility. So of the three structures needing heat, the house being the main concern is currently set up with 120kbtu propane FA, Tundra wood furnace tied in parallel, and a Progress Hybrid on the main level. The Tundra would go away, Progress Hybrid would remain for when I want to look at a fire. Now to my question - plenum in the FA furnace is large, can probably fit a 22x25 or somewhat larger HX. Trying to determine how low I can go on storage temps and get any meaning full output. Output ratings I've found so far are all at 180F input temp, ~21 gpm , and CFM air flow. Looking for a calculator, output vs input temp chart, or even what others have experienced.