If I were to run several loops on both sides of my furnace and cover them / insulate / reflect and have a small pump, do you think that the temperature would get high enough (with furnace on heating), that I could take some of the 'wasted' heat that I am recovering and pump it into a duct with an air / water HE and help warm another area of the house?
I am not looking for DHW. I have my house zoned in 2 sections - the 'bedroom' area gets little or no heat during the day - but if I could capture some of the radiated heat from the furnace and pump it into the ducts for this area of the house, it would be a win / win situation.
In order to get the temps UP (want much more than needed for DHW), I would have all the loops in series, not parallel. Rather than finned tubing, I was going to use 1/2" copper tube, anchored directly to the outside wall of the furnac with conduit clips (I have no problem drilling / tapping into the the furnace shell and like the idea of direct contact to get temps up with non-finned tube. Surface temp of the furnace when running is 340 F or so.
A small circulator would ensure that the water won't boil, along with common-sense expansion and relief devices.
I guess my first question is when you say furnace, do you mean your oil or gas fired hot air furnace?
How often does it run? % of time in a hour?
Pumping water through 1/2" copper tubing you will need to throttle back on the flow. Too much flow you will not have enough residency, too little the water will be heated to its maximum half way through the path of the coil. This is why I like the a supply and return header with pipes in between. So if I supply the coil with 10 gpm that will get divided between each row, so if you would have four rows it would be 2.5 gpm
A heating application is going to take a lot more energy (heat) then heating domestic hot water. But you will not know until you try.