Yanche wrote:Not to put to much cold water on your furnace retrofit project but there is a reason they are called booster coils. As your design analysis showed it takes a hefty multiple water loop coil to get all the needed BTU's into the furnace duct. The referenced Ebay coils are single pipe designs, the 1" pipe will not flow sufficient water to transfer the needed BTU.
I guess I come to a slightly different conclusion:
First the coil in the link I posted is three loops deep (see picture in the link).
If the current furnace NET OUTPUT is 100,000 BTU per hour that can be achieved with a water flow of 10.4 gallons per minute assuming a 20 degree temperature drop through the coil.
10 gallons per minute through 1" pipe is not unreasonable but will likely require a larger circulator pump than a Taco 007.
However, if you are considering purchase of a coil from the link I posted you should seek technical data from the seller--he offers design and planning assistance.