8 X 8 Water to Air Heat Exchanger Hot Water Coil - American Royal For use in your hot air furnace plenum. These are heavy built units! 40K BTU, 3 Row pass system for better efficiency/longer life. 8" x 8" Coil Size: 8" wide, 10" long, 3 1/2" thick box size (not including the 3" manifold) Lifetime Mfg. Warranty Against Defects AND Lifetime Pro Rated Warranty that Covers Customer Abuse! 1" Return and Supply