12 X 18 Water to Air Heat Exchanger Hot Water Coil - American Royal For use in your hot air furnace plenum. These are heavy built units! 90K BTU, 3 Row pass system for better efficiency/longer life. 12" x 18" Coil Size: 12" wide, 20" 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