Do it Yourself Liquid-to-Air Heat Exchangers

Cut six squares of sheet steel to form a rectangle. You can cut them using a plasma cutter. Stainless steel is a good choice because of its anti-corrosive properties, but it may require more-advanced welding skills.

Cut two 1/2-inch diameter holes in the piece, which will become the top of the rectangle, and two 1/4-inch diameter holes in the piece, which will become the bottom. Drill the holes toward the edge of the sheets.

Weld five sides of the heat exchanger together in continuous, airtight weld beads. Make sure only one side is left off, and the top and bottoms are oriented opposite one another.

Weld two sections of 1/2-inch tube steel to the 1/2-inch openings so that the hoses can be fitted and clamped over them. The pieces of tube steel should be 2 inches long, or longer.

Coil copper tubing into continuous tight coils that will fit into the enclosure. Leave at least 1/4 inch of space between one coil and the next. Slide a piece of coupling hardware over head end of the copper tubing before flaring the ends with a flaring tool. Attach mounting hardware in each of the holes, then fasten the flared copper ends with their coupling hardware to holes. This hardware, common to boilers and other metal vessels, will thread tight to the hole, sealing it with a gasket and allowing you to thread tubing to either side of the hardware.

Weld the sixth and final panel on the side of the heat exchanger core, enclosing the copper coils.

Plumb copper tubing from a hot-water source to one of the 1/4-inch fasteners outside the enclosure. Plumb copper tubing from the other 1/4-inch connector back into the hot-water source to be reheated.

Plumb high-temperature vinyl hoses over the 1/2-inch vent tubes, tightening them with hose clamps, and routing them to whatever or wherever you would like to warm.