Okay, it keeps us warm. But just how expensive and complicated can this get? It uses nothing but a roll of common aluminum flashing material and two large hose clamps! The total cost of a passive fireplace heat exchanger is about twelve dollars! Read on:

Go to your local construction supply store and get a roll of aluminum flashing. Ours was about 8 inches wide by 25 feet long. This is enough for two passive fireplace heat exchangers. Pick up two large diameter hose clamps for each heat exchanger you will make. Make sure they will be large enough to go around your metal chimney pipe. You can also use copper flashing if you can afford it. Copper transfers heat better than aluminum, but costs a lot more.

To make your heat exchanger, you will need a water level or similar item that has one edge about an inch wide. The aluminum flashing is rolled out flat on the ground in front of you and you bend the metal over the edge of the level. Anything with an inch wide side can be used to make this heat exchanger. Bend the metal back and forth as seen in the photos below. Keep going until you have enough fins to go completely around your chimney pipe with a little bit of overlap.

After you have finished cutting all the fins and pressing the material flat, you can attach it to the chimney pipe. Take your hose clamps and open them all the way so that you can put them around the chimney pipe. You may need some help to get this step done properly. Now take your homemade passive heat exchanger fins and form them around the pipe. If someone holds it in place for you it makes tightening the hose clamps easier. Put one hose clamp on top and one on the bottom of your heat exchanger and tighten them down firmly to the chimney pipe.