This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The accompanying sketch shows a plan I adopted for conserving the waste heat from my furnace. I found that I was able to put a coil into the smoke pipe, which was about 8 in. in diameter, and thus heat water for domestic purposes. It will be seen that the coil is spiral in shape rather than cylindrical, as the latter would leave a free passage up the center and therefore would not bring enough gases into contact with the coil.
In addition to this coil I have a gas heater near the tank which is used only in case the demand for hot water exceeds the capacity of the coil, which is naturally not as efficient per unit of length as one directly within the fire-pot would be. It has the advantage of not absorbing heat which should go to make steam,"but only that which would otherwise be wasted. The heating surface of the coil is much greater than would be possible within the firepot, which in a measure compensates for its lower efficiency. - Contributed by W. E. Morey, Chicago.