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Energy Saver: Furnace, Boiler & Woodstove Combustion
(continued ...) Some of the problems encountered (discussed from a different perspective earlier) in the high energy efficient equipment are that the condensate from the flue gases includes a number of acidic compounds that destroy … well most all types of metal. Metals happen to be the best materials to use for heat exchangers. There are a few stainless steel alloys, special siliconized cast iron and other exotic materials that can tolerate the acid compounds. These and other engineering problems are the reason that high energy efficient equipment costs more than standard energy efficient equipment.
You will notice high energy efficient equipment installations when you see a pair of PVC pipes sticking out of the wall or through the roof of a house. The flue gases are cool enough to be piped in plastic pipe that shouldn't be used for temperatures much higher than 120 F (49 C). The second pipe is the combustion air intake pipe. The flue gas output temperature from some high efficiency machines is sometimes less than 90 F (32 C). The PVC flue frequently has a drain at its base for the liquid acid water brew to leave.
The basic arrangement of combustible hydrocarbon equipment to extract heat energy is typically a single-pass or multi-pass heat exchanger and whatever radiation that is generated. Even lower on the energy efficiency list is a standard fireplace. I love to sit in front of a wood burning fire place. But on the energy efficiency list, read fireplace as HUGE ENERGY HOLE. The only thing that I can think of that has worse efficiency for purposes of staying warm is an open camp fire.