This post is the second in a four part series on high efficiency gas furnaces. If you haven’t read part 1 yet, I recommend you do before continuing.

History of the furnace (continued)

Throughout the 19th and 20th centuries, hot air circulating and steam systems competed for prominence in the global interior heating market. In fact, we can still see evidence of the rivalry between these two types of systems today. Both systems with boilers and radiators and radiant floor heating systems use the principles of Watt’s design, while central air heating systems use the principles of Strutt and Sylvester’s designs.

"Furnaces 101" illustration. High efficiency gas furnaces

Central heat rises to dominance

While larger structures in the US were often built with boiler and radiator heating systems, it became clear over the years that air circulating systems were far more practical for single family residential structures. As these systems spread across America, the prominence of central heating was cemented.

Quantifying efficiency

These early air circulating systems were very dirty and inefficient by modern standards. In order to understand the efficiency of furnaces, we need to understand a couple pieces of heating industry jargon.

The first is BTU, which stands for British Thermal Unit. A BTU is the amount of energy required to raise the temperature of 1 pound of water by 1 degree Farenheit at 1 atmosphere of pressure. BTUs give us a way to talk about quantities of heat.

The second is AFUE, which stands for Annual Fuel Usage Efficiency. AFUE is a percentile measure of how many BTUs a furnace puts out for a given input. Early furnaces often had AFUE scores of 40% or worse. This means that for every 100 BTU of fuel the user puts into the furnace, there is only 40 BTU of usable output. Obviously, this leaves a lot of room for improvement, since an AFUE of 40% means that the majority of the money the user spends on fuel is going to waste rather than heating the structure.

Check back soon for High efficiency gas furnaces – less pollution, more heat, part 3! In the meantime, check out our page on high efficiency gas furnaces.