Feb 6, 2014 Heating
Gas furnaces come in a variety of sizes and can be used for different applications. By and large, the most common gas furnaces use natural gas and utilize electronic ignition. Electronic ignition gas furnaces are slowly replacing the older style standing pilot furnaces where a pilot light remained lit all the time.
Now, with the newer modern electronic ignition, gas furnaces only use gas when there is an actual call for heat. This adds efficiency because with the older standing pilot gas furnace the pilot remained on even in the summer months. While the heat produced by a standing pilot light is negligible, it is still added heat to the system in the summer when the whole purpose in the summer is to remove the heat from the system rather than add heat like a standing pilot would do.
A gas furnace is rated for efficiency by AFUE or Annual Fuel Utilization Efficiency rating. Furnaces installed before the nineties have efficiency ratings down to 60 percent efficient (or less in some cases). The AFUE ratings of gas furnaces is simply the amount of heat delivered by the gas furnace divided by the fuel used to make that heat.
Another way to look at AFUE is to measure the amount of heat lost up the exhaust stack. If the furnace delivers 90 percent of the heat produced into the dwelling then ten percent of the heat produced is lost up the stack. Even the highest AFUE rated furnace is going to lose heat up the stack.
It is nearly impossible to get 100 percent AFUE out of any gas or oil furnace simply because all oil and gas delivered to a furnace has a small amount of moisture in it. Gas typically has a moisture content of 4 to 5 percent. Oil depends on the quality of oil purchased, how many additives used in the oil, and the integrity of the oil tank and piping system. The end result of all this moisture and additives in the fuels effects the AFUE of the furnace and is the reason why no gas or oil furnace can achieve more than 96 percent efficiency.
The original is here: High Performance HVAC