Size & Efficiency, Why Does It Matter?
A gas furnace has an input rating that is expressed in btu per hour. This is how the size of the unit is expressed. Common sizes are 40000, 60000, 80000, 100000, and 120000 btu/hr. It is absolutely critical that your unit is the proper size for your home. A load calculation is performed to find the right unit for your specific home.
Not all of the heat that is available from the gas is put into the home's air. The ratio of the heat output of the unit to the heat content of the gas supplied is the unit's efficiency or afue. AFUE is short for annual fuel utilization efficiency and is expressed as a percentage. Some common efficiency ratings are 80%, 90%, etc. A higher efficiency unit requires less gas to heat the home and therefore your utility bill is lower.
There are several classes of gas furnaces. Most manufacturers still produce non-condensing models. These units have only one heat exchanger. These units are the least efficient at about 80% and are sometimes referred to as mid efficiency units. These units use metal pipe to exhaust the combustion products from the home. With the price of fuel these days, it is not a good idea to purchase one of these units.
The next class is referred to as high efficiency units. There is no standard as to when a manufacturer can use this phrase to describe their furnace. Therefore, you have to look at the details. In general, for a unit to be considered high efficiency, it should be a condensing furnace. These units have two heat exchangers to get more heat from the unit. Their efficiency rating (afue) is at least 90%. These units extract heat from the exhaust products of the furnace. This produces water which is called condensate. They collect this water and deliver it to a drain system outside the furnace.
The last class is where the units have added features built into them. One such feature is called a variable speed blower motor. In these units a sensor monitors the speed of the blower and adjusts the power supplied to it to provide a desired speed. These blowers are usually quieter and more energy efficient.
Another added feature is where the unit has two gas input rates. These are called two stage furnaces. On the first stage the unit burns less gas and this helps to more closely match the heating load. On the coldest days, the unit can increase it's output by switching to the second stage firing rate. An advantage of these units is that you normally have longer run cycles which can provide more uniform temperatures throughout the home. These units usually have a slightly higher afue rating. Most of the manufacturers combine this feature with the variable speed blower.
Some advanced units, called modulating gas furnaces, have variable gas input rates. This allows them to very closely match the heating requirements of the home. Usually the units have a built in control that, after a preset amount of operation, increases the gas input. This is continuously done to match the load.
The most advanced units are part of a CHP system and produce electricity while they heat the home.