For homes with a natural gas or propane supply, the gas furnace is the heart and soul of the HVAC system. Along with supplying the heating during the winter, it also contains the fan motor that operates during the air conditioning season. You want a furnace to be reliable, and provide comfort, efficiently. First, let's define these terms:
Comfort - The temperature and humidity in your home is matches what you have set on the thermostat, and is even throughout each room in the home.
Efficiency - The system uses the least amount of energy possible to achieve comfort.
Gas Furnace Efficiency: Today's furnaces are now reaching up to 98% efficiency - or in the industry referred to as AFUE - annual fuel utilization efficiency. That means it converts 98% of the energy supplied to it directly into heating your home. At this time you probably have a furnace that is from 60 to around 80% efficient. These furnaces have a metal exhaust vent coming out the top (just like your water heater). The temperature of the exhaust can reach up to (400F!). Essentially, 20% to 40% (or that 400 degree air) is wasted energy going out the chimney. An installation of a new furnace can save you up to 30% on your gas or propane usage. For our climate in Columbus, Ohio, this is typically the largest energy savings you can realize. For this reason, we suggest investing more dollars in a higher end furnace rather than a higher end air conditioner.
New vs Old Heat Exchangers: Old furnaces had ribbon type heat exchangers and natural draft systems limiting their ability to get over 80% efficiency. In the late 1980's, high efficiency heat exchangers were introduced. High efficiency furnaces actually have two heat exchangers, the primary and secondary. The primary is a tubular (similar to the exhaust system on your car) typically made of a high grade stainless steel. The secondary is also made of stainless steel, and is tubular with fin-over-tube design, pulling an additional 10-15% efficiency out of the gas. So much heat is extracted, the gas condenses to a liquid, hence the term "condensing furnace".
Gas Furnace Comfort: Large improvements have occurred on the comfort side of gas furnaces as well. Two of these are multi-stage furnaces, and EC motors.
More than likely your furnace has a single stage. For these systems, when the thermostat calls for heat, the furnace comes on at full capacity. But, the majority of the time it isn't necessary for your furnace to operate at full capacity. When we size a furnace, we size it to keep your home at 70 degrees when it is 9 degrees outside. In Columbus, OH it gets below 5 degrees only 2% of the time. So, 98% of the time your furnace is essentially oversized. The multi-stage furnaces solves this problem. When it is warmer, the furnace will reduce its output to match the required capacity needed. We like to compare this to the heating system in your car. When it is 30 degrees outside and you first get your car, you typically turn the heat on at 100%. But, as the temperature gets closer to your comfort level, you reduce the heat output. Newer vehicles typically do this automatically in order to maintain the temperature set point you have dialed in. With your older furnace, there is only one setting, on or off. The multi-stage furnace will reduce output as come closer to your comfort level. So, how does this translate to comfort? Your furnace will now operate longer, adjusting its output as necessary. The longer it runs, the more likely it will heat all areas of your home evenly, reducing "cold-spots" or rooms where it is noticeably colder than others. Another bonus is the furnace has less "starts and stops". The less "starts and stops" it has, the easier it is on the mechanical components of the furnace, increasing the life span of the components. Think of it like highway versus city miles.
Electronically commutated motors (EC) have been an option in furnaces for about 15 years now. They have three benefits over the standard motors found in most furnaces:
Efficiency - These motors are much more efficient than the motors they replaced. Older motors usually had between 2 and 5 different speeds. When the furnace was in heating mode, the motor might operate in low or medium low speed. When the system is in cooling mode, it would typically kick up to high speed. EC motors essentially have infinite speeds throughout the range. This allows us to dial in the exact output required.
Constant Fan - When you have your thermostat set to "Auto", if the heating or cooling system is not running, the fan is not running. This is good for energy efficiency but not so good for comfort. After all, your fan moves the air around your home and brings it through the filtration system. If the fan is not on, it will not perform that task. With an EC fan, you can run the fan in a low speed mode when the system is off at a very low energy cost. Now, you can keep that filter doing its work all the time.
Dehumidification - In the summer, we are not only trying to escape that 90 plus degree heat, we are wanting relief from the high humidity. Your air conditioner not only removes heat, is it also a big dehumidifier. The EC fan has an option to increase humidity removal during times of high humidity in the home.
Gas furnaces range in price (installed) from about $1800 for a builder's grade, 80% efficient with a single stage motor up to $5000-$6000 for a super high efficiency modulating with a variable speed motor and a fantastic warranty.