If you’re in the market for a natural gas or propane furnace, don’t overlook residential furnaces (figure 1).

Figure 1: Residential gas furnace

The Lennox SLP 98, shown here, has a variable-capacity gas valve and a variable-speed blower.
Photograph of a Lennox SLP 98 residential gas furnace.

Even though they’re designed for a different market, residential furnaces have a lot to offer commercial buildings:

  • They’re relatively inexpensive and readily available with a variety of options because manufacturers make larger quantities of residential furnaces than they do commercial furnaces.
  • Residential furnaces can accommodate nearly any zoning scheme since they’re available in small sizes.
  • They beat commercial furnaces in efficiency.

What are the options?

Size A residential furnace’s size, or heating capacity, is quantified in terms of Btu per hour of gas input. One Btu is equal to the amount of energy it takes to raise 1 pound of water 1 degree Fahrenheit—about the amount of heat given off by completely burning a single kitchen match. With a few exceptions, residential furnaces are available with inputs that range from 15,000 Btu per hour to 150,000 Btu per hour.

Efficiency The efficiency of residential furnaces is expressed as annual fuel utilization efficiency (AFUE), which accounts for actual operating conditions. Federal law sets the minimum AFUE available at 78% for most furnace types. The furnaces with the highest AFUE available are slightly less than 98% efficient.

To find the AFUE of any given furnace, look at the yellow-and-black EnergyGuide label on the furnace (figure 2). You can also check the AHRI Directory of Certified Product Performance or consult the product manual.

Figure 2: Furnace efficiency label

The Federal Trade Commission requires that gas furnaces carry this label. In addition to showing the unit’s AFUE, the label also shows how the furnace compares to other brands in the same input range.
Image of an EnergyGuide label for a gas furnace. Its AFUE rating is 82.7, between the least efficient (78.0) and most efficient (96.8).

Combustion air source Furnaces may draw the air they require for combustion from either inside the heated space or directly outside. Drawing air from directly outside—typically through a plastic pipe that protrudes through an outside wall—is more efficient and safer. Sometimes this method is referred to as sealed combustion because the gas is burned in a chamber that’s closed to occupied space. Sealed combustion virtually eliminates any risk that combustion gases could leak into occupied space; however, it requires some complicated installation techniques, so check the manufacturer’s installation instructions carefully.

Blower speed control Most furnace blowers operate at a constant speed. Although you can adjust the speed by changing wiring configurations, it never varies during furnace operation. Premium-efficiency furnaces, however, have blower motors that sense how much airflow is required at any given moment and modify fan speed accordingly. These furnaces not only save electricity but also are much quieter than their constant-speed counterparts are.

How to make the best choice

Pick a size that’s just right Sometimes, heating contractors oversize furnaces so they can quickly specify a model and guarantee it will maintain comfort conditions. However, their customers are then stuck with an oversize furnace that’s noisier, less efficient, and more expensive than an accurately sized one.

Follow the procedure from ASHRAE’s handbook to calculate the heat load served by the furnace.

If your building is modern and has a tighter building envelope (better windows and insulation so it doesn’t lose as much air)—or if you’ve recently performed envelope retrofit upgrades—you may consider purchasing a smaller, leaner furnace that uses less energy.

Visit the Energy Star website You can find a list of certified products on Energy Star’s Furnaces page.

Consider a premium-efficiency furnace Premium-efficiency furnaces feature sealed combustion and variable-speed blowers. They cost more but may be worth it.

Check the manufacturer’s installation instructions Before buying a furnace, check the manufacturer’s instructions to make sure you can correctly install it in your building. Many furnaces use venting systems that were virtually unknown a decade ago to achieve high efficiencies. Check with prospective contractors beforehand, as some may be less familiar with these newer venting systems. Also, keep in mind that premium-efficiency furnaces condense water out of combustion gases, and that condensate stream must be properly disposed of.

What’s on the horizon?

Instead of burning gas in a central location and distributing the heated air throughout the building, future heating systems may use many tiny distributed gas heaters, each designed to heat a small space.

The Pacific Northwest National Laboratory (PNNL) developed a MicroHeater that’s about the size of a pack of cards and is capable of combusting about 4,000 to 120,000 Btu of input gas per hour (figure 3). A good rule of thumb for furnace sizing is 45 Btu per square foot for cooler climates, so PNNL’s MicroHeater would be capable of heating a space up to 2,600 square feet. You can incorporate these MicroHeaters into baseboard heaters and connect them to gas supply mains with flexible tubing and quick-connect fittings. Such a system would eliminate the thermal and electrical losses associated with furnaces, which can easily account for a quarter of their operating costs.

Figure 3: MicroHeater

The MicroHeater, developed by researchers at PNNL, is lighter and 10 times smaller than conventional combustors. An array of modules can heat a house efficiently and could reduce ducting and zoning energy losses by 45%.
Photograph of a PNNL MicroHeater next to a penny for scale. The depth of the heater is about twice the size of the penny.

Who are the manufacturers?

There are far more furnace manufacturers than we can list here. For a complete list, search for furnaces in AHRI’s Directory of Certified Product Performance. Here are a few industry leaders:

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by the authors, nor does any content in the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of the technology-related programs mentioned herein.

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