Manufacturers balance consumer-demanded comfort with government-mandated efficiency in the next generation of gas furnaces.
It’s that time of year again. Halloween’s right around the corner, winter’s well on its way, and all over northern United States furnaces are kicking on. In many houses, it will be natural gas furnaces that efficiently supply the warmth and comfort. The latest U.S. Census data shows that the market share for gas heat in new construction, single-family, detached homes rose from 44% in the mid-1980s to 67% by last year — a clear indication gas-appliance manufacturers have succeeded in channeling new technologies into functional convenience for consumers
MAKING GAS MORE EFFICIENT
Most modern furnaces use sealed combustion, which means all combustion air is pulled from the outdoors and all flue gases are expelled outdoors. This eliminates interaction with indoor air and reduces the risk of backdrafting — cold outside air making its way into the furnace and house. To move air through a sealed system, especially the heat-exchange section, furnaces rely on an inducer fan. Other common features on mid-efficiency furnaces are hot-surface igniters, multiport in-shot burners, and aluminized-steel serpentine heat exchangers.
The multiport burners use several orifices, or ports, to mix air and gas for maximum heat. They work with the inducer motor to shape and place the flame in the throat of the heat exchanger. Furnaces that don’t use inducer fans, like the Lennox WhisperHeat furnace, still use steel-ribbon burners. These burners use a natural draft to vent flue gases and burn properly.
Hot surface igniters do away with pilot lights, which waste gas and require an exposed flame. Most igniters use silicon carbide with a tungsten element, which are fragile and prone to early failure if they aren’t handled delicately and provided a stable working voltage. To avoid that problem, engineers at Trane have switched to a silicon nitride igniter. “It’s more durable and resists water, dirt, and other trash that might have contaminated silicon igniters,” says Tim Storm, furnace product leader at Trane. “We also have an ignition controller that varies the igniter’s voltage so it maintains the same temperature regardless of whether the house voltage is 90 or 130 V.”
Engineers at Carrier take a different tact. They worked with their igniter supplier to design a more robust silicon- carbide version. “We’ve had success with our reformulated igniter,” says Dan Dempsey, director of heating product development at Carrier Corp. “And it doesn’t require the significant amount of electronics to control voltage that nitride igniters do.”
Aluminized steel is the metal of choice for heat exchangers. A thin aluminum coating protects the steel from corrosion, a constant threat since flue gases are extremely acidic. The Sshape of many heat exchangers slows the flue gases on their way through the exchanger. Slowing the gases lets more heat transfer from the gases to the exchanger. Initially, many exchangers were welded together. Manufactures quickly discovered that welds burned away the aluminum coating, leaving an opening for corrosion to start. And once in use, welded joints creak and pop as they expand and contract. To solve this problem, most manufacturers use crimping to make exchangers. Carrier goes one step farther and folds the metal edges so there are no flanges or hems acting as cooling fins on the outer edges. “If the surface of the heat exchanger gets too cold, it will condense moisture out of the flue gases,” says Bob Peitz, senior manager of heating products. “Folding eliminates the fin effect.”
To make their furnaces efficient, most manufacturers use a two-stage gas valve with two levels of heating — one around 65% of full gas flow and the other at 100% of full flow. To precisely meet the air demands of the two combustion rates, two-stage furnaces also have two settings on the inducer motor. In operation, two-stage furnaces spend almost 80 to 90% of the time on the lower setting, with the higher settings reserved for extremely cold days or quickly heating relatively cold interiors.
Running at the lower setting lets the furnace delivers more stable temperature control. Cycling on and off at a higher setting creates big temperature swings, along with a lot of noise. Running the burners and fans on low, on the other hand, generates less noise. And for humidity control, it’s better to run the furnace longer and let it condense out more moisture.
Although there are only two operating settings on two-stage inducer motors, some manufacturers still refer to them as variable-speed motors. Trane’s XV-80 furnace, for example, is called variable because the motor starts slowly and increases speed until it reaches operational levels. “It gives a soft start, which prevents the noise and whoosh of a sudden start,” notes Storm. “Also, when the unit is just circulating air, it cuts back to 50% of normal airflow. This makes it quieter, increases the efficiency of electric air filters, if one’s installed, and it draws less power.”
To power its inducer fan, Carrier uses a brushless dc motor from General Electric with its own internal intelligence. By monitoring the back-emf from the unfired phase of the three-phase motor, the motor senses the load on the system, which correlates to airflow. The motor increases or decreases its output to maintain airflows specified at installation. This lets the motor compensate for changing duct loads and filters that get dirty and restrict airflow. “In the past, you would determine what airflows were needed to heat and dehumidify a house without really knowing the static pressures inherent in the ductwork,” says Peitz. “To avoid freezing up an indoor coil, you’d pick an airflow much higher than what the system really needed to operate best. Since the motor compensates for changes, you don’t have to set it up for airflows way beyond what they should be.”
It’s a relatively simple step to convert the basic mid-efficiency furnace design into high-efficiency or condensing furnaces: Just add a secondary heat exchanger to capture more of the heat from the flue gases. “After passing through the first heat exchanger, the flue gases are still around 400°F,” notes Storm. “Adding another heat exchanger cools the combustion products down to about 125°F, below the dew point and condensing moisture out of the flue gases. The furnace makes use of this latent heat and boosts efficiencies to 90% and higher.”
On the downside, flue gases are no longer hot enough to go up a vent on their own and a fan is used to push them outside. The condensate, mostly water, also must be collected and piped to a drain. The condensate is very acidic. If vented up a masonry chimney (in which the cement is basic), it eventually destroys the chimney. One solution uses PVC piping to send the flue gases out of the house. The secondary heat exchanger must also be built to resist corrosion. Although most manufacturers use high-grade stainless steel, such as Allegheny Ludlum’s 294-C, for this purpose, Carrier installs a polypropylene liner. Carrier has also refined its secondary heat exchanger for greater efficiency. While others use plate-fin exchangers with 1⁄8-in. clearance between fins, Carrier uses 1⁄4 to 3⁄8-in.- clearances. “The tighter clearances tend to trap dust and debris, which decreases the exchanger’s efficiency,” says Dempsey. “With our larger gap, the exchanger is more tolerant of stuff that is not stopped by the filter. It also reduces internal static electricity, which cuts down on dirty cooling fins.”
Since two-stage furnaces are more cost effective in delivering heat, you might expect appliance manufacturers to be working on infinitely variable furnaces. “We’ve found that designing gas burners to work over such a wide range, as well as the blowers, is a big challenge,” points out Dempsey. “And we’re not sure consumers would get any benefit from it. On our two-stage models, for example, if we ran the blower any slower, electric air cleaners would produce excessive ozone levels. Running slower also causes problems for the motor bearings. And reducing the amount of heat runs the risk of creating uncomfortably cold drafts.”
One future issue the gas industry has addressed is emissions. “The EPA hasn’t targeted appliance-sized emission sources yet, but we believe at some point they will,” says Larry Brand, team leader of the Gas Research Institute’s residential space conditioning section. “We want to be sure we can do burner design well enough to lower NOx levels and meet regulations.”
The institute used a split combustionzone design to lower emissions. In the hot zone, GRI researchers reduced the oxygen to stop NOx formation. In the cold zone, they added enough oxygen to burn CO out to CO2. “This let us meet South Coast Air Quality Management District regulations of 40 nanograms per joule for emissions and equipment,” says Brand. It also didn’t affect the furnace’s overall efficiency or add more than a few dollars to component costs.
As contractors install more high-efficiency furnaces, making them more chimney friendly becomes a larger gasindustry issue. One promising method combines furnace and water-heater venting, which dilutes the flue gases and lowers the dew-point temperature for the combined gases. A lower dew point means the gases won’t condense inside the chimney, ruining masonry and draining onto basement floors. Other fixes, especially for new construction, are to install metal or plastic chimney liners, or vent the gases out a metal or PVC duct.