Skip navigation
Machine Design

Heating with Gas

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

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.

Making installation easy and inexpensive
Putting gas furnaces or other gas appliances in a house once required routing heavy black-iron pipe from the gas hookup to the gas-powered unit. Black pipe is difficult to run through walls and around joists, with each twist and turn requiring another fitting. And each joint needs to be sealed and checked for leaks. A recently developed piping product, corrugated stainless-steel tubing (CSST), is changing all that.

The flexible tubing is made of stainless steel covered with a yellow polyethylene jacket. (Yellow is the international color for gas piping.) It comes in a variety of dimensions. Parker Hannifin Corp.’s line of CSST, for example, is made in 3⁄8, 3⁄4, and 1-in. diameters, small enough to fit inside walls, floors, and roofs. Since CSST is flexible, it requires fewer fittings to snake around corners, and any necessary connections can be made without heavy equipment or pipe threading. A 250-ft roll weighs about 37 lb, light enough for a single worker to carry and install.

Although CSST is comparable to black pipe in material costs, the labor and installation costs drop 24 to 66% in new construction, and 72% in rehab projects, according to studies by the Gas Research Institute. The gas industry hopes the lowered installation costs possible with CSST will help convince home owners with one or two gas appliances, mainly a furnace and water heater, to add more, perhaps another heater in the attic or a gas-log fireplace.

Pulse technology
Unlike most other furnaces, the Lennox Pulse 21 uses pulse technology to wring as many Btus out of natural gas as is it can, achieving an AFUE (annual fuel-utilization efficiency) of up to 96.2%, compared to 70% for older furnaces and 90% for most newer ones. Besides its unique combustion process, it features automatic shutdown, which shuts off when it senses a blocked flue or other abnormal operating condition. The Watchguard feature automatically resets ignition after a power outage or interruption in the gas supply. It also has a heavy, cast-iron and stainless-steel heat exchanger which fully contains combustion and makes for efficient heat transfer. The spark igniter acts much like a car’s spark plug, eliminating the wasted gas and exposed flame of a pilot light.

Heating and cooling with gas
In searching for an economical way to cool houses using natural gas, the gas industry, represented by the Gas Research Institute, together with Battelle Laboratories, York International Corp., York, Pa., and Briggs & Stratton Corp., designed and built the Triathlon system. The system uses two units, one indoors and the other outside. A conventional vapor-compression refrigeration cycle provides heating and cooling, with the compressor run by a cleanburning natural-gas engine. The outside unit contains the engine, compressor, traditional condenser coils, an auxiliary heater and most of the controls, so all combustion occurs outside the house. The indoor portion includes a radiator and blower fan.

The heart of the system, a single-cylinder, water-cooled, 5-hp engine dubbed the Marathon, was developed specifically for the Triathlon by Briggs & Stratton. It uses leanburn technology and heavy-duty components for a long life and increased efficiency. Controlled by a microprocessorequipped “smart” thermostat, the engine has 17 different speed settings from 1,200 to 3,000 rpm to meet the cooling and heating loads. The indoor blower, an electronically commutated motor from GE, also has 17 settings — between 400 and 1,400 ft3/min — and the engine and blower speeds are matched to maintain efficiency. “Some high-efficiency heat pumps can cool or heat a house quickly, but then they stop, and so does circulation, dehumidification, and air cleaning,” says Barry Swartz, product manager. “The Triathlon, on the other hand, runs for longer periods, but at lower speeds so it continues moving, cleaning, and dehumidifying air while it cools.”

For cooling, the Triathlon functions much like an ordinary heat pump, pulling heat out of the house and moving it outdoors. It uses a standard refrigerant cycle with R-22, an HCFC not as allegedly harmful to the ozone layer as CFCs and used by all major air conditioning companies. R-22 is scheduled to be outlawed early in the next century.

Heat pumps have gotten a bad reputation for warming indoor spaces mainly based on “cold blow.” They heat by forcing 85°F air through ducts and since humans have a body temperature of about 98°F, that air doesn’t really feel warm and the house feels drafty. Triathlon’s engine carries a recuperator, a tube-and-shell heat exchanger, to capture engine waste heat. That heat is added to what’s being pumped indoors. It raises the air temperature 10 to 15°F, making the house feel more comfortable. On really cold days, an auxiliary gasfired heater kicks on to add more heat to circulating air.

To keep the system quiet, York engineers concentrated on the outdoor unit where the reciprocating engine is mounted. It is installed inside an insulated box, with the exhaust sent through the recuperator, which also absorbs noise. Exhaust then passes through a silicon-lined hose to a muffler and through another silicon-lined hose to the outside. Silicon lets the hoses handle high-temperature exhaust gases and absorb some noise. The inlet duct is a maze of insulated baffles that feeds into a filter then the carburetor. The overall system is comparable in noise to competing high-efficiency electric heat pumps, except at the unit’s highest heating or cooling setting, which it shouldn’t run at for more than 20% of the time no matter how hot or cold.

With its novel configuration, Triathlon has an AFUE (annual fuel-utilization efficiency) of 126%, compared to 95% for the most efficient gas furnaces. “That’s because AFUE looks at total Btu output from gas divided by total Btu input. And we seem to be getting more out than we’re putting in because the unit not only makes heat, it moves heat from outside to indoors,” says Swartz. “And in furnaces, which just make heat from gas, there’s always some losses so their AFUE is never over 100%.” To compare it against electric heat pumps, Triathlon has an HSPF of 14. (HSPF stands for heating seasonal performance factor, the estimated seasonal heating output in Btu divided by the seasonal power consumption in watts. It serves as a rating for comparing air-source heat-pump performance) The unit’s HSPF rating is 40% higher than the most efficient electric heat pump on the market. The system also lets homeowners take advantage of lower summer gas rates available in some parts of the country. “The approximate cost of an installed 3-ton system ranges between $7,000 and $10,000, which is more expensive than a conventional system,” notes Swartz. “But you can recoup the difference quickly with lower gas rates, gas company rebates, and the system’s overall efficiency.”

© 2010 Penton Media, Inc.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.