All About Furnaces and Duct Systems
Most forced-air heating systems are still poorly designed and installed
UPDATED on October 2, 2014 with more information on duct system design.
Many different appliances can be used to heat a house, including boilers, water heaters, heat pumps, and wood stoves. However, most homes in the U.S. are heated by a forced-air furnace.
These devices are connected to ducts that deliver heated air to registers throughout the house. Different types of furnaces are manufactured to burn a variety of fuels, including natural gas, propane, oil, and firewood. The most common furnace fuel in the U.S. is natural gas.
In Europe, where furnaces are almost unheard of, most homes are heated by a boiler that distributes heat through hot-water pipes. Unlike Europeans, however, most Americans insist on central air conditioning in their homes. It’s easier to provide whole-house air-conditioning in a home with a duct system. Once you have a duct system for cooling, it’s cheaper to install a furnace for winter heating than to install a boiler with a separate distribution system.
Even though the smallest available furnaces are often oversized for a high-performance home — a problem I addressed in a 2009 article, Heating a Tight, Well-Insulated House — furnaces still have virtues that are hard to ignore. They are inexpensive, widely available, and easily serviced by local HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors. For many North American homes, they are a logical way to supply space heat.
When it comes to fuel efficiency, residential furnaces in the U.S. are divided into two main categories: so-called “medium-efficiency” furnaces and “high-efficiency” furnaces.
Furnace efficiency is usually calculated using a laboratory procedure that measures an appliance’s “annual fuel utilization efficiency,” or AFUE. This calculation accounts for heat losses up the chimney, heat losses through the appliance jacket, and heat losses due to on-and-off cycling, but it doesn’t account for electricity use (fan energy use) or heat lost through the distribution system (ductwork).
AFUE can be calculated for boilers as well as furnaces, and is used for appliances that burn many different types of fuel.
Low-efficiency and medium-efficiency furnaces
The usual definition of a “low-efficiency” furnace is one that is less than 75% efficient. The reason that you can no longer buy a low-efficiency furnace is that the federal government now requires residential gas-fired furnaces to have a minimum efficiency of 80%. (The minimum efficiency for oil-fired furnaces is 83%, except for oil-fired furnaces designed for installation in mobile homes, which have a minimum efficiency of 75%.)
Medium-efficiency furnaces have efficiencies in the range of 80% to 82%. The line between mid-efficiency and high-efficiency furnaces is not arbitrary, but marks the division between appliances with distinct operating characteristics. Mid-efficiency furnaces are designed to keep flue gases hot enough to avoid any condensation of flue-gas moisture, while high-efficiency furnaces deliberately encourage the condensation of flue-gas moisture.
It is technically difficult to manufacture a furnace with an efficiency between 83% and 89%, so none are available in that range. Furnaces with "in-between" efficiency have sporadic condensation of flue gases, and this condensation causes corrosion problems. Furnaces with an efficiency of 90% or more wring so much heat out of the flue gases that the furnace exhaust can be vented through PVC pipe, a material which is more resistant to corrosive condensate than the stainless-steel vent pipe that would have to be used for the hotter flue gases that would occur in a furnace with an efficiency in the tricky 83% to 89% range.
High-efficiency furnaces (also called condensing furnaces) have AFUE ratings that range from 90% to about 97%. These furnaces have a secondary heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. where the moisture in the escaping flue gases is condensed. This phase change from water vapor to liquid water releases heat, improving the unit’s efficiency. Condensing furnaces must be hooked up to a drain that can dispose of the liquid condensate.
A high-efficiency furnace costs more than a mid-efficiency furnace. However, the venting system for a high-efficiency furnace may cost less than the chimney required for a mid-efficiency furnace.
Most condensing furnaces burn either natural gas or propane. While condensing oil-fired furnaces exist, the devices have a mixed reputation. According to some HVAC specialists, oil-fired condensing furnaces require frequent cleaning.
Single-stage, two-stage, and modulating furnaces
The simplest furnaces are single-stage furnaces with single-speed blowers. If the furnace is rated with an output of 60,000 Btuh, that is the furnace’s output whenever it is running.
More sophisticated two-stage furnaces can operate at two different output levels. Most of the time, these furnaces operate at a lower Btuh output; the higher output is only needed on the coldest days of the year.
Modulating gas furnaces are more sophisticated than two-stage furnaces. They include an automatic fuel valve that varies the amount of fuel delivered to the burner. Many modulating furnaces also include a variable-speed blower motor (usually an electronically commutated motor, or ECM) which (like the automatic fuel valve) ramps up and down in response to heating demand. Since modulating furnaces can match the heating demand precisely, they provide more even heat than single-speed furnaces which operate with a stop-and-go jerkiness.
Oil-burning furnaces are less flexible than gas furnaces. While it’s fairly easy to design a gas valve which varies the amount of fuel delivered to the burner, oil burners have a nozzle that is optimized for a single firing rate at a fixed Btuh output. That’s why oil furnaces are usually single-stage furnaces.
Condensing furnaces are power-vented, so they include at least two fans: an air-handler fan that distributes warm air through the home’s ductwork, and a power-vent fan to move exhaust gases through the flue pipe.
Most, but not all, condensing furnaces are “sealed-combustion” furnaces — meaning the burners pull outside air into the combustion chamber through plastic ducts to feed the fire’s needs. Sealed-combustion furnaces don’t use any indoor air for combustion. The main advantage of a sealed-combustion furnace (compared to an old-fashioned atmospherically vented furnace) is that a sealed-combustion furnace is much less likely to suffer from backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney. problems. (Backdrafting occurs when a powerful exhaust fan — for example, a range hood fan — depressurizes a house enough to draw combustion fumes down the chimney and back into the house. For more information on this issue, see Makeup Air for Range Hoods.)
A low thermostat setting may void your furnace warranty
Energy advice columnists routinely advise owners of vacation homes to turn down their thermostats when the homes are unoccupied. For example, the “Home Energy Saver Answer Desk” at a website maintained by the Environmental Energy Technologies Division at Lawrence Berkeley National Laboratory was posed this question by a reader: “How can I save energy in my second home, which is unoccupied a large part of the year?”
The LBNL experts responded, “For cold-climate homes, turning the heat off (or at least way down) while away is a natural starting point. … Turning the heat way down (e.g. to 40-45 degrees) should provide adequate freeze protection at much-reduced cost.”
As it turns out, homeowners following this advice are not only at risk of damaging their furnace; they are at risk of voiding their furnace warranty. The problem was brought to my attention by Jonathan Beers, the residential services manager at Madison Gas and Electric Company in Madison, Wisconsin, and his colleague Mark Faultersack, the manager of multifamily services. “I had a conversation with a customer — the guy had a vacation home in northern Wisconsin,” Faultersack told me. “When he wasn’t there, he kept his furnace at 50 degrees, and his Carrier furnace failed — the heat exchanger rotted out.”
If you read the fine print on the installation instructions for Carrier condensing furnaces, you'll find this statement: “This furnace is designed for continuous return-air minimum temperature of 60°F db [dry bulb] or intermittent operation down to 55°F db such as when used with a night setback thermometer [thermostat]. Failure to follow these return air limits may affect reliability of heat exchangers, motors and controls.”
Intrigued, I contacted the Carrier Corporation and asked whether setting one’s thermostat to 50°F would void the warranty on a Carrier condensing furnace. Here was Carrier’s official response: “For optimal performance, Carrier Corp.’s 58MXB gas condensing furnace should be operated with return-air temperatures no lower than 60°F and no higher than 80°F. To support appropriate return-air temperatures, Carrier recommends that the 58MXB furnace be set within the range of 55°F to 80°F. Return-air guidelines and detailed operating instructions are included in the 58MXB owner’s manual. Failure to operate the furnace according to the owner’s manual could affect the furnace’s reliability and void the factory warranty.”
The bottom line: condensing furnaces are more efficient than non-condensing furnaces, but their efficiency comes with the added risk that low return-air temperature can contribute to the condensation of corrosive flue gases in the primary heat exchanger.
During the 1950s and 1960s, fuel was so inexpensive in the U.S. that most heating contractors routinely installed leaky ductwork. In many areas of the country, contractors still install ductwork in vented crawl spaces or vented attics; since these locations are outside of a home’s conditioned envelope, the conditioned air that escapes from leaky ductwork in these locations is gone for good.
To make up for the fact that leaky duct systems waste large amounts of energy, HVAC installers usually install oversized furnaces with huge blowers.
In the 1980s, energy-efficiency advocates responded to the nation’s leaky duct crisis by establishing training programs to encourage HVAC installers to seal duct seams. After three decades of training, these programs are beginning to bear fruit in some areas of the U.S. Unfortunately, the gospel of airtight ductwork hasn’t reached every corner of the country, and many HVAC contractors are still installing ductwork the way their grandfathers did in 1964.
Here is a list of the most common duct design and duct installation errors:
- Trying to design a duct system without performing a room-by-room heat loss calculation. For more information on this issue, see Saving Energy With Manual J and Manual D.
- Locating ducts outside of a home’s thermal envelope (for example, in a vented attic or vented crawl space). For more information on this issue, see Keeping Ducts Indoors.
- Failing to provide a return-air pathway from every room in the house back to the furnace. For more information on this issue, see Return-Air Problems.
- Undersizing return air ducts. (Return air ducts should be at least as large as supply air ducts.)
- Using framing cavities like stud bays or panned joist bays instead of ducts to move supply air or return air.
- Failing to seal duct leaks. For more information on this issue, see Sealing Ducts and Duct Leakage Testing.
Designing a duct system
The best way to design a duct system is to follow the Manual D method developed by the Air Conditioning Contractors of America (ACCA). The use of Manual D presupposes that you have already performed room-by-room heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. and cooling load calculations using Manual J.
Unfortunately, most HVAC contractors install systems without performing Manual J and Manual D calculations.
To help educate yourself on the elements of duct system design, and to double-check the expertise of your HVAC contractor, you may want to learn a simplified duct design method like the one outlined in “Trouble-Free Forced-Air Heat” by Gary Bailey. While the method described in Bailey’s article is no substitute for the Manual D design process, it is probably better than the method used by many HVAC contractors.
Bailey’s article includes a chart showing the cfm capacity and Btuh capacity of different duct sizes. The chart assumes that the maximum distance from the furnace to a supply register is 60 feet; this maximum allowable duct length must be decreased to account for each elbow in the duct run.
A standard duct size used to serve individual registers in residential forced-air systems is often 6-inch round galvanized duct, which can deliver 100 cfm and 7,400 Btuh of heat. Bailey advises, “Size individual room ducts based on a room-by-room heat loss calculation. Size the trunk line to carry the total cfm of all the branch ducts. Step down the trunk line to maintain air velocity, making sure that each new trunk section has the capacity to carry all the branch lines coming off from that point to the end of the trunk.”
For more information on designing residential duct systems, see:
More ducting tips
Every branch duct running to a register needs a balancing damper. These dampers are adjusted as part of the commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. process to make sure that each room gets the design air flow.
In general, undersized ducts cause more problems than oversized ducts. If your duct system is undersized, air flow will be constricted and the furnace may not be able to remove heat fast enough to prevent damage to the heat exchanger.
Return air ducts need to be as large as or larger than supply air ducts. Most residential HVAC systems have undersized return ducts; when in doubt, make them bigger.
Galvanized ducts are always preferable to flex duct. The corrugations in flex ducts cause turbulence that reduces airflow through the duct; moreover, flex duct is hard to keep straight and well supported. For maximum efficiency, ducts should be as straight and as short as possible, with a minimum of elbows. Whether you choose galvanized ducts or flex ducts, make sure to install enough duct hangers to prevent sagging.
Traditionally, supply registers were usually located near exterior walls, in an attempt to counteract the chilling effect caused by winter infiltration and the radiational cooling that occurs when warm bodies lose heat to cold window glass. If you are building a tight house with thick insulation and high-quality windows, however, it’s possible to install supply registers on interior walls. This strategy results in shorter duct runs that operate more efficiently than longer ducts extending to a building’s exterior walls.
It should go without saying that duct seams should be sealed with mastic and duct systems should be checked for leakage with a Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage..
Furnaces leak, too
Unfortunately, furnaces and furnace plenums often leak as much as some duct systems. If your furnace is located inside your home’s conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. , these leaks may not matter very much. But if your furnace is located in a garage or vented attic — a bad idea, by the way — leaky furnaces waste energy.
Brand-new furnaces and air handlers are delivered from the factory with leaky seams. As typically installed, furnaces also have leaks between the furnace and the plenums. In a study conducted by the Florida Solar Energy Center (FSEC), 69 furnaces and air handlers were measured for leakage. On average, 5.3% of system airflow was leaking at the furnace or air handler. (Of course, additional leakage occurred in the homes’ duct systems.)
Commenting on the research, Philip Fairey, FSEC’s deputy director, noted, “In most cases the units as shipped from the factory contain seams that leak. Some factory seams are gasketed, but in many cases they could be better.” The solution: feel for air leaks, and seal any accessible seams with aluminum tape or mastic.
Here’s a checklist of the steps you need to take to create an efficient, high-performance forced-air heating system:
- Perform a room-by-room heating load and cooling load calculation.
- Avoid the temptation to buy an oversized furnace. Specify a furnace that meets your home's design heating load, without tacking on a "safety factor."
- If you live in a cold climate, specify a condensing furnace.
- Locate the furnace in the center of your basement or in a mechanical room near the center of your house.
- Design the duct system using Manual D.
- Locate all ducts within the home’s thermal envelope.
- If the house has high-performance windows and a low rate of air leakage, locate supply registers on interior walls.
- Keep duct runs short and straight, with as few elbows as possible. It’s better for ducts to be slightly oversized than undersized.
- Minimize the use of flex duct. If flex duct is installed, support it with an adequate number of hangers, and make sure the duct runs aren’t twisted, crushed, or pinched.
- Design a return air system with multiple return air grilles rather than a single central return.
- Plan for a return air path from every conditioned room back to the furnace’s return air plenum.
Don't forget to consider other options
After reading this article, you may know more about furnaces than you used to. Before specifying a furnace for your next project, however, remember that other options exist. An increasing number of high-performance homes are heated and cooled with two or three ductless minisplit heat pumps.
Martin Holladay’s previous blog: “Justin Fink’s Canned Spray Foam Tip.”
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