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Musings of an Energy Nerd

All About Furnaces and Duct Systems

Most forced-air heating systems are still poorly designed and installed

No! Don't put your furnace in the attic! Furnaces should be installed in your basement or a mechanical room near the center of your house — not in a vented attic, vented crawl space, or garage. Sadly, this illustration comes from the web site of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI). I have no idea why this system has no return air ducts.
Image Credit: AHRI

UPDATED on March 2, 2017 with information on the Dettson furnace rated at 15,000 Btu/h.

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 contractors. For many North American homes, they are a logical way to supply space heat.

[Author’s note: Since this article was written, a Canadian manufacturer has begun selling a gas furnace rated at 15,000 Btu/h. For more information, see Finally, a Right-Sized Furnace.]

Defining AFUE

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

High-efficiency furnaces (also called condensing furnaces) have AFUE ratings that range from 90% to about 97%. These furnaces have a secondary heat exchanger 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 backdrafting 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.

Ducting mistakes

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 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 commissioning 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 Blaster.

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 space, 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.

Summing up

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.”

Click here to follow Martin Holladay on Twitter.


  1. Douglas Horgan | | #1

    Condensing Oil Furnaces technically exist
    Great article as usual, Martin.
    I remember seeing a condensing oil furnace at AHR Expo a couple of years ago. While I've never seen one 'in the wild' (and I wonder how much cleaning they require) you can find them.
    Clicking around the web a bit more, one can find forum discussions about the need for proper setup, frequent cleaning, and possibly even special fuel for these units.
    All in all, a good heat pump probably makes as much or more sense. I was just standing next to a Carrier multistage unit yesterday in 9 degree temperatures, it was running along just fine. When I last did the math on a heat pump, it was less expensive to run than a propane furnace. (9 HSPF, $0.13/kWh; 95% AFUE, $2/gal)

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Douglas Horgan
    Thanks very much for the feedback and correction. I have edited my article to reflect the information you provided on oil-fired condensing furnaces.

  3. Kevin Dickson, MSME | | #3

    Supply and return ducts?
    Martin, you state that " 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."

    Fair enough, but wouldn't you still have to locate the return air ducts near exterior walls, which would negate the savings? What's the best strategy, low supply and high return or vice-versa?

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Kevin Dickson
    Traditionally, supply registers are installed low in heating-dominated climates, and are installed high in cooling-dominated climates. Studies have shown that either high returns or low returns can work well with either type of supply system.

    In a tight, well insulated house, the location of the supply registers isn't as critical as it is in an old, leaky house. Even in a heating-dominated climate, supply registers that are located near the top of an interior wall work well, as long as the registers or diffusers are chosen to have a good "throw."

    Again, assuming that you have supply registers with a good throw installed high on interior walls, return grilles can also be located near the interior of the house, with no performance penalties.

    Choosing the right supply registers is an art unto itself, deserving of a dedicated article.

  5. Steve Gordon | | #5

    I'm a small remodeling
    I'm a small remodeling contractor. HVAC has been my weakest area. This article explains a lot.

    This is a fantastic blog.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Steve Gordon
    Thanks for the feedback. I'm glad that GBA is proving useful.

  7. User avater GBA Editor
    Martin Holladay | | #7

    If I appear unresponsive...
    I will be on vacation from January 25 through February 1. So if anyone posts new questions, and wonders why I'm not answering promptly... now you know.

    I'll try to address unanswered questions on February 2 or 3.

  8. Aj Builder, Upstate NY Zone 6a | | #8

    Mini splits are in....
    Mini splits are in.... worrying about ducts... is out.

  9. Liam Branigan | | #9

    Just Getting into the Energy Auditing field
    I've been working for a mid size home performance / energy efficiency company for about 5 months. This is a great article and explains many of the problems I have noticed when repairing faulty duct work (especially things like running ducts through I-joists with sheet metal). Thank you for the great article!

  10. Terry Lee | | #10

    Minisplit VS Conventional HVAC
    My question has to do with Marc Rosenbaum recent Q&A blog on minisplits, an answer he gave below. Seems the issue can be having to leave room doors open compromising mommy and daddy’s privacy, etc, and whole house sizing(BTU’s through walls, etc) he presented some calculations on I appreciate but would not want to reply on. Manufacturing data seem unreliable too. I think if each room cannot be temperature controlled doors shut (which is one of the big advantage/selling points to potential clients aside from lower operating cost, from a builders stand point, all is lost. Others like “tricking the thermostat with heat from xmas light bulbs or you cell phone ” he suggest to set the min temp , those sort of tactics won’t sell. When it comes to comfort people rather pay the price for a proven system. So what is being suggested in the last paragraph, a hybrid forced/single zone mini air ducted that moves air from hot to cold room with doors shut, or ground fans he suggest? If so have we just lost the benefits of no ducts, associated duct problems, and installation cost? I noticed some companies offering more and more air handlers so maybe technology is moving quick here and this is a better alternative to the convention HVAC?

    When your heating and cooling loads in a net-zero home are already smaller than the capacity of anything but a single wall-mount ductless minisplit, how do you balance correct sizing with adequate distribution?

    Answer 3 — Leigha’s question in Comment #17 is a variation of the first question in a climate that will need cooling as well as heating. The house sounds like a single-story. With the bedrooms to the north in this very well insulated passive solar house, I’m assuming that they don’t have a ton of glazing, which in this case drives heat gains from the sun, as well as heat loss. So I might be comfortable using just a single unit in the main space if the occupants understood that the bedrooms will be cooler than the main space in winter and warmer in summer, and if the rooms met my 1,000 BTU/hr criterion mentioned above.
    Having said that, the system you mentioned that incorporates a wall cassette for the main space, and a ducted unit for the bedrooms would be a step up in occupant satisfaction for many people, because the bedrooms are now able to have their own setpoint. A ducted unit with the temperature sensor built into the air handler (the stock setup) averages the return temperature from the bedrooms to decide how much heating or cooling to provide. You also have the opportunity to incorporate some good filtration in that system.

    Here’s another approach: do the whole house as a single-zone ducted system. With a strongly passive solar design, the whole-house ducted system can serve to redistribute air from the warmer side of the house to the bedrooms.

  11. Elizabeth Kormos | | #11

    Whole house single zone in tight house doesn't work
    We built our tight home this last year and have a gas fired hydronic hot air system supplemented with solar thermal. We were told a single zone would work fine. The house is two stories in the front with our home offices and a guest room upstairs, an open plan living area below and a one story master suite wing in the back. The doors to all the rooms are usually open. The system, a NUair Enerboss, has an integrated HRV with separate intake ducting. The thermostat is located in the hall between the living area and the master suite. This winter we discovered to our dismay that the main living area was considerably cooler than the thermostat and that upstairs was uncomfortably warmer (80 degrees with a thermostat of 72). Also when we use the wood stove downstairs the master suite becomes uncomfortably cold and the upstairs temperature cools down a bit. We just had the HVAC firm completely block the trunk line to second floor to test if mechanical zoning would improve the situation which it did. The upstairs is now within 2 degrees of the thermostat with outside temps in the single digits. Essentially the second floor doesn't require any heat. We assume the opposite will be true in the summer when the second floor will need the most cooling. Our plan is to have mechanical dampers and thermostats controlling the second floor and the master separately from the living area.

    In retrospect I think we would have been better off with 3 minisplits but we got the solar thermal system and gas furnace at no cost by participating in a NYSERDA program to test the effectiveness of adding solar thermal to gas fired hot water systems.

    Our advice is that if you go with an air system in a tight house you should design the system for zoning unless the house is a very simple single floor open plan home.

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Terry Lee (Comment #10)
    You have raised several issues in your discussion of minisplits versus conventional ducted forced-air systems. It's fair to say that there are advantages and disadvantages to both approaches.

    If a customer wants consistent temperatures from room to room, even when the doors are closed, then a well designed, well installed ducted forced-air system probably makes more sense than a single ductless minisplit in the living room.

    That said, few forced-air systems are well designed or well installed. Poorly designed and leaky duct systems are responsible for tremendous quantities of wasted energy. Anyone interested in achieving very low energy bills should consider whether a ductless minisplit or a ducted minisplit might satisfy their needs.

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Elizabeth Kormos (Comment #11)
    Thanks for sharing your experiences. You're right that it is difficult to maintain even temperatures if a two-story house is heated and cooled with a single-zone forced-air HVAC system. I agree with your conclusion that a few ductless minisplits would have worked better for you than the system that was installed.

    For more information on this issue, see Keeping Cool in a Two-Story House, including the comments posted below the article.

  14. John McKenzie Jr. | | #14

    Condensing Oil Furnaces & 2 Stage Oil Furnaces

    I really enjoyed this article. For areas in the New England states where oil is the primary fuel for heating, ThermoPride branded furnaces (specifically the OHC95 & OH6 2-Stage) could be the best options for homeowners looking to conserve energy. Even in a new home, where energy efficient building practices are on the top of the list.

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