The Highest Efficiency Rating Is Found in Condensing Furnaces

Bird's Eye View

Warm-air furnaces are the most common type of central heat

The majority of central heating systems installed in new houses in the United States include a warm-air furnace. The most efficient furnaces have annual fuel utilization efficiencies (AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. ) topping 95%, considerably higher than the federal minimum AFUE of 78%. In standard furnace designs, with average efficiencies of about 80%, flue gas temperatures are higher. Although standard furnaces avoid the problems associated with corrosive condensate, they have higher fuel costs. Condensing furnaces are the most efficient and have a AFUE of at least 89%.

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Key Materials

Ducts leak, and so do furnaces

Although many builders are aware of the problem of duct leakage, few builders realized that new furnaces and air handlers include 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. 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: seal accessible seams with aluminum tape or mastic.

Design Notes

Keep duct runs short by placing furnace in a central location

Furnaces perform best in a central location that keeps ducts short and away from outside walls. At the same time, they need to have good access to an outside wall or roof for installation of combustion air intake and exhaust pipes. Furnaces need to be installed where they won't draw contaminants into the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. of the home. Although a garage may seem like a convenient location for heating equipment, it would be best to create a separate furnace room to keep carbon monoxide and other fumes out.

Ducts for warm-air furnaces take up a lot of space — important to consider when laying out walls and floors. Open-web floor trusses make running ducts easier, and can be combined with ventilation air systems. Dedicated vertical passages should also be provided for supply and return ducts. A vertical chase can also be a huge benefit for plumbing supplies, plumbing vents, electrical runs in addition to ventilation ducting. Registers are usually located under windows to stop convection currents, but in a tight, well insulated house with high-performance windows, this may not be necessary.

Exposed ducts save materials and energy. Borrowed from commercial and institutional design, exposed ductwork can save resources by eliminating the need for additional ceiling material. It also ensures that ducts are inside the conditioned space — an important consideration for energy efficiency. Any open plan home can benefit from this approach. This is more common in contemporary homes but traditionally styled timber frames are often built this way too.

Builder Tips

Duct design and installation both crucial

Why spend all that money on a high-tech furnace if you're going to squander the energy savings with inefficient ducts? Metal ducts sealed with mastic will make sure all the hot air gets where its going without picking up mold and dirt along the way. Running all of your ductwork inside the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. is another way to minimize heat losses.

Make sure the air has an easy path back to the furnace to be heated again. If you don't use multiple return registers, include jumper ducts or transfer grilles to prevent pressure differences between rooms.

A furnace needs air to burn fuel too. If you're building envelope isn't very tight, you can rely on natural air infiltration for combustion air. Otherwise you need to provide a dedicated outside air source to keep the furnace from using up the oxygen in the house — or worse — backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney. exhaust gasses. The best option is a sealed-combustion furnace with its own outdoor air supply.

Code Issues

Outside airClick for slide show

Regulations cover duct size and materials

General requirements for forced-air furnaces can be found in Section 2442 of the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.. Additional provisions related to combustion make-up air and venting can be found in Sections 2407 and 2427, respectively.

Supply and return ducts should be sized to total at least 2 sq. in. for every 1000 Btus of output capacity provided this doesn’t exceed the manufacturer’s own requirements (2442.2). Outside and return air requirements must conform with Section 2442.5.

Duct material and installation must conform with Chapter 16. Ductwork must be adequately supported (1601.3.2) and made substantially airtight with tape, mastic, gaskets or other approved material (1601.3.1).

Illustration: Code Check Plumbing 3rd Edition. click to buy .


Change filters for higher efficiency not clean air

According to a study done by the Canada Mortgage and Housing Corporation (CMHC), the furnace filter you use has little effect on indoor air quality. That’s mainly because a typical furnace runs only about 20% of the time, and even if you used a 50% efficient filter, it would remove just 10% of particulates (50% of 20%).

However, if your furnace filter is dirty, changing the filter will help your heating system run more efficiently. Changing the filter is an inexpensive way of assuring that airflow is not restricted by a dirty filter.


LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. Up to 3 points based on furnace efficiency rating under EA6 (Energy & Atmosphere).

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 7 — Energy Efficiency: up to 17 pts. based on superior system efficiency of furnace and climate (703.4.2).


Venting efficient furnaces is simple

Furnaces with efficiencies of 89% or above are “condensing” or “modulating-condensing” designs. Condensing furnaces use 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. to condense water vapor in flue gases and capture heat that would otherwise be sent up the chimney. Because flue gases are cooled, they can be direct-vented with plastic pipe rather than piped into a masonry or metal flue. The corrosive condensate from the heat exchanger is drained away.

Modulating designs use automatic fuel valves to regulate the amount of fuel burned based on space heating demand. With the addition of variable-speed motors, these furnaces produce just the amount of heat that’s required, unlike less sophisticated designs with single- or two-speed blower motors.

Condensing furnaces are power-vented. They are usually equipped with a sealed-combustion burner that draws combustion air from the outside into a sealed chamber where it is burned. Sealed combustionCombustion system for space heating or water heating in which outside combustion air is fed directly into the combustion chamber and flue gasses are exhausted directly outside. eliminates the risk that combustion fumes can be pulled into the house when an exhaust fan (for example, a range hood fan) depressurizes the house.

Worth the higher cost. Rapidly rising fuel prices make high-efficiency furnaces cost-effective in cold climates, despite the fact that they are more expensive than standard models. The cost of residential natural gas has nearly doubled since 2000. Fuel oil, a major source of home heat in the Northeast, has increased nearly fourfold in cost over the same period.

In warm climates (for example, in Southern California, New Mexico, and the Gulf Coast states), upgrading from a standard furnace to a condensing furnace is not currently cost-effective.

Furnaces also burn wood. Several manufacturers produce residential wood-fired hot-air furnaces, including Alpha American (Pallisade, MN), Blaze King Industries (Penticton, British Columbia), Fire Chief (Fenton, Mo.), Falcon Manufacturing (Winnipeg, Manitoba), Newmac (Debert, Nova Scotia), PSG (Quebec, Quebec), and U.S. Stove Co. (South Pittsburg, TN).

Both stand-alone and add-on models (designed to work in tandem with a gas or oil furnace) are available.


All About Furnaces and Duct Systems

American Council for an Energy Efficient Economy worksheet on estimating savings by switching to a high-efficiency heating appliance:
http://www.aceee.org/consumerguide/heating.htm - new

Image Credits:

  1. John Hartman / Fine Homebuilding
  2. Andy Engel/Fine Homebuilding
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