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Furnace

Components | Combustion Components | Controls | Air Distribution | Choosing a Furnace for Optimal Performance | Low-Efficiency | Mid-Efficiency | Condensing or Sealed Combustion | Potential Savings for a Furnace Upgrade

Quick Facts

  • AFUE (Annual Fuel Utilization Efficiency) measures the percent of fuel which is converted to useful heat
  • The most efficient furnaces achieve much higher AFUE by using more heat exchangers to extract heat from exhaust air
    • Low AFUE: 55%-65% (no longer sold but present in many homes)
    • Mid-range AFUE: 78%-85% (the minimum sold today)
    • High-efficiency AFUE: 90%-97%
  • Energy consumption is also impacted by the efficiency of the fan motor which blows the heat through the duct system
  • It is more cost effective to seal and insulate a home’s building envelope and duct system before investing in a high efficiency furnace, since a smaller furnace can suffice in a home which retains heat better
  • Higher total efficiency and increased comfort can be attained by combining space heating with water heating in a hydronic forced air system

Components

Furnaces have several functional components; each can impact a different aspect of system efficiency.

Combustion Components

Unlike low AFUE furnaces (which are no longer sold) mid and high efficiency furnaces increase efficiency by better controlling air flow through the furnace (for more complete combustion) and by using more extensive heat exchangers to transfer combustion heat to household air.

Burners

As the name indicates, this is where fuel is burned to produce heat for the system.

Heat exchanger

Combustion gases from the burner travel through the a heat exchanger, a metal structure whose function is to absorb combustion heat and transfer it to air which blows over the heat exchanger and is distributed throughout the house, thereby delivering heat.

Draft inducer

The oldest furnaces have a wide opening to ambient air around the furnace which is pulled up the venting chimney and dilutes the flue gases.  Newer mid-efficiency furnaces have a fan which induces the flow of exhaust gases up the chimney instead of relying on the natural draft created by rising heat.  The highest efficiency furnaces are sealed combustion systems which use outside air for combustion and exhausts combustion gases directly outside, eliminating the need for a draft inducer.  A draft inducer also eliminates the concern of backdrafting and spillage of combustion gases.

Venting

Most furnaces vent through a metal flue.  High efficiency condensing furnaces extract much more heat from exhaust gases and can thus vent them through a plastic pipe such as PVC schedule 40 (according manufacturer’s specifications).

Low- mid-efficiency furnaces use atmospheric combustion, where gas combustion and exhaust are open to the ambient space where the furnace is located (such as garage, crawlspace of hallway closet).  In contrast, high-efficiency furnaces use sealed combustion, in which the combustion chamber as well as any draw of oxygen or venting of exhaust gases is completely separated from the ambient space, by use of a supply and exhaust pipes which are run to the outdoors.  This makes sealed combustion much safer than low- or mid-efficiency since sealed combustion reduces or eliminates dangers from backdrafting or infiltration of gas, carbon monoxide, or other combustion gases.

Pilot light

The oldest and least efficient furnaces have a standing pilot light which ignites gas which flows to the burners when the furnace is turned on by the thermostat. A pilot light burns continuously, even when the furnace is not running.

Controls

Newer furnaces also conserve energy by using an electronically controlled gas valve and ignition system instead of a standing pilot light, so no gas is consumed while in “standby mode”.  Some furnace models have firing controllers which are dual stage, triple stage, or modulating, meaning that the furnace fires as a function of the heat needed (instead of always firing full blast); this can increase comfort by keeping a more steady indoor temperature. The newest top tier models are “communicating” , meaning that the thermostat, furnace, and air conditioning condenser communicate electronically with each other to optimize performance and to indicate when maintenance is needed.

Circuit board controller

Modern mid-efficiency and condensing furnaces have a circuit board which is turned on when the thermostat calls for more heat.  It initiates and controls the whole firing sequence from turning on the fan motor to preparing the electronic ignition to opening the gas valve for combustion in the burner.

Just like modern higher efficiency cars, higher efficiency furnaces have more complex circuit boards which manage varying fan speed and burner firing in order to constantly maintain the highest level of fuel efficiency, electrical efficiency, and comfort

Ignition

Modern furnaces attain higher efficiency by using electronic ignition instead of a standing pilot light to ignite gas which flows to the burner.

Air Distribution

The furnace brings air from the duct system, blows it over the heat exchanger and back out to the duct system and can be referred to as the “air handler”.  Air movement is controlled by an electronically controlled fan which blows cool interior air over the heat exchanger, warming it and delivering it back to the home through the duct system.  Regular maintenance to keep the filter and fan free of dust and debris can assure that the furnace functions at its highest efficiency level.

Fan and motor

This electrically controlled fan propels the air through the forced air system.  Higher efficiency can be attained by regularly having the fan blades cleaned and by choosing a furnace with a multi-speed or variable speed motor.

There are two kinds of fan motors which work in different ways and have different impacts on energy use and comfort.

  • Single Speed (PSC) motors are the most common:
    • How it works: the motor runs at a constant speed which is designed to deliver sufficient airflow at an optimal static pressure (air resistance in the ducts)
    • Comfort: duct systems commonly have more air resistance than these motors are optimally designed for, since the motor cannot adjust to this the result is less airflow and circulation of conditioned air, meaning less comfort
    • Energy use: at higher levels of static pressure these motors actually use less power to run, meaning lower fan motor energy use in systems with poorly designed duct systems, however, this comes at a cost of decreased comfort (since heated or cooled air is not properly circulated through the house), often causing occupants to set the thermostat even higher in the winter or lower in the summer, meaning higher overall energy use
  • Variable speed (ECM) motors are found in the highest efficiency furnaces:
    • How it works: the motor is set to maintain a constant level of airflow, regardless of static pressure, so in duct systems with high air resistance the motor will run harder to maintain optimal airflow
    • Comfort: since airflow, which is necessary for mixing the air in a house to maintain comfortable, even temperatures, is never sacrificed these motors can make a house feel comfortable even with ducts that restrict airflow
    • Energy use: since the motor must work harder, energy use increases with increased static pressure; in a house with poor ducts fan energy use will be quite high despite increased comfort, while fan energy use in a house with well designed ducts will be quite low meaning that the same comfort comes at a lower cost

These differences underscore the fact that when seeking to improve HVAC comfort or efficiency, it is vital to fix the whole system (including the ducts), not just replace an outdated furnace with a newer, high efficiency model.  Since they consume much less energy when there is less air resistance, variable-speed motors should be paired with a well designed duct system to ensure comfort and energy efficiency. Also, when seeking to include whole house air filtration in an HVAC improvement, variable speed motors can be a particularly good investment since they consume one eighth the maximum power when delivering one half of the maximum design airflow.

Filter

All air which goes through a forced-air system should pass through a filter every time it cycles through the system.  The filter collects dust, lint and debris which eventually impedes airflow and makes the fan motor work harder or deliver less airflow to the house.  Basic 1” media filters should be cleaned and changed multiple times a year to maintain optimal furnace efficiency.  For better indoor air quality it is also possible to combine a furnace with an air cleaning air filter made to remove even smaller particles from the air.

Basic filters are primarily designed to protect the equipment from dust and debris. Basic 1” media filters designed to improve indoor air quality will typically also have a high resistance to airflow (this airflow resistance is also known as static pressure).

Choosing a Furnace for Optimal Performance

There are three main types of furnaces:

Each has different characteristics which affect efficiency.  Potential savings from upgrading to a new furnace depend on the annual run time and efficiency level of the furnace being replaced, as well as that of the new furnace.

Highest efficiency (AFUE close to 97%) furnaces include:

Search for Energy Star Qualified furnaces.

Low-Efficiency

The least efficient of these furnaces range from 55%-65% AFUE; more efficient model reach 68%-72% AFUE. They use simple technology and are common in homes built before the 1990s.

Standing Pilot Light

This continuous flame is a source of inefficiency because gas is continuously burned, even when heat isn’t being delivered to the home.

Heavy Heat Exchanger

The mass of the heat exchanger takes longer to heat up and cool down and the shape doesn’t create enough surface area for heat exchange, leading to significant cycling losses:

  • When the furnace fan comes on, it blows cooler air because heat is first absorbed by the heat exchanger before being transfer to the home
  • When the furnace fan turns off, heat is trapped in the heat exchanger and not transferred to the home
  • Every time the furnace “fires” the heat exchanger expands as it heats up and then contracts as it cools. This repeated cycle stresses the metal heat heat exchanger and in many cases causes it to crack, creating a dangerous CO leak.

Draft Diverter

The natural draft of hot combustion gases draws air through the draft diverter and creates a flow of hot air up the chimney vent. This is a major source of inefficiency because:

  • Heat in the exhaust gases is lost
  • The amount of air for combustion is not controlled, leading to incomplete or inefficient combustion

The diverter is a safety device installed on atmospheric appliances to allow:

  • The draft to divert into the building if the vent becomes blocked.
  • Add dilution air for draft efficiency (also called a draft hood)

While such a device may be useful in some situations, it can be quite dangerous on an older furnace if the heat exchanger is cracked and there is carbon monoxide flowing into the furnace exhaust.

Chimney Venting

The combustion of this type of furnace is vented through a metal chimney through the roof or through an exterior wall.  Since the combustion process is open to outside air as well as to the ambient air around the furnace, the furnace is susceptible to back drafting, in which exhaust gases are pulled back into furnace, introducing carbon monoxide and other noxious gases into the home.  A draft diverter exacerbates this risk.

Mid-Efficiency

These furnaces range from 78% to 85% AFUE.  They are the least efficient furnaces on the market, but attain higher efficiency than conventional furnaces with a few basic improvements.

Tubular Heat Exchanger

The tubular shape of these heat exchangers provides more surface area to provide for more effective heat exchange and resulting in more heat being delivered to the home.

Electronic Ignition

Instead of having a standing pilot light which is ready to ignite fuel when the furnace comes on, mid-efficiency (and high efficiency condensing) furnaces use an electronic ignition system.  The ignition process is controlled by a circuit board which turns on the fan, prepares ignition, and releases gas when the thermostat calls for more heat.

Draft Assisting Fan

Instead of relying on natural heat induced draft to vent exhaust gases and bring new air into the burners for combustion, a draft fan is used to pull exhaust gases out at a controlled rate, pulling combustion air into the heat exchanger at the same time.  This leads to more complete combustion.

Chimney Venting

The combustion of this type of furnace is vented through a metal chimney through the roof or through an exterior wall.  Since the combustion process is open to outside air as well as to the ambient air around the furnace, the furnace is susceptible to back drafting, in which exhaust gases are pulled back into furnace, introducing carbon monoxide and other noxious gases into the home.
A draft assisting fan reduces the risk for this by creating air pressure which induces the air to flow out through the chimney instead of back into the house.  However, while this reduces the risk of back drafting carbon monoxide back through the furnace and into the home, it does not eliminate it since combustion is still an open process.

Condensing or Sealed Combustion

Condensing furnaces generally qualify for an Energy Star rating and range from 90%-97% AFUE.  A condensing furnace keeps some of the improvements of mid-efficiency furnaces, such as electronic ignition and tubular heat exchanger, but adds precision engineering which can squeeze up to nearly 20% more fuel efficiency out of the furnace (comparing 78% AFUE for mid-efficiency to 97% for the highest AFUE condensing furnaces).

Secondary Condensing Heat Exchanger

In addition to a primary tubular heat exchanger, condensing furnaces have a second heat exchanger which wrings much more heat energy out of the combustion gases, causing them to cool to about 100 ° F and condense to a liquid state. Mid-efficiency furnaces specifically avoid condensing combustion gases because they are not made to withstand the acidic content of the condensed gases.  However, in condensing furnaces the metal of the heat exchanger is made of treated metal to resist corrosion from the acidic condensate.  This leads to the jump in fuel efficiency over mid-efficiency furnaces.

Sealed Combustion Plastic Venting

Since the combustion gases are cooled and mostly condensed before being exhausted from the furnace they can be vented through a normal plastic PVC pipe; the liquid condensate is drained through a separate pipe.  There is no risk of back drafting with a condensing furnace because the combustion is a sealed process.  Instead of being supplied by ambient air, air for combustion is piped to the burner from an exterior combustion air intake. Exhaust is also directly vented through plastic pipes to the outside.  Condensing water heaters use the same process and also eliminate risk of back drafting and achieve similar efficiency gains.

Higher efficiency options

All condensing furnaces have the above characteristics which are sufficient to attain 90% AFUE.  Higher AFUE ratings as well as improved electrical efficiency and comfort can be reached with the following technologies, assuming the duct system is properly designed and sealed:

  • Multi speed and variable speed fan motor: Instead of running at full speed, the air handler fan runs only as fast as is necessary to deliver the appropriate amount of heat, as specified at the time of installation.  The fan is set to deliver a certain airflow and the circuit board controller calculates the appropriate speed for the fan to attain the proper airflow, given the static pressure (air resistance) in the rest of the duct system, which can significantly increase fan efficiency at low speeds, compared with a non variable motor. Variable speed fan motors are great for applications including whole home air filtration and/or ventilation when the fan runs most of the time, as they consume one eighth the power of a single speed motor running at 1/2 speed in fan only mode.
  • “Two stage”: Similarly to a multi speed fan, a two stage furnace has two firing rates; full firing for the coldest exterior temperatures and a lower level for days when less heat is needed.  This increases fuel efficiency somewhat and especially improves comfort by avoiding noticeable temperature fluctuations.
  • Modulating burner: These furnaces reach the highest levels of efficiency by using a circuit board to manage fully adjustable fan speed and burner firing in order to constantly maintain the highest level of fuel efficiency, electrical efficiency, and comfort. A properly designed and sealed duct system is always required to deliver total system efficiency, indoor air quality, and comfort.

Potential Savings for a Furnace Upgrade

When comparing the same heat output, replacing a lower efficiency furnace with a higher efficiency furnace can yield the fuel savings indicated below.  Savings can potentially be much higher, however, since, older furnaces are usually oversized (meaning they burn more fuel during operation because they are rated to deliver more heat than the home needs).  A home that has been air sealed and insulated will usually need a much smaller furnace to deliver sufficient heat and retain the heat longer leading to lower fuel consumption and costs.

Annual Estimated Savings for Every $100 of Fuel Costs by Increasing Heating Equipment Efficiency*

Existing System AFUE New/Upgraded System AFUE

—— 55% 60% 65% 70% 75% 80% 85% 90% 95%
50% $9.09 $16.76 $23.07 $28.57 $33.33 $37.50 $41.24 $44.24 $47.36
55% —— $8.33 $15.38 $21.42 $26.66 $31.20 $35.29 $38.88 $42.10
60% —— —— $7.69 $14.28 $20.00 $25.00 $29.41 $33.33 $37.80
65% —— —— —— $7.14 $13.33 $18.75 $23.52 $27.77 $31.57
70% —— —— —— —— $6.66 $12.50 $17.64 $22.22 $26.32
75% —— —— —— —— —— $6.50 $11.76 $16.66 $21.10
80% —— —— —— —— —— —— $5.88 $11.11 $15.80
85% —— —— —— —— —— —— —— $5.55 $10.50
90% —— —— —— —— —— —— —— —— $5.30
* Note: for the same heat output. Source: energysavers.org.

Important Disclaimer
Product examples are for informative purposes only and mention of any product does not constitute a recommendation or endorsement of any product.