Visual Guide to a High-Efficiency Condensing Furnace

high efficiency furnace
© Rheem 2015
  • 01 of 06

    Conventional vs. Condensing Furnaces

    High efficiency condensing furnaces use PVC p

    Troubleshooting and repairing your furnace is much easier if you know what type of furnace you have. The two main types of gas furnaces in the home are the conventional furnace and the newer high-efficiency condensing furnace. The technologies used in these furnaces directly impact their energy-efficiency. This is represented in their AFUE rating or Annual Fuel Utilization Efficiency. Only condensing furnaces achieve the highest ratings of over 90 percent AFUE, with some reaching over 98 percent AFUE.

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  • 02 of 06

    Anatomy of a High-Efficiency Condensing Furnace

    components of a high efficiency furnace
    Natural Resources Canada (base graphic)

    While the condensing furnace is different in its design and repair needs, it does have some similarities to a conventional furnace. Like a conventional furnace, it takes in cold air from the house and runs it through an air filter, it moves the air with a circulating fan, and it has a gas burner with electronic ignition (although condensing furnaces may have more electronic controls).

    The main difference between a conventional and condensing furnace is how the furnace handles the exhaust gasses from the combustion process. Both types of furnace have a primary heat exchanger, or combustion chamber, where heat from gas burners is exchanged to heat the air in the system before it is blown throughout the house. With a conventional furnace, the hot exhaust gasses from the combustion chamber go directly into a metal flue and are vented to the outdoors. The exhaust gasses are still very hot, and all of that heat is wasted. 

    With a condensing furnace, the combustion exhaust gasses are passed through a secondary heat exchanger that draws much of the heat from the gasses. As the gasses cool, they condense to form water and carbon dioxide (which together form carbonic acid). The water (called condensate) drips out through a drain pipe, and the remaining flue gases are vented to the outdoors through a PVC (plastic) pipe. The fact that you can use plastic for the vent pipe demonstrates how cool the gasses are. 

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  • 03 of 06

    Air Intake and Circulation

    furnace blower compartment with a heat exchanger

    A condensing furnace generally operates the same way as a conventional furnace in recycling the air in your home. But high-efficiency furnaces often incorporate a few technology improvements:

    • Heat recovery ventilator (HRV): Just as with a standard furnace, a condensing furnace uses return air from your home to be filtered and re-heated by the furnace. Most of this air is simply recirculated over and over, but in addition, a small amount of fresh air usually is pulled in through cracks around windows and doors and other areas. When a home is well air-sealed, or "tight," it may warrant an optional fresh-air intake that pulls air from outdoors into the furnace. 

      A heat-recovery ventilator, or HRV, is an optional device (separate from the furnace) that works as an air-to-air heat exchanger. It uses the stale, heated indoor air to pre-heat incoming fresh outdoor air before it gets to the furnace.
    • Air filter: Most condensing furnaces (and some conventional furnaces) use high-efficiency air filtration media to improve indoor air quality.
    • Electric blower motor: Condensing furnaces may have one of two types of blower motors: a standard permanent split capacitor (PSC) motor (commonly found on conventional furnaces), or a variable-speed, direct-current electronically commutating motor (ECM). The latter is used with two-stage, or modulating, furnaces and is more energy-efficient than a standard motor. 
    • Blower compartment: Blower compartments on high-efficiency furnaces are usually insulated to retain heat. Conventional furnaces usually are not insulated.
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  • 04 of 06

    Fuel Combustion

    gas burners in furnace

    Condensing furnaces are similar to conventional furnaces in the area of fuel combustion, which includes a gas burner, electronic ignition, and a combustion chamber. However, there are some differences in how air is delivered for combustion as well as in burner gas valve technology.

    • Gas valve: Conventional furnaces often use a single-stage burner gas valve, which means the burner has one "on" stage. With condensing furnaces, it is common to find at least a two-stage or dual-stage burner gas valve with electronic controls that allow the burner flame to be at either high or low settings, depending on the level of heat required.

      The most efficient system includes a modulating, or variable-capacity, gas valve, and electronic control system for the burner, paired with an ECM-type blower motor. This allows for fine adjustments to the burner setting and blower motor speed for improved temperature control and energy-efficiency. 
    • Electronic ignition: Condensing furnaces use electronic ignition systems for maximum efficiency and reliability.
    • Combustion chamber: Unlike conventional furnaces, a condensing furnace uses a sealed combustion chamber and direct-vent combustion air. The combustion air intake is directly vented from the exterior of the home to the furnace. This means the furnace does not take air already heated by the furnace and use it for combustion air.
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  • 05 of 06

    Heat Exchangers


    The extraction of useful heat from the fuel combustion process is where a condensing furnace truly separates itself from a conventional furnace. 

    • Primary heat exchanger: The primary heat exchanger on a condensing furnace is similar to that of a conventional furnace. It is a system of specially coated steel tubing.
    • Secondary condensing heat exchanger: The secondary heat exchanger is made up of small tubes that receive the exhaust gases once they have gone through the primary heat exchanger. Here, more heat is extracted from the exhaust gases, and as a result, the gases are cooled to the point that they condense into water and carbon dioxide. Because water and carbon dioxide from a slightly acidic condensate called carbonic acid, the secondary heat exchanger must be made of stainless steel to resist corrosion.
    • Condensate drain line: The carbonic acid condensate from the secondary heat exchanger is drained via a PVC pipe and usually discharged into a floor drain. 


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  • 06 of 06

    Venting System

    PVC vent screen

    The flue gas exhaust temperature from a condensing furnace is fundamentally different than that of a conventional furnace. The condensing furnace flue exhaust is relatively cool and can be vented with a plastic vent pipe without use of a metal chimney vent.

    • Draft inducing fan: Condensing furnaces, like many conventional furnaces, use a draft-inducing fan and pressure switch.
    • Plastic flue gas vent: The flue gases from a condensing furnace can exit through an ABS or CPVC pipe because of their low temperature (around 100 degrees F or less). They usually vent out through a wall of the home, at least 12 inches above grade or above the anticipated snow level.