The first condensing furnaces were introduced in the mid-1980s, and it’s likely that most of those original systems have already been replaced. Of course, there may still be a few left in existence, especially if their homeowners took good care of them, regularly replacing their filters and having the units maintained properly.

For those homeowners who are finally looking to replace their 20-plus-year-old condensing furnaces, they may be in for a surprise as today’s models offer higher efficiencies as well as many more features than their predecessors. New controls, more efficient motors, and better heat exchanger designs are just a few of the features that now come standard on most condensing furnaces.


Condensing furnaces can offer higher efficiencies and better comfort, but they also require more care than non-condensing furnaces. Which is why contractors should make sure their customers are scheduling maintenance on their furnaces every fall and spring, said Brian Vinsant, product support specialist, residential heating equipment, Rheem Mfg. Co. “In the spring, it’s important to get the A/C coil cleaned, because a dirty coil can impact the operation of the furnace, such as heat rise and maximum outlet air.”

In addition, during regular maintenance, contractors should verify that all hoses are secure and fully-seated and inspect for worn or cracked condensate hoses, said Vinsant. “Contractors should also look for visible signs of water leakage at hose connections, internal vent connections, and any metal surface below these connections.”

Burners may also need to be cleaned as wasps, spiders, and other pests will sometimes build nests inside the vestibule of the furnace during the summer months, noted Vinsant.

“Be sure to look for any type of rust or corrosion that may have accumulated on the burners due to a condensate leak that might have gone undetected at some point during the life of the furnace,” he said.

Indeed, cleanliness, along with proper combustion, will keep a condensing furnace operating at peak efficiency for a long time. “The secondary heating coil, motor cooling inlet openings, blower wheel vanes, the cooling coil, air filter, and ductwork system all need to be free of dirt and dust to ensure proper airflow across the heat exchangers of a condensing furnace,” said John Remley, product manager of residential furnaces and packaged products, Nortek Global HVAC.

To ensure proper combustion, contractors need to make sure that the vent and combustion pipes are unrestricted, well supported, and leak free, said Remley.

“If there are low NOx rods, contractors should make sure they are intact, structurally sound, and free of rust and corrosion,” he said. “The inshot burners should be checked for the same deterioration. Lastly, contractors should verify/adjust gas pressures in the particular application according to the manufacturer’s instructions and perform a combustion analysis to confirm that the furnace is operating properly and has the proper combustion air volume.”

But even condensing furnaces that are properly maintained will likely lose efficiency over time and eventually stop working.

“As a unit ages or has not been maintained, some loss of efficiency is expected,” said Dwayne Hall, product service coordinator of residential heating, Lennox Industries Inc.

Factors that can reduce the efficiency of a furnace include incorrect incoming and manifold gas pressure; a dirty heat exchanger; clogged air filters; intake and exhaust pipe recirculation; and an unclean blower motor and wheel, to name a few, said Hall.

“There are also many factors that could cause a condensing furnace to stop working, such as a plugged condensate management system, plugged pressure switch hoses, an obstruction in the intake or exhaust system, heat exchanger restrictions, and incorrect gas pressure,” he added.

Hall stressed that contractors should not upgrade motors or controls or alter any part of an older condensing furnace in an attempt to optimize its performance or efficiency.

“The furnace was designed, manufactured, and certified per industry standards for safety and proper operation.”

Vinsant agreed, noting that a replacement part that is not approved by the manufacturer may void any remaining warranty. In addition, while a replacement part may be a more efficient alternative, like an electronically commutated motor (ECM) versus a permanent split capacitor (PSC) motor, existing controls may not be compatible with the new components.

“Components must be tested to ensure they are a safe alternative and provide equal or better performance,” he said. “Ideally, a furnace design incorporates components that optimize performance resulting in a safe, dependable, and affordable product for the customer.”


If homeowners have neglected the maintenance on their condensing furnace for too long, they may just need to consider replacing it.

“There is only so much a contractor can do to bring an older condensing furnace back to life,” said Remley. “Most of the furnace can be cleaned; however, bringing a heat exchanger back to an operable condition is not possible if sooting has occurred. Sooting is the end of a heat exchanger’s life because it is constructed in such a way that it cannot be thoroughly cleaned and brought back to an acceptable operating condition.”

While most homeowners do not relish paying for a new furnace, they will likely be pleased to see how much the technology has changed over the years.

“Not only have the efficiencies improved, but the product warranties have improved as well, which delivers a greater level of consumer and dealer confidence,” said Jill Murphy, senior product marketing manager, residential heating, Lennox Industries Inc.

Even if a condensing furnace is still operable, if it’s over 15 years old, homeowners may want to consider replacing it with a new, energy-efficient model, which can save money and provide better comfort.

“By replacing an older furnace that is 60-percent efficient with one that is 98-percent efficient, homeowners can save approximately 40 percent on energy bills and up to $11,469 over the life of the system,” said Murphy. “Energy calculators can help consumers compare the savings of different high-efficiency systems and determine whether to repair or replace an older unit.”

In addition to higher efficiencies, the latest condensing furnaces offer new technologies, such as enhanced diagnostic systems with LED fault codes, which allow for quicker servicing for the contractor and homeowner, said Vinsant.

“The higher tier furnaces even have smart home automation abilities, such as Rheem’s EcoNet® control center, which allows the unit to ‘talk’ and troubleshoot using Wi-Fi-connected features like alerts and notifications,” he said.

Modern condensing furnaces offer other innovative features as well, including stainless steel heat exchangers; improved inshot burner designs; controls to reduce igniter failures; two-stage operation; modular gas valves; and more efficient motors, said Remley. “The bottom line is that today’s condensing furnaces lower operating costs for homeowners while delivering true comfort solutions in both the heating and cooling season.”  


To keep both condensing and noncondensing furnaces operating at peak efficiency, Lennox Industries Inc. offers contractors these maintenance tips:

  • Check furnace wiring for loose connections, voltage at the indoor unit, and amperage of the indoor motor;
  • Check the condition of the belt and shaft bearings if applicable;
  • Inspect all gas pipe and connections for leaks;
  • Check the cleanliness of filters and change if necessary;
  • Check the condition and cleanliness of burners and heat exchanger, and clean if necessary;
  • Check the cleanliness of blower assembly, and clean the housing, blower wheel, and blower motor if necessary;
  • On induced draft furnaces, inspect the combustion air inducer and clean;
  • If the furnace is equipped with a condensate drain and trap, inspect for leaks and cracks. The drain and trap must also be cleaned, and the trap must be primed with water. Inspect the rubber hoses connected to the pressure switches for cracks or loose connections, and replace as necessary. Remove the rubber hoses from the cold end header box, and inspect for any blockage — clean as needed. If strainers are installed in the hoses, remember to remove and clean before reinstalling the hoses;
  • Evaluate the heat exchanger integrity by inspecting the heat exchanger per the Air-Conditioning, Heating & Refrigeration Institute (AHRI) heat exchanger inspection procedure;
  • Ensure sufficient combustion air is available to the furnace. Fresh air grilles and louvers (on the unit and in the room where the furnace is installed) must be properly sized, opened, and unobstructed;
  • On condensing furnaces, inspect the furnace intake and exhaust pipes to make sure they are in place; structurally sound; without holes, blockage, or leakage, and check that the exhaust pipe is sloped toward the furnace. Inspect terminations to ensure they are free of obstructions and are structurally sound;
  • Inspect the furnace return air duct connection to ensure the duct is sealed to the furnace. Check for air leaks on supply and return ducts, and seal where necessary;
  • Check the condition of the furnace cabinet insulation and repair;
  • Perform a complete combustion analysis during the furnace inspection to ensure proper combustion and operation. Consult product literature for proper combustion values; and
  • Verify operation of CO detectors and replace batteries, as required.

Publication date: 11/27/2017