How Does a 95% Furnace Work? - Smart Inspector Science

By Tom Feiza, Mr. Fix-It, Inc.

The efficiency of forced air furnaces has changed dramatically. From roughly 1920 to 1980, the naturally drafted gas furnace was king. It operated at about 50% to 65% efficiency. With the flue gas maintained at about 350 degrees F, products of combustion and steam went right up the masonry chimney, and there were no condensation problems.

However, in the 1980s, energy costs increased dramatically, and everyone expected an energy shortage. Changes that included flue dampers, electronic ignition, better heat exchangers and new control systems created incremental improvements in efficiency.

Condensing furnace: one giant leap in efficiency

Later came condensing furnaces, with dramatic improvements in efficiency (Photo: Condensing Furnace – 3 Controls, (3 firing rates)). The furnace pictured here is 96% efficient, with three firing rates and all ECM (electronically commutated motors) variable-speed electric motors. With this furnace, 96% of the energy remains in the home, and only 4% goes up the chimney.

Condensing Furnace - 3 Controls

Today’s condensing furnaces use a secondary heat exchanger. It captures more heat by condensing the flue gas from 350 to 120 degrees and condensing the steam into water (Illustration H096 – Btu – Condensing Furnace). These furnaces use two PVC pipes for venting, making them easy to identify.

BTU, Condensing Furnace

Condensing steam captures the latent heat – that is, heat from the change of phase from steam into water – at a rate of 970 Btu per pound of water. If a furnace uses about one therm of natural gas (100,000 Btu), combustion creates about one pound of steam, and we capture 8,080 Btu per pound. In a larger home, we could capture ten times that amount of energy over 24 hours. Efficiency has increased from 60% to more than 90%.

Issues have been resolved

Initially, the heating industry had lots of issues related to the slightly corrosive condensate. Manufacturers and repair workers had never dealt with corrosive water before. Now, about 30 years since modern condensing furnaces were introduced, most of those issues have been solved.

As a bonus, the highest efficiency furnaces use ECM (electronically commutated motors), which turn at a variable rate, providing greater efficiency and better comfort as well as substantial savings on electrical power.

Home inspectors should love this development, because the heat exchanger is sealed and not visible. But still, you must document the type of furnace, run the system, remove the access covers, and check for signs of water leaks. Any type of leak in the furnace indicates a problem.

Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through, he provides high-quality marketing materials that help professional home inspectors boost their business. Copyright © by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.

If you find this information helpful, you may want to catch Tom’s presentation: “The Science Behind Great Home Inspections” and “Report Writing – Describe That Defect.”

Leave a comment

Please note, comments must be approved before they are published