More HVAC Equipment Changes on the Way

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The United States Department of Energy (DOE) finalized energy efficiency standards for residential gas furnaces in late 2023.  These new standards are slated for implementation in late 2028.  The last time the DOE raised the efficiency rating of gas furnaces was in 2007, when the minimum efficiency for gas fired furnaces was raised from 78% to 80% efficient.

Let’s look at what efficiency means in this context.  In simple terms, if a furnace is rated at 80% efficiency, then 80% of the energy is available for use in heating a home and 20% is wasted to the environment in the form of exhaust.  This efficiency rating does not consider other HVAC system inefficiencies such as lack of insulation, damaged insulation or leaks in the air duct system of a building.

The new rule finalized by the DOE will raise the minimum efficiency from 80% to 95% efficiency.  To achieve an efficiency of 95%, we have to apply a little science.  Combustion of natural gas or propane produces water vapor as a byproduct.  This is why you sometimes see a furnace exhaust chimney “smoke” or “steam” in cold weather.  The water vapor that is in gaseous form is being condensed by the cold outdoor air, creating the plume you can observe from a chimney.

To achieve 95% combustion efficiency, we must condense this water vapor in the heat exchanger of the furnace, recovering the latent heat of evaporation/condensation from the water vapor and allowing this heat energy to be used to provide heat to the building.  This is why these high efficiency furnaces are called “condensing furnaces.”

So this is a win for consumers right?  Wasting less of the heat energy in natural gas or propane to the environment is a win on several fronts right?  If more of the fuel you purchase actually went to heating your home, instead of up the chimney, then you should hopefully use less fuel, resulting in savings for the consumer.  A reduction in the use of fuel gas should reduce the greenhouse gas emissions to the environment.

Well like most things in life, the devil is in the details.  The products of combustion, or exhaust, from the furnace must be vented to the atmosphere for safety reasons, with carbon monoxide being present in most exhaust.  Furnaces rated at 80% efficiency typically utilize the natural buoyancy of the exhaust with a chimney or vent vertically to the exterior of the building.  The fireplace in your home works the same way, smoke and products of combustion rise up the chimney to the exterior of your home.

Condensing gas furnace

Condensing furnaces “condense” the water vapor present to recover the available heat, and this makes the exhaust less buoyant.  Therefore, condensing furnaces use a fan to force the exhaust out of the building, often through a horizontal vent to the exterior.  Fans are not required in non-condensing 80% efficiency furnaces. Also, the condensate is corrosive, and the heat exchanger material of construction must be upgraded as well to prevent premature failure. This extra sophistication in the furnace is not free and adds to the initial cost of the furnace.

The “chimney” or vent materials must be made of different materials for condensing vs non-condensing furnaces for the same reason as the heat exchanger.  Most non-condensing furnaces utilize galvanized steel vents, and they last a long time in this service. However, by design a condensing furnace “condenses” the water vapor in the exhaust.  This condensate or liquid water, will rust standard galvanized vents, causing them to rapidly fail, potentially allowing combustion byproducts such as carbon monoxide into the building.  Therefore, the material of construction for a vent utilized on a condensing furnace must be resistant to the chemical attack created by the condensed water vapor.  Typically, PVC is utilized in this case.

PVC concentric vents

If a building has an 80% efficient furnace installed, and the owners choose or have to replace it with a 95% efficient furnace, then the vent piping will have to be replaced with material such as PVC that is compatible with the condensate present in the exhaust of the condensing furnace. This will add both material and labor expense to the installation.

In 2024, the American Gas Association (AGA) and National Propane Gas Association (NPGA), along with others, filed a petition in court seeking a reversal of this new standard due to numerous concerns, including the additional first equipment cost as well as the additional upgrades necessary to an existing non-condensing furnace to allow a condensing furnace to be installed.

This case will play out in the court system with all due speed that we have come to expect. In the meantime, building owners and homeowners should be informed of the changes coming to make educated decisions when replacing furnaces.  Changing from a condensing to a non-condensing furnace is not a drop-in replacement, and owners should be prepared for the expenses associated with this upgrade.

Chad Jones, PE, CFEI, CVFI, CMSE has a Bachelor of Science in Mechanical Engineering from Clemson University. Chad has over 25 years of engineering experience including mechanical, process, and manufacturing engineering. This work has included equipment design, machine safeguarding, cost estimating and safety compliance. Chad also has over 10 years of commercial, industrial, and residential HVAC and plumbing design experience. A lifelong auto and motorcycle enthusiast, Chad is accomplished in the maintenance, repair, and modification of vehicles and engines. Chad is a Certified Fire and Explosion Investigator, Certified Vehicle Fire Investigator, and IFSAC certified Firefighter II in Greenwood County, South Carolina.

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