Furnace Efficiency

Posted by Jim Harless on Wed, Nov 28, 2012 @ 09:11 AM

High Efficiency Furnace Vent

With new efficiency rules going into effect next year for gas furnaces, this is a good time to discuss furnace efficiency and how those rules will impact homeowners.

First, let's look at the differences between standard efficiency gas furnaces and high efficiency gas furnaces.

Standard efficiency gas furnaces are generally rated at or near 80% A.F.U.E. which stands for "Annual Fuel Utilization Efficiency". This means that 80% of the heat produced through combustion is used to heat your home while the remaining 20% is lost with the combustion products through the flue. The "flue" is the vent that escorts the combustion products out of your home. Those combustion products include steam (water), carbon dioxide, carbon monoxide, and various other gases. The flues for the standard efficiency gas furnaces are typically one of two types. They are "Mason" or "B-Vent". Mason flues are brick or masonry block chimneys that vent the combustion products to the roof. B-Vent flues are double-walled metal pipe that also vent the combustion products to the roof. Both "Mason" and "B-Vent" flues may be "common vented". Which means they provide venting for more than one gas appliance. Typically, you will find a furnace and water heater "common vented". 

High efficiency gas furnaces, also known as "Condensing Furnaces", generally fall between 90% and 97% A.F.U.E.  For example, if your furnace is rated at 95% A.F.U.E., then 95% of heat produced through the combustion process is used for heating your home, while only 5% is lost through the flue.  High efficiency condensing gas furnaces differ from the standard efficiency models in several ways.  The primary difference is the manner in which they are vented.  Since the heat lost through the flue of a high efficiency condensing furnace is a lot less than the heat lost through the flue on a standard efficiency furnace, the steam is much cooler on the high efficiency condensing furnaces.  So much cooler that the steam condenses into liquid water.  This is a big problem for venting into masonry or b-vent flues.  The solution is to vent the flue gases through water-tight, sealed pvc piping.  This requires a dedicated pvc flue and does not allow for a "common vent" option.  The pvc exhaust and intake (when necessary) are usually sidewall vented as shown in the picture above.  The high efficiency gas furnaces also require a condensate drain to remove the water produced by the furnace. 

 Now that we have an understanding of some of the differences between standard efficiency gas furnaces and high efficiency condensing gas furnaces, let's look at the new "Direct Final Rule" enacted by the U.S. Dept. of Energy that will be taking effect May 1, 2013.

This "Direct Final Rule" is known formally as "Residential Furnaces and Central Air Conditioners and Heat Pumps Direct Final Rule" can be found here :


 Also, you can find info on this site to bring awareness to the end consumer :


The most important impact of this rule that will affect residents of the state of Ohio and other northern states is the requirement that the minimum efficiency for new furnaces will be 90% or higher.  This means that the standard 80% efficiency furnaces will no longer be allowed to be installed after May 1, 2013. 

When I first heard of this rule, a problem immediately came to mind.  Some residences do not have an exterior wall near the furnace to pipe the sidewall vent required for the high efficiency condensing gas furnaces.  In particular, multi-family dwellings such as apartments, townhomes, and condos typically have a centrally located utility room with no access to an exterior wall for the sidewall vent.  I see this creating an additional expense to the homeowner in addition to the normally higher cost of installing a high efficiency condensing gas furnace.  The piping will need to be installed thru living rooms, kitchens, etc... and then soffitted to enclose the piping from view.  Other alternatives would involve replacing an existing standard efficiency gas furnace with an electric furnace and/or heat pump.  Personally, I would not consider that an improvement, just a way around the sidewall venting issue.  I am hoping that an ammendment to rule will be coming in the future, but I'm not holding my breath.  I'm all for conserving our energy, but feasibility must be considered.  I would think that excluding multi-family dwellings from the rule would go a long way in helping alleviate the problems that will arise from this rule.  Also, maybe a provision for certain single family homes without a good option for the sidewall venting as well.  And, then only for retrofit installations.  New builds can plan the high efficiency model furnaces into the contruction.

 There is more I could say, but I think this will suffice.

If you have an older standard efficiency furnace and have no access to an exterior wall, then now may a good time to consider installing a new standard efficiency model while there is still time left.  The last day to install a standard efficiency furnace is April 30, 2013.

In light of everything mentioned here you would think I don't like high efficiency furnaces, but not so.  I always look the high efficiency option first for my customers and give them all the of the pros and cons to allow them to make an informed and educated decision for themselves.  Usually, the only downside to the high efficiency models is the higher cost.

If you live in the central ohio area, you can call us for a free estimate to replace your existing furnace at 614-837-5062.

-Jim Harless III


Topics: Heating and Cooling Tips, Furnace Efficiency, furnace repair