Furnace Efficiency?


Meteorological Autumn is here and calendar Autumn starts in just 14 more days. Where has this summer gone?  I know some are very happy to see it go! Now this does not mean A/C season is over but, it is time to start thinking about the inevitable — Winter!

Now is the time to think about replacing those old, inefficient furnaces with energy smart new one. That brings us to the subject of this post — Furnace Efficiency.

Have you ever noticed that the old standing pilot and spark ignition furnaces appeared to be rated at 80% efficiency?  You look at the data plate and it reads 100,000 Btu input and 80,000 Btu output or bonnet capacity.  This is 80% efficient, RIGHT? Well, yes and no!  In the past, furnaces were rated by STEADY STATE EFFICIENCY which meant that if the furnace ran 24 hours a day the entire heating season, it would be 80% efficient.  This rating never took into consideration normal cycling or milder outdoor temperatures.  So, based on this type of rating, it WAS 80% efficient.

Today’s furnaces, however, are rated by A.F.U.E.’s. This is a measure of the annual fuel utilization of the furnace.  It takes into consideration cycling on and off, weather, and other factors so the 80 % or up to 98% rating is a lot more accurate.  It is a truer measure of the efficiency of the product.

Why am I telling you this? Well, a lot of dealers still use  the rule of thumb and size a replacement furnace based on the input of the existing furnace and then get themselves into trouble.  They say, ” the old furnace was 100,000 Btu’s input so I replaced it with a 100,000 Btu furnace.” the problem here is now the furnace is grossly oversized. Then there are dealers that base the new furnace by using the output of the existing furnace. It still would be oversized (just not as bad) because, in reality, the older furnaces, based on A.F.U.E.  were really only 55% to 60% efficient.  To make matters worse, if a 92 – 98% efficient furnace is used to replace the old furnace and it is sized based on the input or out put, now we really have a over-sizing problem.

An over-sized furnace will heat the space but the end-user will be sacrificing both comfort and efficiency. The furnace will “blast” a lot of warm air into the space, over heating it, and then is off for an extended period of time.  Because of this, there are larger temperature swings in the space and the occupants feel warm and then cold and then warm and so on.

The best way to properly size heating equipment is by using a load calculation program such as Manual J or Right J.  With homeowners installing newer energy-efficient windows, insulating attics and walls, and weather proofing their homes, the sizing of the old furnace is even more out of line and to replace it based on the old rating is even more absurd.  It is a disservice to the customer because they are not getting the true fuel efficiency and comfort they think they are paying for.

So keep this in mind when you are quoting or replacing an existing furnace.  By taking the time to do a proper sizing, you are giving the customer what they are paying for, you are eliminating possible warranty calls due to erratic cycling, and you are providing the proper comfort that the furnace was designed to give. It can also give you the edge in getting the job by showing the customer that you are a “professional” and are truly concerned with their needs and comfort.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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2 Responses to Furnace Efficiency?

  1. trachsel says:

    Great article and well said!

    [cid:image002.jpg@01CD8B7D.9D122F80]Dan Trachsel
    Technical Specialist
    Johnstone Supply
    503.519.4413

  2. Thank you for sharing excellent informations. Your web site is so cool. I am impressed by the details that you have on this site. It reveals how nicely you perceive this subject. Bookmarked this website page, will come back for extra articles. You, my friend, ROCK! I found just the information I already searched all over the place and just couldn’t come across. What a perfect web site.

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