Heating Efficiency and Proper Sizing


This weekend I got an email from my nephew who lives in New Hampshire. He recently moved into a home and they finished the basement and attic. Now, he had a contractor come in and he was telling him that he needed  an additional 50,000 BTU larger boiler to heat the house. Since he knew I retired from heating and air  conditioning, he wanted my opinion. My first question to him was, ” How did the contractor determine that he needed an additional 50,000 BTU’s to heat the new areas?” Then I asked, ” Did he do a load calculation? Was he basing it on how many feet of new baseboard radiators he was installing? What water temperature is the existing boiler operating at?”  The existing boiler may be large enough to handle additional baseboard radiators. I told him that basements usually are not included in load calculations so that could reduce the sizing the contractor was suggesting. Depending on the water temperature the boiler was operating at, he may be able to go from say, 160 degree water to 180 degree water to handle the additional load. My nephew said that the contractor just said he needed a new LARGER boiler.

My nephew’s boiler is gas-fired hot water. I told him that possibly replacing the existing, inefficient boiler with energy smart new one might be the way to go but there are a few things he needed to consider. That brings us to the subject of this post — Heating Efficiency and Proper Sizing.

Have you ever noticed that the old standing pilot and spark ignition furnaces / boilers 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 / boilers 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 / boilers, however, are rated by A.F.U.E.’s. This is a measure of the annual fuel utilization of the furnace. So to put this into perspective a 60% furnace is 60% efficient because 40% of the heat it creates goes up the flue. An 80% sends 20% up the flue. An 90% sends 10% up the flue and so on.  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 / boilers based on the input of the existing equipment and then get themselves into trouble. They say, ” the old equipment 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 or boiler is used to replace the old furnace or boiler and it is sized based on the input or out put, now we really have a over-sizing problem.

An over-sized heating unit will heat the space but the end-user will be sacrificing both comfort and efficiency. With a furnace, it 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. Boilers are more forgiving because they heat water that is circulated through baseboard/radiators. The problem here is it just wastes energy to give the heat to an area.

Now, what I told my nephew was that the best way to have properly sized heating equipment was by having the contractor use a load calculation program such as Manual J or Right J. I told him if he was going to make a major investment in new equipment, he really needed to have it sized right the first time.

So back to the subject of this post, with homeowners installing newer energy-efficient windows, insulating attics and walls, and weather proofing their homes, the sizing of the old furnace / boiler is even more out of line and to replace it based on the old rating or “rules of thumb” or guesstimates, 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 heating unit. 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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13 Responses to Heating Efficiency and Proper Sizing

  1. Your thoughts on efficiency and sizing for furnaces is quite skewed and you border on the line of “not really knowing what you are talking about”.

    Efficiency of most gas furnaces are mostly based on design factors of the furnace. The efficiency of “most” older style furnaces were 60%. Current low end furnaces are 80% and condensing furnaces are 90% and all the way up to 97%.

    Efficiency has nothing to do with how much it cycles. It has to do with how much heat goes up the flue. This is how you achieve efficiency (of a furnace). If you don’t run the furnace at all you could say you’re 100% efficient while listening to your teeth chatter.

    So to put this into perspective a 60% furnace is 60% efficient because 40% of the heat it creates goes up the flue. An 80% sends 20% up the flue. An 90% sends 10% up the flue and so on. Now the only way it sends this excess heat up the flue is if you are running it.

    Sizing is important to a degree, but many modern middle and higher end furnaces are now 2 stage. This captures more efficiency to a degree because you burn less gas, however in these lower stages the furnace will run longer. The advantage is less draft and more even temperatures with in the structure.

    With me pointing out these flaws really illustrates why you need to be really careful who you talk to, who you listen to and more importantly who you hire to work on your HVAC system in your home.

    About the author of this post:
    My name is Ray Austin with Austin Air Companie serving Katy, Texas and some surrounding areas. I am a Licensed HVAC Contractor with over 20 years experience in HVAC. I have an AOS Degree in HVAC/R, NATE certified in HVAC Repairs & Installations, Universally Certified to handle ANY refrigerant.

    • Ray — thank you for your comments. I agree that cycling has nothing to do with efficiency — it has to do with comfort levels. The longer a furnace runs, as with 2-stage equipment, the more comfort you have due to less cycling. Staging does not affect efficiency either (at least not yet since D.O.E. does not rate by staging).
      I still profess the use of Proper Sizing techniques such as Manual J for residential equipment. I have “approved” your comment so it will appear after the post on the blog and people can see your comments. Hopefully, that is acceptable to you. If not, I will remove it from the post.
      I appreciate people taking the time to respond/comment on these posts.

      • York Central, Thank you. The article reads much better now. However, another point I can bring up is Manual J calculations can be easily skewed and typically more so in regards to air conditioning needs. My market is very far south as Katy, Texas is a suburb of Houston so heating season for me is quite quick.

        For those that happen to read these pages, A Manual J, is a tool to calculate the load of a structure and must be used properly or the use it provides is of no use at all. I demonstrate in a video on my website how easily a Manual J can be manipulated. Why would someone do this? A salesman already knows you think a bigger unit is better. (When it’s not)

        The confusion really sets in when a properly sized unit (AC condenser) is physically bigger in size than the unit it replaces. So one must understand when reading this when we talk about sizing we are talking about Capacity of the system… not the physical size of it.

        As time goes on I think the residential market is moving toward a 3 tier based sizing scheme. The systems will run with algorithmic controls, run longer cycles more efficiently. Because these systems will run based on demand of the structure this will somewhat reduce the risks of over sizing. Additionally the 3 tier scheme would reduce the amount of product the manufacturers must produce. We’ve started to see this happening already to a degree in 2 speed Condensers.

    • If you re-read the post, I have removed the sentence that said AFUE considered cycling. Thanks again for point that out

  2. Allen Rice says:

    York Central, i agree with you about the older equipment being rated by steady state running, when cycling the old boilers had so much mass that it could take up to 10 min. before the circulator would turn on. And the same was true about hot air furnaces but only about 2 min. So these would short cycle most of the heating season here in the north east because units were sized for the coldest day of the year and still are. And that may happen only 5 days the entire winter. And today short cycling is still a big problem. The efficiencies may be there going up the flue, but it shortens the life of the equipment, and uses much more power. So short cycling actually lowers the AFUE dramaticly because power consumption is included in the ratings today. Motors use much more power starting up, instead of running constantly. This is also one of the reasons the industry is going to Variable speed (load matching) equipment. If it never completely turns off the efficiencies are higher, and comfort levels are greater.
    Allen Rice
    Johnson Controls Inc./York
    Branch Service Manager Edison NJ

    • Allen — thanks for the comments. I agree and understand that almost all equipment is oversized due to “design conditions” for the occasional extreme weather. Load calcs are still the way to go though to at least get the equipment close to properly sized for the remainder of the season.

  3. Richard Parker says:

    Nice post, keep it up.

  4. I am aware of that situation but he was correct in the fact that cycling of heating does not affect efficiency in the AFUE rating like it does for cooling with the SEER rating criteria. I did make that correction and let it go at that.
    thanks for being a loyal follower of my posts. — it is appreciated

  5. Gentlemen:
    this is a forum for learning. I would appreciate refraining from “arguments”. As I said, Ray was right in pointing out an error in my post and that is always helpful. If you wish to engage in personal discussions, please contact each other via your own web sites or blog sites.

  6. Yorkcentral, I am sorry for going off topic. I was merely defending myself and my reputation.

    Thank you for at least allowing me to clarify things.

  7. Eric Miller says:

    I am only a maintenance tech but I have a question. What you said above I agree with, sizing IS important. We have had several residential heating systems replaced over the years and I have started requiring heat load calculations by the contractors. Before that it often was just replaced by matching the BTU output number. Even now though, when systems are being replaced with equipment that is sometimes significantly smaller, the systems still aren’t running most of the time when the outdoor temps are down near design temp. More than once I have gotten called in for a no heat call at a location and when I got the system working again, it still wouldn’t run continuously.

    One example on an oil fired hot water system that I do know to have been replaced by the rating plate method, on a very windy and bitterly cold night with all zones calling and the house 15 to 20 degrees below thermostat setting the system still wouldn’t run more than 50%. And even starting out with the indirect fired domestic tank completely cold it still wouldn’t run maybe more than 80%, no priority zone. Another one that HAD a heat load calc done before replacement with a reputable brand variable fire rate gas boiler with outdoor reset, still will only run maybe 2/3 of the time on a no-heat recovery. Seems oversized to me.

    Is there any way I can figure out roughly what size system is really needed just from run time, outdoor temperature, a bit of wind condition info at the time and maybe deltaT on the furnace supply and returns? Any other info I am missing that would be needed in the analysis or is it not possible to engineer from existing conditions? I am willing to build or purchase the necessary data logging equipment to do the system analysis. What am I missing or why don’t the contractors’ calculations seem to come out closer? I also would be curious how the radiation is designed and what I should expect for recovery times Answer these and I have some forced air furnace sizing questions. 🙂

    • Load calculations depend on the person’s knowledge of performing it properly. there are lot of factors that can be changed like design conditions Example – in Chicago we use 70 degrees indoor at -10 outdoor. If you just change that to -15 or -20 degrees outdoor, you change the sizing. Do contractors know the real insulation in existing walls? Do they look in the attic to see what is actually up there? Even with a “best guesstimate” using Manual J, it is better than just guessing or using rules of thumb. Run time will not tell you how to size. Now, with hot water heat, the type, size, etc of the radiation/radiators can d
      aid in determining boiler loads. IBR (Institute of Boilers and Radiation) has tables that can tell you how much heat the radiation will produce based on water temperatures and doing a “radiation count” can help determine the amount of heat needed to handle the existing radiation.

  8. Eric Miller says:

    Sorry for the delay getting back, I don’t get to look at this every day. To your statement that the contractor may not be checking the parameters of the house, I would defend them in our situation, I have been the one to take them thru the house to collect their data and I know they were diligent. They also are reputable, have been around for a long time and pride themselves on correct sizing of retrofit systems. I also understand design temps and how those details are applied. Around here we use 72* indoor and 0* outdoor. I still think there should be some way to gather data on an existing system along with outdoor parameters and get a good idea on correct system size. On that second system I exampled, I can tell from the controller information how many boiler cycles and the runtime hours and I really think the house is undersized in radiation capacity. It has had approx 72,000 cycles and 13,000 hours runtime in four years but that includes an indirect fired hot water heater. Maybe I should just build the monitoring equipment and see if I can get measurements that correlate to parameters that I know. Thanks for your answer.

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