To Manual J or to NOT Manual J - Retrofit Sizing
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  1. #1

    To Manual J or to NOT Manual J - Retrofit Sizing

    How about some comments on your thoughts on Manual J and Retrofit Sizing?

    I know this statement will cause an immediate uproar in the HVAC Community and it seems I am cutting off my nose to spite my face, because I make my living from Manual ‘J’. Everywhere on the Internet you will find information about the, “Must Have Manual ‘J’, for ALL furnace and air conditioner installations. This is one of those issues easier said than installed and sold. The Internet is the greatest source of information the world has ever seen, BUT everyone needs to keep in mind, not everything on the Internet is the gospel truth which should be believed unconditionally and general statements may not fit your particular situation.

    ACCA Manual ‘J’ is the best, and most widely accepted method, for sizing Residential HVAC equipment. Is this true for the replacement, retrofit market? Maybe, but maybe it isn’t. I will explain the reasons I believe this may not be the case. I have over 35 years of experience in the HVAC field and the very consistent truth is, equipment is over-sized and duct systems are under-sized. If I had a nickel for every home I have visited where a 4 or 5 ton air conditioner was installed with 8” x 16” trunks, 12 to 14 - 4” x 10” supplies and 4 - 6” x 30” returns, in a 2500 sq.ft. home, I would be a rich man.

    Manual ‘J’ requires accurate information concerning the construction components used to build the home. Accurately determining the insulation values in fully finished walls, correctly determining 15 year old window U-Values and SHGC, finding out what the actual insulation values are for a ceiling, or how about a vaulted ceiling/roof combination, is next to impossible without extensive research, effort and time. Even after you have done the research, put forth the time and effort, you will find older windows have no NFRC Ratings, the ceiling insulation has been extensively crushed, and the settling factor is unknown in the walls and floors. Even in the new construction industry, it has been proven the insulation values are not what the ratings state, installation practices are less than optimal and the overall effective R-Values are not what was documented, predicted, or expected.

    Also keep in mind that 40% to 80% of the cooling loads are directly from solar gains through the windows. Changing window SHGC values from 0.35 to 0.45, can change the cooling loads of a home by more than 20%. These best guess values for the Manual ‘J’ could make the loads on a 3000 sq.ft home vary from 2 tons to 5 tons. So what do you do?

    Is this the size you will put in? Most of the reputable contractors who went through all the efforts to provide the Manual ‘J’ would say, “This is the size you need and is what we are going to install for you”. Would they be correct and would you have a system which functions perfectly? MOST LIKELY NOT!

    What went wrong with these methods? I did everything possible as an educated consumer to select only a Manual ‘J’ providing, reputable contractor? The answer is, your duct system is incapable of providing the equipment capacity the best guess Manual ‘J’ proved I needed. For air conditioning systems, airflow directly determines capacity. If the airflow is not being delivered to support the size of air conditioner installed, the capacity will be reduced, or even worse, the equipment will fail. This can lead to very short equipment life and hefty repair bills, not to mention excessive utility bills.

    What are the choices? Either you upgrade the entire duct system to meet the Manual ‘J’ loads or you decrease the air conditioner size to meet the duct capacity. The most common retrofit will fit into the latter category, the air conditioner and furnace need to be sized to the existing duct system capacity to provide the most efficient operation. This practice will also greatly extend the life expectancy of the equipment.

    These are the reasons I developed Right-Size to size air conditioners and furnaces for the retrofit/replacement market. Manual ‘J’ loads are easily manipulated when used to select equipment for existing homes. The data entered is consistently inaccurate and the end results will reflect this best guessed equipment sizing. I know of very few homeowners who would purchase equipment from any contractor if they were told they would have to make extensive changes to the ducts system, in their fully finished basement, to meet the Manual ‘J’ loads. It is extremely rare that a duct system in any home over 10 years old will have the duct system sized correct to the equipment currently installed. On the other hand, a good share of existing duct systems, with a few simple modifications, can achieve air flows to size equipment that will maintain the desired indoor temperatures for over 95% of the cooling or heating hours. This is not the ACCA Design recommendation of 1%, but for existing homes, it will typically be much more comfortable and efficient than it was with the existing grossly over-sized equipment.

  2. #2
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    So basically your selling software?
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  3. #3
    Join Date
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    south louisiana
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    wouldn't it be worthwhile to know the values of the wall insulation
    based on age of home & wall construction types used in your area?
    determining R-value of attic insulation isn't difficult.
    learn types of insulation, R-value per inch. then measure
    high & low spots and average out R-value for calc entry.

    same with windows. find out shgc & ufactors of single pane
    metal, wood windows. add to that double pane clear metal.
    then windows with low e coatings, argon gasses & conductive
    vs non conductive window frames.

    these are things I leaned long time ago doing weatherization work.

    things you don't mention that effect sizing of equipment are
    air tightness of house, and duct system. this...again to me..
    is where I see so much flexibility in sizing of systems.
    changing from average to tight reduces size of system
    where as leaky to average increases size of system.


    I've found that most software programs can be manipulated
    to produce the answer that the programmer wants it to have
    if they use it long enough to find the areas that allow
    manipulation.
    not saying it is right...but when you see
    a 2000 sq ft house with a 5 ton system with a load calc
    to back it up...something is off in the entries.

    like poster above I wonder if you are selling your software
    or is there another point you are making?
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  4. #4

    Manual 'J'

    Quote Originally Posted by jimj View Post
    So basically your selling software?
    Thank you for your comment. I have worked in this industry for over 35 years and the one issue I always found to be true was salespeople were either very good at selling, or very good at sizing equipment. But unfortunately, they never seemed to be good at both, except in very rare cases. This is what made me think a software product could help, so I looked everywhere for one to fit the bill. I found nothing that would help the salespeople size the equipment correctly to the existing duct and tell them how to fix a duct system that would only support two tons of cooling, when a three ton unit was required. The Manual 'J' they did told them they needed this size air conditioner and that is what they had installed. It was only after the installation I would have to visit the home because the air conditioner froze up. When this happened, and it happened often with 30+ salespeople on the road, I had to fix the system on the companies dollar rather than the homeowners. I tried to teach the salespeople that the Manual 'J' was not always the best way to go but they would state the Manual 'J' had to be done to get the rebates.

    So there is the problem I tried to solve. I am not a large corporation, I am a one person show and I had an idea which may help. The reason I posted what I did was to find out what other HVAC people have encountered concerning salespeople and Manual 'J' retrofit sizing. Has the company you work for or own, ever incorrectly sized an air conditioner or furnace because of the results provided by a Manual 'J' which were incompatible with the existing duct system? Do all the salespeople you have met have the skills to determine if a duct system is correctly sized for the equipment they are selling and can they determine how to fix a duct system if it is incorrectly sized? Do you use Manual 'J' to size retrofit equipment or do you use some other method? I am only trying to determine if the software I designed has any value in the HVAC Field and if it could be useful for some people. It is more of a fact finding post rather than a sales pitch which you made it sound like. So far to date I have sold 4 copies of this very new software and it hasn't even come close to paying for the months of time and effort I have put into it. Please let me know what you think about Manual 'J', retrofit sizing and salespeople. If you know of any other software which does what mines does, would you please let me know what it is called so I can look at it? Thank you for reading my post and all comments are welcome information I can certainly use to either forget the idea entirely, or continue the pursuit to better the industry.

  5. #5
    Absolutely, I agree with all your comments but really, the bottom line is if the current duct system can't handle the size the Manual 'J' requires, what do you do then? If the basement is completely finished and the duct system can handle two tons of cooling, but the Manual 'J' states it needs three tons, what do you do in this case? Do you install the two ton or do you install the three ton? Do you tell the homeowner you need to completely redo the current duct system and provide a price to do it? Would you be able to sell the job with that approach when the next three contractors tell the homeowner this is the size they need and they don't have to fix the duct system because the Manual 'J' they did stated it was the correct size? Would a salesperson even know if the duct system was incorrectly sized and if they did, would they know how to fix it? My post was a fact finding mission only and the text was from a page on my website. I am trying to find out how other people in the industry size retrofit equipment. Is it strictly by Manual 'J' sizing, by duct sizing, by a combination of the two? Or is it by looking at the house from the sidewalk? What do you think on this issue?

    Your comments and thoughts are important for my fact finding. I certainly agree with the software manipulation part. I can make a Manual 'J' say just about any size I want and the homeowner would have zero clue it was incorrect. The software I developed uses counts, register sizes, grille sizes and trunk sizes to determine the size of the furnace and air conditioner which could be installed on the existing duct system and it can be manipulated for sure, but not only could a homeowner figure this out fairly easy, but why would a salesperson do it to sell equipment that would not work? The problem with Manual 'J' is the data collection time and complexity of the software, is well above the effort and learning curve the typical salespeople I have seen would put into the calculation. Do you agree with this statement? I have personally field tested my software to see how well it would work. During the field testing I made many changes to the software get to the point were it would work on better than 98% of the existing duct systems. This is typically a much higher average than a salesperson could achieve on their own. It takes less than 15 minutes to enter in the required information and it has proven to be pretty accurate concerning what size the duct can handle. I would bet this is a much higher percentage than the Manual 'J' a typical salesperson would do. Do you think this statement is true? This is really what I am trying to determine with my post.

  6. #6
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    Has the company you work for or own, ever incorrectly sized an air conditioner or furnace because of the results provided by a Manual 'J' which were incompatible with the existing duct system?
    Well, I'm a believer of not only Manual J but also Manual D. Do most salesman even do a Manual J? Not in my world. Dog and pony shows I see every day. Duct design, you have to be kidding?

    Spend some time on this site and you will see home owners begging for a contractor to do a load of any type on there home.

    . I tried to teach the salespeople that the Manual 'J' was not always the best way to go but they would state the Manual 'J' had to be done to get the rebates.
    Most manual J's that I have seen have not been done correctly, so that in it self is a joke. So I guess what I'm saying what you have said is not to far from reality.
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  7. #7
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    It takes less than 15 minutes to enter in the required information and it has proven to be pretty accurate concerning what size the duct can handle.
    Its here I think your a little a** backwards, Why would you size your system by duct size? You have a four ton load and a two ton duct system so you put in a two ton?
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  8. #8
    Thank you again for your comments. This is my experience also. You can view some of the work I have do at my website- www.AuthorityAir.com I just thought if this is the general way things are done may be my software would be of some value. But honestly, I really don't know, so I made the post to HVAC Talk to get some input from real, live, in the field techs.

  9. #9
    This could be true, but what would you do in this case and how would you sell the job to the client? Would you tell them they have to rip their ceilings in the basement down to fix the duct system? Would you be able to sell the job against the competition? Could you convince the homeowner what you say is correct when other contractors state something else? How would you prove to the homeowner you are correct and the other guys are wrong? This is what I am trying to figure out for myself.

  10. #10
    Quote Originally Posted by jimj View Post
    Its here I think your a little a** backwards, Why would you size your system by duct size? You have a four ton load and a two ton duct system so you put in a two ton?
    Sorry, I posted this wrong. This reply, "This could be true, but what would you do in this case and how would you sell the job to the client? Would you tell them they have to rip their ceilings in the basement down to fix the duct system? Would you be able to sell the job against the competition? Could you convince the homeowner what you say is correct when other contractors state something else? How would you prove to the homeowner you are correct and the other guys are wrong? This is what I am trying to figure out for myself." Goes with your comment above.

  11. #11
    Join Date
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    so its really the question...how do I convince the ho that my service is better, more accurate,
    and actually the correct application to the ho's issues?
    some you can convince, others will never get it.

    it is about solving ho's issues & problems IMO.
    only a handfull of customers will tear out ductsystem & install correctly
    designed & sized duct system. others will opt to do the many things we
    see in the field.

    I've always thought that the level of service some of us provides puts
    us into a niche market.
    I get calls after the original hvac contractor has done all he knows to do
    and problems are not solved. then I go in and find the problem & solve the problem.
    this is what moves one from parts changer to problem solver.
    instead of replacing whatever failed...asking why it failed..finding the answer
    and solving the issues.

    not every ho goes this route. rather they upsize system to overcome
    comfort issues. few even consiter duct loss or incorrect sizing or design.
    how many hvac companies own flow hoods?? just think about it.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  12. #12
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    Moore, Oklahoma, United States
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    Oversizing is a huge problem in our industry, bigger is better rules the roost. Rarely will you see AC under 3 tons/Furnaces under 60k installed correctly. Just as rare is a furnace 60k or less that's a condensing model, or an AC under 3 tons thats more than 13SEER. You won't find many contractors going more than 750sqft per ton no matter how new or tight the house is. Homes built in the 60's are getting 500sqft per ton, contractors have issues with accepting newer homes are insulated 50% better.

    http://acrightsize.com/files/10_reas...g_persists.pdf

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