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Community and Q&A

Origin of the 99% Load Calculation

john_m1 | Posted in Energy Efficiency and Durability on

From what I understand the Manual J accounts for 99% of the situations your HVAC system will need to handle.

where does the 99% come from,
could I spec a new or replacement system at 95 and 5 or 90 and 10?

It would seem you get a more affordable system and a much smaller environmental footprint.  If things got really bad you could add a room space heater in the winter or go to the movies in the summer.

Though the equipment would run harder that would only be true for that portion that exceeds design capacity.  You also would be getting something, just not 68 deg at 40% RH.

It’s only been two generations that we collectively have decided that every home needs to be air conditioned.  To get the efficiency we can size smaller systems or ask for less HVAC.  Where we are now is getting more and more exotic technology but what is the cost of achieving that 99 and 1% metric?

It seems we can achieve significant reductions in climate impact by a policy change.  Under current code (i will look later) the 99 and 1 may be required but that is a policy choice not an inevitability.


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  1. Jon_R | | #1

    > I understand the Manual J accounts for 99% ...
    Where does this come from? Using 99% input data doesn't imply that the results are sufficient for 99% of conditions.

    Generally, no, you aren't going to save much if anything by downsizing equipment (and it can go the other way).

  2. Expert Member
    NICK KEENAN | | #2

    Durable goods like houses and cars present a policy problem, because they pass through many owners in their lifetime, but the first owner makes decisions that all of the subsequent owners have to deal with. So I understand the public policy justification for mandating standards that the market wouldn't otherwise deliver.

    In addition, municipalities have an interest in keeping people from building houses that no one else will want. As the collectors of property tax they in effect have an ownership interest in the property, and as such it is very much in their interest that housing be built to a minimum standard. Lenders have a similar interest. I'll add that it is very un-green to build something and then have to tear it out and redo it later for a subsequent owner.

    So there are strong policy arguments for requiring mechanical systems that satisfy the desires of almost everyone. If a house has air conditioning, there is no requirement that you use it, nor that you set the thermostat at a particular level. On an individual level, I don't see anything in your proposal that couldn't be accomplished through judicious use of the thermostat.

    While the energy code has made great strides in the past two decades, where I would like to see more regulatory focus is on performance-based standards rather than processed-based ones. Something more like fuel economy standards for cars. Currently there's no disincentive to build a house as large as possible, and a standard like ACH50 is easier to meet with a larger house than with a smaller one.

    1. john_m1 | | #3

      Thanks DC Contrarian. That was a very thorough and eloquent answer. I agree with almost all of that. I would also suggest having each municipality decide how to tackle these complex issues is probably not the best way to get us to where we need to be.

      As for turning down the thermostat, I am there personally. Kind of like wearing mask. We don't really believe in collective action to obtain collective benefits.

  3. BirchwoodBill | | #4

    I am not an expert, but I have a manual j spreadsheet for a room by room load. In my neck of the woods, the design temp is -13f which covers 99% of the time. The worst case was -29f and most of the time during the winter it is above 10f.

    The other war of looking at it, is 99% of the manual j have inflated numbers because people like to oversize than right size things.

    1. john_m1 | | #5

      That is exactly right, and the is the gist of my question, 99% means three days out of the year. Why three? Why not five or ten or zero.
      I totally agree on the over sizing. There is a new home near me about 4000sf. Two zones with what looks like separate ducts to bring in fresh air. Two monster outdoor units.
      Personally I would go with a smaller unit and have a whole house dehumidifier. 80 degrees indoors is tolerable with low RH.
      I put a small mini split this past summer on my first floor. 12k for a 12k window unit. The difference is unbelievable. No J, but it probably would have recommended 18k. Its about 800sf with lots of glazing south. More than anything it was the drop in RH that made Mini Split the killer that it was. Its doing ok in the winter, but in no way can replace the boiler.

      So this experience is part of the genesis of my question. The J probably would have recommended the 18 or even a 24 (you know for a safety margin)
      There are so many things wrong with what I did but in no way do I regret it. They unit removed so much humidity that the whole floor was more comfortable.

      I just want to know why its 99% and not something else. No one has really answered that question. Most people who buy a house (with Central AC) and hire someone to do a manual J. They will either wait until the unit breaks or go for years complaining about the poor performance or replace it based on someone's recommendation using some rule of thumb and looking at the bonus plan that they are on.

      We can also go in the other direction and say why not zero or 99.9. Why that particular number?

      1. Expert Member
        NICK KEENAN | | #6

        During cooling season dehumidifiers just waste energy.

        99th percentile is a rule of thumb, I don't think there's any science behind that number, it's just that experience has shown that sizing that way gives good performance for most people in most situations. Engineering is all about compromises.

        The problem with radical undersizing is, what happens when someone else lives there? Maybe they'll adopt your aesthetic, or maybe they'll say "I don't like to sweat." Either they'll get a bunch of window units and space heaters, or they'll rip out the system you put in and put in one that they like. Neither option is green.

        Again, what's so terrible about sizing the system to 99% and then adjusting the thermostat to your comfort?

        1. john_m1 | | #12

          I have zero issue adjusting the thermostat, and I do. Before my kids were born went from 62-68. My house is in the stone age for HVAC, 80% boiler, with oversized radiators (look up fresh air movement from the last pandemic)
          Again trying to understand why THAT number.

          1. Expert Member
            NICK KEENAN | | #15

            It was just made up.

          2. john_m1 | | #16

            The 99% may have been made up, but its not necessarily wrong its just not well supported.
            To you or anyone else here. Dan Holohan from on his podcast (Dead Men Tales) talks about how the fresh air movement, in the wake of the Spanish Flu Pandemic, affected how we size our rads. My house was built in 1916 and the largest radiators are all underneath windows. They probably weigh in the neighborhood of 400-500 lbs. In the thinking of the day that was considered proper.
            I am amazed at how many people through decades left the size of those radiators unchallenged. When they tear a place down here, to the sticks, at least half those radiators go.

      2. Expert Member
        NICK KEENAN | | #7

        >The J probably would have recommended the 18 or even a 24

        Until you actually do the J your whole premise makes no sense.

        1. john_m1 | | #8

          Thanks again. Is 90 or 95 radically undersizing. It fills the bill 90 or 95% of the time. I have an 1800sf house and I suspect a proper J would recommend 48k, though 4 tons seems excessive.
          Oversizing, as we all know, is bad as you will never dry the space properly.
          Agree that the J answers my situation, but the answer is why 99%. And that's my point. I also agree that turning the thermostat down or up is the far easier thing to do.

          Lstiburek said there was no science behind the .6ACH50 for passive house, but as much as I respect him, I have to believe otherwise, but if true then why .6?
          99% provides a framework. It doesn't sound though that I would even save anything if I sized at lower percentages. Completely agree on the compromises, but that is based on money, or comfort.

      3. Patrick_OSullivan | | #9

        > That is exactly right, and the is the gist of my question, 99% means three days out of the year. Why three? Why not five or ten or zero.

        Like most things in life, perhaps it's somewhat arbitrary. But you have to design to something, always.

        Most systems are oversized so as to avoid customer complaints. Since we can't even seem to get systems right sized, it's perhaps a bit premature to consciously undersize (in most cases).

        It's also worth noting that it probably doesn't make sense to think of a 99% design temp in units of days. Temperatures fluctuate widely during the day in some climates, so a more reasonable approach (assuming we all at least agree that discrete math is more appropriate than continuous math) is an hour by hour analysis. 1% of year on hourly basis is 87.6 hours. Another way to think of that is 29.2 days having a 3 hour period where your system cannot satisfy the load.

        Hypothetically, that could mean a 99% designed system is insufficient for a 3 hour period each day for an entire month. Is that acceptable? Maybe if it's in the dead of winter and you're under covers anyway, you don't care. Maybe if it's the dead of summer and you're sweltering mid-day, it's really annoying.

        In reality, the capacity of systems is quantized into relatively big chunks (12,000, 18,000, 24,000, etc. Btu/h). I suspect the difference between these quantizations is likely to exceed the error bars around 95 vs 99% in most cases, unless you are exactly on the margin.

        If you get close and use a good system that can modulate, that's worth a lot more than quibbling over 95% or 99% design temps. There's already far too much noise in the system (meaning the measurements and assumptions) to be overly concerned.

        1. Expert Member
          NICK KEENAN | | #10

          Well put, and I'll add that among those hours that are outside the 99% temperature range, the most common amount they will be out by is one degree. Second most common will be two degrees. And so on. For the most part these excursions will be so small as to be unnoticeable. Very rarely will they get to be five or ten degrees. And when that happens, it won't be for very long. Houses have heat capacity -- they weigh on the order of magnitude of 50 tons -- and they take time to heat up and cool down. So if you design for the 99% temperature you're unlikely ever to have comfort complaints.

          1. john_m1 | | #13

            Great points

        2. john_m1 | | #14

          I love the comment about noise. My actual career is supply chain and I had to explain this concept to executives that you couldn't buy 15.6 widgets like the multi-million $$forecast system wanted you too, either 12 or 24. Or you could but any savings would be more than offset by the cost of breaking packs.

          On your hypotheticals you maybe western facing stucco wall heats the house up so much in the summer its not worth an extra ton to heat the one room on that side of the house that is occupied during the day.
          I like to challenge assumptions, Seems we went in two generations from accommodating ourselves to the worst part of summer/winter, to saying our standard is to basically never feel uncomfortable.
          Most of us on these pages will be willing to turn the thermostat down. Most of us not on these pages won't put much thought into it. they only ever think about when their system is broken (on the hottet day of the year) and the guy who comes out who's been doing it add's your rooms up, takes your comments, and adds a factor and then sells you an oversized system.
          If you start talking about Manual J,S,D,T you will probably never make the sale. You get like 15 minutes to grab their attention. I am sure it happens but how many HVAC contractors come in for a system replacement and start talking about how the ducts are improperly sized and need to be replaced. Sadly consumers/homeowner largely do this to themselves.

          I will continue to read these comments but I want to thank everyone for your very excellent answers.

  4. BirchwoodBill | | #11

    My Manual J are for sizing hydronic and heat pumps not ductwork. In looking at boilers, they don’t modulate low enough and don’t handle water going down to 83f to 103f. Right now my house is heated with a heat exchanger and a 70 gallon DHW tank that has the heat storage capacity for dealing with the excursions.

    As for HVAC if air handler has a VFD for the blower drive and a ECM for the chiller water, it can modulate low enough to move air slowly across the coil allowing it to dehumidify without making the house too cold. Right sizing the duct work means lower resistance and head loss. Again proper air sealing, insulation and good windows will lower the loads for the ventilation system. Just make sure to update the manual J with numbers coming off the blower door testing. Every mechanical contractor does a blower door test right?

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