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How to Cheat on a Manual J Load Calculation

How To Cheat On A Manual J Load Calculation

Sometimes a house just needs a lot more air conditioning than building officials want you to put in, right?*  It’s better to be safe than sorry, so having extra heating or cooling capacity can be a good thing.  Today I’m going to show you how to cheat on a Manual J load calculation, which gives you the heating and cooling loads for a house.

I’ve done quite a few load calculations since my first one in 2004.  I’ve looked at a whole lot more in the process of doing quality assurance for energy efficiency and green building programs.  I’ve also done many presentations and training classes on this topic.  And HVAC design is what we do every day at Energy Vanguard.

Plus, I’ll divulge a little secret here.  I got really good at cheating back in 8th grade.  But what I’m going to share with you here isn’t about putting together a cheat sheet that you hide under your leg as I did for that one test.  I’m going to show you the tried-and-true tricks used by the pros!

Cheating like a pro

The first thing to know is that some Manual J inputs have a bigger effect than others.  Those are the ones to manipulate because a little cheating can go a long way.  Here are my top parameters to take your swindling to the next level.

Windows.  Even a good window is significantly worse for heat transfer than a mediocre wall.  A window with a U-value of 0.25 has an R-value of 4.  That’s pretty good…for a window.  But the surrounding wall will have at least R-13 insulation in it.  And there are five good ways to cheat here:

  • Area – Double the window area, and both the heating and cooling loads will jump.
  • U-value – Got a house with U-0.25 windows?  Enter them as 1.25, and watch those load blow up.  (Higher numbers mean higher loads.)
  • Solar Heat Gain Coefficient (SHGC) – Same as U-value.  Increase those SHGC-0.2 windows to 1.0.  This increases your sensible cooling loads only, but who’s complaining.
  • Overhangs – If the windows have overhangs, “forget” to enter them and your cooling load goes up.
  • Orientation – One way to get extra load here is click the button in your software for worst-case orientation.  Another is simply to put all your windows on the west side of the house.
Window specifications on an NFRC label
Window specifications on an NFRC label

Infiltration rate.  Airtight homes have lower heating loads because they lose less heat through air leakage.  They have lower cooling loads on both the sensible and latent parts because they bring in less outdoor air, which adds latent load in humid climates.  Bump those numbers up, and the heating and cooling loads both increase.  If the builder says the house will be at 3 air changes per hour at 50 Pascals (ACH50), try 8 ACH50.  Those numbers look similar, and the building official will probably miss your deception.

Ducts.  This is a good one because it’s like windows:  You can change them in more than one way to get higher loads.

  • Duct leakage – Many codes require testing for this, but new homes don’t get a duct leakage test before you have to do the Manual J, right?  Crank up that leakage for more load.
  • Location – This is an easy one—and sneaky, too.  Just enter the duct location as unconditioned attic.
  • Insulation – Less insulation on the ducts means higher loads.  Make them uninsulated for maximum effect.
  • Area – Bigger ducts have more surface area for more heat loss or heat gain.  Double or triple the supply and return duct area.

Occupants.  This one can help you a lot, but only on the cooling load.  Each occupant adds 230 BTU per hour of sensible load and another 200 BTU per hour of latent load.  Need another two tons of cooling capacity on your report?  Just put in 56 occupants.  Boom!  You’re there.  Now that’s how to cheat on a Manual J load calculation!

Appliances.  This is a really easy one.  Just add a couple thousand BTU per hour of appliance loads to six rooms in the house, and you’ve added a ton of sensible cooling load.

Design temperaturesThis is the first place most people think to cheat on the loads.  Drop that winter (99%) design temperature from 15 °F to -15 °F or the summer (1%) design temperature from 92 °F to 99 °F, and the loads increase.

Those are some of the easiest places to jack up your Manual J heating and cooling loads, but there are plenty more.  Any parameter can be hacked to give you those higher loads you deserve.

Is it right?

Chances are, you can submit fraudulent Manual J reports to a great many building departments in the US and get away with it.  Building officials have responsibility for a lot of facets of the construction process, and they know more about some areas than others.  Reading over load calculation reports is one of the weakest areas of most building officials.  So, you’ve got that going for you.

But is cheating on these reports OK?  Here are some of the consequences of boosting the loads without justification:

  • The larger equipment will cost more.
  • The air flow will be higher, and you’ll have more ductwork, bigger ducts, and more vents in the rooms as a result.
  • If you don’t expand the duct system, the duct resistance (as indicated by total external static pressure) will be higher.
  • The system will probably be noisier if the duct system isn’t designed appropriately.
  • The oversized equipment will short cycle.  The main problem with that is that the people in the house will be less comfortable.

So, yeah, I can show you how to cheat on a Manual J load calculation.  But guess who else finds this article when they go googling:  building officials.  And actually, they’re the ones I’m writing this for.  The numbers I used in my examples above are wildly exaggerated.  If someone submits a Manual J with 56 occupants, for example, that would be easy to spot.  But if they do a smaller amount of cheating in a bunch of parameters, it’s a bit harder to find.  Look closely, building officials!

I really did get good at cheating in 8th grade.  To make a long story short, it didn’t help me at all.  I did it only because I detested the teacher.  I spent my time in the morning before class started writing that cheat sheet I put under my leg, but I didn’t look at it at all during the test.  Just writing out the information was enough for me to remember it for the test.  I did put myself at risk of getting caught, though.

Anyone looking to cheat on Manual J load calculations is perhaps putting themselves and their businesses at risk, too.  But worse, they’re doing a disservice to the people who have to live in those houses.  Instead of going to the effort of cheating on the Manual J load calculations you do, learn the HVAC design process and do it right.  And if you don’t want to learn, hire a third-party HVAC designer.  We’re happy to help with that.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the summer of 2022. You can follow him on Twitter at @EnergyVanguard.


* Those of you familiar with this blog and my views on load calculations know already that the opening here is facetious.  Those of you without that knowledge know it now.  Keep reading and all will become clear.


Related Articles

How to Read Manual J Load Calculation Reports

Got Manual J? Don’t Assume It’s Correct

What a Load Calculation Does NOT Tell You


Photo of building with a multitude of mini-splits by douglemoine from, used under a Creative Commons license.


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This Post Has 28 Comments

  1. Great article, although when I started reading I thought what has happened to Allison!? Sloppy loads or loads done intentionally with incomplete information are a huge black eye on our industry. As houses get tighter and HVAC contractors keep doing it the same old way they are asking for a trip to Texas Department of Licensing and Regulation meeting (and fine or suspension) if they are in Texas. After I sold my business, I helped out at TDLR for a short time in an investigators assistant role. Over half the cases I looked at were load related. Garbage in, garbage out cost contractors not only money but also damaged reputations.

  2. For those of us doing preliminary design calculations (downsizing a house) – do the calculations in MS-Excel and then accidentally double the number when using the function to summarize the result. That way a 2000 square foot house shows up at 56K BTU per hour.

    Also embedding a coefficient in a formula is also a good technique.

      1. Not cheating but a software design error!

        For my PGH, I use the rule of thumb of 7.5 BTUh per square foot to help catch mistakes. 2000 sq. ft. (with basement) should be around 30K BTUh. That helps find design flaws in spreadsheet – so I can size the heat-pump.

        1. for sure a rule of thumb of any kind is cheating William as shown in the acca manual do’s and don’ts. its this very thought that a certain btuh/per square foot can produce usable info for hvac design is exactly what Allison is bringing attention to, only the acca manual (full version) can accurately model a building for hvac and is never verified by a simple outdated rule of thumb guess.
          funny to me that a simple spreadsheet would be used to “verify” or double check a result… is silly, as the acca manuals require hundreds of input points for even a tiny home (can be as many as 52 formulas per room to produce an acca compliant design). simple spreadsheets are guesses, there is no way to perform adequate exposure diversity calculation using any type of spreadsheet – AED is the prerequisite to manual j and determines which of the two manual j calculation procedures are performed on the home (peak or average procedure) – each of the manual j procedures produces a different size duct system and different room air flow values.

          1. My calculations are only preliminary – but they factor in the square footage of the wall, the NRFC numbers, and the infiltration numbers for the windows being specified. PGH calls for an ACH of 1 or better. I use the rule of thumbs for visual inspection of the numbers to make sure there are no outliers. My calculations are more based on heat transfer mechanics.

            I use two spreadsheets – (1) Block Load and (2) Zone-By-Zone load. The zone-by-zone spreadsheet is much more useful because that points out weaknesses in the house design (blue prints). My spreadsheet is only for preliminary load calculations for designing the framing and assembly’s of the house (budgeting). Once the house project starts, Energy Vanguard can do a proper design of ductwork. I am just making sure that the floor truss and bulkhead have enough room to run the ventilation duct (dehumidification / recirc / fresh air) for the AIr Treatment Unit (ATU). For instance, I had to take room out of a closet so I can fit a supply and return duct. For blue-print design there is nothing wrong with using the Energy docs “duct-sizing” to make sure you have enough room.

            Have done the same thing for the radiant ceiling panels, Warmboard and the concrete slabs. The heat loss tells me how many panels or loops I need per zone and the flow rate for the pumps. So far, I have been able to down a size heat-pump size – because I incorporate heat-loss and air sealing principles into the architectural design process. Just finished redesigning the roof for one-section – so the ridge beam is now within conditioned space.

            Perhaps, I have read too much of Allison, Andrew Ask, John Siegenthaler and Robert Bean.

  3. Isn’t cheating a bit harsh? That implies foreknowledge and “willful intent”.

    Consider a practitioner who only takes a couple days training in manipulating the software. He/she never owned the manual and never sat in class for a week, hanging on every word from Hank Rutkowski. And yet after a couple days of software training, these trainees can navigate the software and become self proclaimed experts.

    Section 2-5 of Manual J states, “The first thing to understand about load estimating software is that it may not always do what Manual J stipulates…..Software is a powerful tool, but it is not magic. If Manual J software is not used correctly, it will produce an inaccurate and misleading answer, and those who have not studied Manual J (the 600+ page paper version) will not know the difference.”

    If one doesn’t know the right way, then how can they be convicted of cheating?

    My experience has shown no one has to intentionally cheat to inflate loads. It’s simply a natural consequence of the lack of training.

    1. Danny: I do believe that lack of training and experience is the bigger problem with incorrect load calculations, but I’ve seen some that looked mighty suspicious. But the real point here isn’t about cheating. I’m trying to educate building officials and anyone else who reviews Manual J reports on what kinds of things to look for.

  4. A,
    I am not sure you are aware of this. I was told several years back that ACCA gathered data from competent practitioners to use in directing their focus on research, training etc. I have no clue if they still do it. But it is my understanding the data consistently showed cooling loads on a typical home have averaged +/- 1,000 sq ft per ton for nearly 20 years.

    Dont misunderstand what I’m implying. No one is saying use this 1000 ft2/ton to size equipment. It is simply a binary screen to determine if additional investigation is required.

    I routinely share this metric with code officials in my state and feel pretty confident it will identify the poorly trained and the cheaters. If the load shows <1000 ft2 per ton, then there is reasonable suspicion to warrant further investigation.

    1. Danny: Thanks. I didn’t know about that. I’ve been doing that same thing for more than a decade. In the article below from 2011, I wrote:

      “If they want to do ENERGY STAR new homes, though, and their Manual J reports are coming to me, they should know that I’ve got my own rule of thumb – 1000 square feet per ton. When that 2000 square foot house comes to me with a 3 ton air conditioner, I have no choice but to look further because they didn’t pass my rule-of-thumb test.”

      I Use an Air Conditioner Sizing Rule of Thumb!

  5. Absolutely. 3 ton for 2,000 sq ft sounds like the crawl space door method of sizing. (For those unfamiliar with that process, you install the biggest unit you can fit through the crawl space door).

  6. Since ’83 (yes folks the 1900’s) i have produced >62,000 hvac acca designs – by the book(s).
    never had a problem, but many question “why such a small hvac unit”, as few hvac companies or engineers in my area are qualified to produce an acca compliant design, Hank is correct and thanks Allison for highlighting it in this article!. – so i produced this many years ago to help home owners, builders, and architects see what a fake hvac design would look like – i had a library of hundreds of these fakes produced locally by unqualified hvac designers/ spotting a fake hvac or energy design:
    and showing that a simplified duct heat load calc like ashrae 152 or the Hank duct charts method can be off by more than 50%, only a scaled duct drawings should be used to determine the duct wall exposure amounts (for manual j duct transfers) and heat rise amounts (used to fine tune manual S) and not a duct guesstimate like used in the 80’s and 90’s with manual j 6th or 7th editions – there simply is no reason to guess on the manual j duct loads when intelligent cad with smart duct figures will most accurately quantify the results – available in easy to use software

    If you are currently using a “designer” or “service” that does not perform a complete design package, be aware their partial design effort, also known as “calc’s only”, will lack accuracy. Many HVAC contractors and less than professional design services offer these expensive partial calculations that take just a few minutes to generate – but a complete accurate HVAC design package requires several hours of intelligent cad design to generate. Don’t leave your work to chance – demand a complete HVAC and energy design package that includes the code required HVAC ACCA “best practice” design Manual’s J,S,ZR,T, and especially the ACCA Manual D scaled duct design. A complete HVAC design effort ensures your building is code compliant and comfortable.

    i have tracked many blogs at this site, and nearly every bad idea being made by hvac contractors – a short time ago one hvac installer claimed that ductless mini splits were the way to go on any new home!
    I feel for Hank and all the time and effort he spent on the acca design manuals , manual zr and manual j show you should not use ductless mini splits unless they are the correct size of the single room they serve, the room geometry is within the air flow pattern of the air handler, and that ventilation air is provided by a separate ventilation dehumidifier (ventilation air can not be supplied by fans, opening windows, building envelope leakage, etc – must be mechanically induced by the hvac system) .
    each time i see the “my way ac guy”, one who does not follow the acca manuals exactly as written, i can see why the hvac industry is a mess……follow Hanks books on design, install properly, be in business a very long time and make the end user comfortable as defined in manual J.
    when instructing a student, my training classes take about 6 to 9 months (hundreds of hours) before a new designer can take on even a simple one story home. often times my acca compliant designs are used as a guide on how to correct a poor install (mostly oversized equipment with a duct “design” based on a duct -u -lator!) no good reason to guess or cheat any acca manual, Hank is correct – his books work every time – and Allison is very familiar on how the hvac industry cheats these calcs – so any acca user should read the “do’s and don’ts” of manual J so that the calculations are precise

  7. Ah, code officials. I’ll digress.

    When I first started in construction, I had goose bumps before each inspection, I feared them like my 8th-grade geography teacher (she was ruthless, but also probably the best teacher I had). That was then. After only a short while I realized most code officials in most jurisdictions were spread thin, overworked, and on a tight schedule for each inspection. How could a final inspection on a 5000+ sqft house take only 5 minutes? The house was perfect in all departments?

    Maybe one good approach to Manual J is not trying to satisfy the code officials, but to produce a load calculation that will roughly duplicate if done with another piece of software. Load calculation is just the beginning, especially considering that the calculation only comes up with maximum reasonable loads under harshest exterior conditions for the given climate zone.

  8. Loads should be calculated based upon the Acca guidelines
    what if someone wants a 2nd unit for gatherings .

    1. Kevin: That’s what ACCA says you should do to handle additional internal loads from parties. That way, the main system is sized appropriately for the normal loads the house experiences the majority of the time.

    2. Indeed. I typically specify a ductless mini-split for the public area of the home if the owner indicates frequent large gatherings (more than, say, 5 or 6 additional occupants). I select from among several ductless air handler styles per owner preference: 4-way ceiling cassette, floor standing, wall mount. Only AC is needed for this application, but depending on the model/series, I may end up specifying a heat pump just to get the desired size & efficiency.

  9. The dilemma of teaching code officials how to properly interpret Manual J can be easily resolved. Simply encourage a special interest group to lobby the code writers to adopt NC’s solution. Amend the code.

    “For permitting, inspections, certificate of compliance or certificate of occupancy, verification of Calculations for HVAC Systems – ACCA Manual D, ACCA Manual J nor ACCA Manual S calculation submittals and review shall not be required”
    Section 312 NC Mechanical Code

    Problem Solved!

  10. This might be a bit off topic, but what does Manual J (or S) say about sizing heat pumps? Size for cooling, take what you get in heating, and make up the rest if necessary with strip heat? Or should you size for heating with no strip heat and live with too much cooling capacity in some cases?

  11. “ERRORS”? New build in progress 2004 ft2 ranch with a bad Manual J / duct plan. Builder won’t address it and waiting on HVAC contractor response. Permit MJ8 errors:
    -Building orientation incorrect
    -Actual equipment airflows not entered
    -Cooling equipment performance data at actual design conditions do not reflect Carrier’s expanded performance data at actual airflow
    -Static pressure entered as zero
    -Wrong winter outdoor design per local code
    -Indoor heating/cooling design temps should be 70/75 degrees instead of 72/74
    -The indoor winter relative humidity design point should be 30% instead of 35%
    -Full basement with 9′ ceilings when its 8′ ceiling and partial crawl space
    -Infiltration includes non-existent fireplace, average construction quality instead of something in tight to semi-tight range (blower door test on these houses range 2 to 2.5 ACH50)
    -Glazing missing Low-E glass and porch overhang, extra windows, oversized windows in one room, no inside window coverings, no foreground reflectance
    -Overstated loads from 60 CFM for ASHRAE ventilation
    -Loads heating/cooling 54,952/20,588(4,092 latent)
    -My MJ8 says loads 49,093/17,143(2,140 latent)

    Specified Carrier 59SC5B 80,000 BTUh with 2 ton A/C when my own MJ8 shows only need 60,000 BTUh with 1.5 ton A/C. In model of this house at part load full sun 82 degree day A/C short cycles 8 minutes with no dehumidification 56.5 RH 55.4 DP. Above full load full sun 97 day runs short cycles 10 minutes with minimal dehumidification 60.3 RH 58.4 DP in low humidity Chicago June weather.

    Duct work appears to be sized at 760 CFM design airflow with fixed .07 friction rate instead of actual CFM and calculated friction rate. In model return air is overly restricted with insufficient intakes and strangling the furnace on A/C with return duct 0.6 iwc so TESP estimated 1.1 to 1.5 iwc. A/C fan set about 1600 CFM! Heating CFM set such that high limit trip may occur with overall excessive static pressure.

    I reviewed 4 MJ8 reports on 4 different builds of this same house with slightly different variations of most of the same errors which makes no sense. It screams careless at the very least.

    Can you say code violations?

  12. The saga continues. I have learned the hard way that just because a village requires a Manual J and duct plan, and, purports to have a current building code including requirements for ACCA Manuals J, S, D and T doesn’t mean that they will actually follow and enforce it.
    – The submitted Manual J and duct plans are both essentially works of fiction containing numerous obvious errors and thus were clearly not reviewed by the building department.
    – The builder won’t accept that it is a code violation because the village inspectors approve the work.
    – The HVAC contractor says they are doing exactly what they were contracted for.
    – Cheating? This goes well beyond that since everyone involved is looking the other way.
    – Ironically, the village actually has local code amendments including a lower than ACCA specified winter outdoor design temperature and yet accepts a Manual J without it.
    – The HVAC contractor head engineer claimed that the supply trunk was plenty large enough for two tons of cooling but the installation crew sets a cooling fan speed around 1400 CFM! Pairing that with a return trunk smaller than the supply trunk makes for a pretty noisy system that is strangling the blower.
    – After pulling four different Manual J and duct design permit submittals for the same house design, each with multiple obvious errors, I can safely say that it looks like nobody official cares about the problem.

    Requiring independent review of HVAC design submittals with post installation inspection seems to be the only way to address this as someone else noted earlier.

    Coming up with a plan B to deal with this mess is certainly challenging!

  13. Mike F,
    I have a theory that very few code official have the skill to understand the ACCA process. As some say, those you can do and those you can’t become code inspectors.

    Perhaps part of the plan b solution is inverters which modulate frequency from the actual building load imposed on the evaporator coil using temperature or pressure inducer employing refrigerant/temp as a surrogate. That might address the oversizing issue. But the duct design is still an unresolved issue.

    1. Danny G,

      I quite agree with you regarding lack of HVAC knowledge among many code inspectors. The village in question contracts out for electrical and plumbing inspections but handles the rest on their own. ACCA attempts to provide a solution for this by providing questionnaires for code inspectors to measure compliance but few seem to be aware of this resource. Doubtless the inspectors would cite a lack of time and manpower for real HVAC submittal evaluation.

      Regarding duct design, I have a notion for how they could simplify system evaluation: static pressure measurements. Check total external static pressure compared to equipment ratings. Anything substantially outside of equipment design specification means that the installation does not conform to code. The majority of the time, it will be due to undersized ducts. For my system there is absolutely no design allowance for the optional media filter. I requested a friction rate worksheet from the HVAC contractor and their response was that they do not share their design details. They went on to explain how the ducting was properly sized for a .08 friction rate. At that point, it became obvious that they only use their Wrightsoft package to generate a local code required Manual J and duct design drawing that is not reality based and totally ignore the duct design capabilities of the software. One cannot share design details that were never created. As a result, I also hold the HVAC contractor responsible for what, in my opinion, is negligent work in violation of code since they are obviously not following ACCA or any other recognized rigorous design standard. I am unfamiliar with ASHRAE standards but seriously doubt my as yet uncompleted system would meet their standards either.

      The builder shares responsibility here for enabling bad practices. I suspect that economics also come into play here. The builder received my protests as my wanting custom installation and refused to recognize my code objections because “they have built houses for years without major HVAC warranty claims” and “it passes code inspection”. Unfortunately, since I am not directly in the trade I am not given standing in the builder’s eye’s. My dad was in the trade and was trained in balancing too. I was exposed to much of Hank R’s knowledge that way so I actually do understand how badly this HVAC job is being executed.

  14. Mike,
    I like your idea about static pressure being used to validate proper design. Most manufacturers’ installation instructions state to measure and record it on their start up sheet. And since installation instructions are a mandatory code requirement, it should not be an unrealistic requirement. But, that would require the system be up and running before final inspection. In my hood, the house doesn’t get permanent power until the final inspection is approved, unless the builder requests and pays for temporary permanent.

  15. Granted the static pressure idea isn’t perfect but it would work for some jurisdictions. Some places actually do require submittal of the friction rate worksheet for forced air systems. The ACCA brochures for code officials point out to ask for this information. That has the potential to raise awareness. I am irritated that it is so difficult to get inspectors, builders and HVAC contractors on board with doing it correctly. As long as the builder can find an HVAC contractor willing to cut corners (read costs) on duct design the problem is perpetuated. The Wrightsoft software does all of the heavy lifting for duct design if you let it. Unfortunately many give it bad input thus the old acronym GIGO (Garbage In Garbage Out). Don’t even get me started on cheap poor duct design like the outdated plenum/trunk tee or bullhead junction being installed on my system with around 100 feet of excess (in)effective length!

    One thing you said intrigues me: “… since installation instructions are a mandatory code requirement”. That is an interesting angle. Is there actually something in the code that requires them to follow manufacturer instructions? I don’t recall reading that point anywhere. A reference to that specific thing would be useful. I see multiple instances of not following manufacturer installation instructions in my impending system.

  16. Mike,
    Our code is based on ICC, I think the 2012 version. M1401.1 provides, “Heating and cooling equipment and appliances shall be installed in accordance with the manufacturer’s instructions and the requirements of this code”.

    I don’t think it takes much maneuvering to get to the code requiring it. Another example is the requirement for Manual D but no mention of ZR. But wait, there is a reference in D saying zoning systems should be designed in accordance with ZR. So in my “non liar”, I mean “non lawyer” mind, ZR would be required by code via the reference. Same is true for installation of flex duct referring to ADC guidelines which is in lots of flex duct installation instructions.

    But at the end of the day, I dont think relying on code enforcement will make any difference. It has to be market driven. Change will occur when the consumer demands better design. Sadly, these days, the consumer doesn’t demand it up front and gives it no thought until a failure that cost them money, money, inconvenience or makes a family member sick. As Emerson said, no one studies geology until the day after the earthquake.

  17. Regrettably, consumers are mostly unprepared to recognize problems let alone push for change in an area as complex as HVAC. I see too many examples of people in the HVAC trade who demonstrate insufficient understanding so how is the general consumer to even have a chance? Look over the Internet and see how many “knowledgeable sources” say that you should size sheet metal duct according to a fixed friction rate of .08! My new house HVAC is being installed by a large local contractor having a head engineer making that very statement to me!

    Thank you for that useful code reference. It might yield a solution to my situation.

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