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Community and Q&A

Manual J sticking point with vendor: mean temperature value

Gwisejr | Posted in General Questions on

I’ve had an issue with my builder and their HVAC vendor. They way oversized my system by doing the following items:

1. Wrong Window U-Values 0.58 instead of the correct 0.34
2. Not adding any window blinds
3. Not including the conditioned crawlspace
4. Using 0.4% instead of 1.0% mean location temp
5. Intentionally orientating the house to show the highest loads instead of using the correct orientation.

I hired ProCalcs and they exposed all these items over time.  The last sticking point is that they are insisting on using the 0.4% Mean temp value and I need to understand if the industry standard is 1.0% and if so, under what conditions it is justified to change it to another value. 

Right now my builder is wanting me to sign a $300 change request mainly to include the following verbiage:
Client is requesting an HVAC system smaller than the size recommended my the HVAC contractor.  HVAC contractor is recommending a 2.5 ton system on main level (3 ton air handler, already installed  pared with a 2.5 ton heat pump condensor) and a 1.5 ton system on the second (2 ton air handler  is installed, to be paired with a 1.5 tone heat pump condensor). Client is requesting a 2 tone system  on the main ( causing the current unit to me removed and replaced.) Due to this undersizine on the  main level (based on the clients calculation of 91 degree outside tem0), SHC will not warrant system operating properly.

ProCalcs shows that using all the correct inputs and using 1.0% mean, the recommended tonnage is 1.5 on Main and 0.9 on second floor. The vendor started out at 3.0 on Main, 2.0 on second. After all the corrections and leaving the Mean temp at 0.4%, they ended up at 1.7 ton on main and 1.2 ton on second floor. With my forcing them to use the 1% mean, they come down to 1.5 on Main and 1.1 on Second.

So, I would like someone if possible to tell me under what conditions is it acceptable to use one temp mean setting over the other? I’d like to stand on firm ground when I push back on this.. 

Thanks for all who have been providing feed back on my questions the last few weeks.

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  1. this_page_left_blank | | #1

    I wouldn't even waste time on this. Kick them to the curb and find someone else. They're asking for you to pay $300 for them to do the job properly, and essentially prevent any warranty claim. If I was spiteful and had the time and money to spare, I'd let them install the system they want and then barrage them with comfort complaints due to the oversizing.

  2. silkwj | | #2

    I don't think your issue is 0.4 vs 1% here. These HVAC guys sound awful, I wouldn't use them at all. Even if they install what you want, it's probably not going to be a pleasant experience, not to mention getting maintenance done down the road (if they do that).

  3. Gwisejr | | #3

    I am very much inclined to kick them to the curb but My builder hired them and unless I can really provide proof that they are just not doing the job right, I can't fire them myself..

    I really just need to understand the building science standard on this Local Mean Temp setting.
    I need to know if there are any valid reasons form them using this 0.4% instead of what I understand to be the standard of 1%..


    1. silkwj | | #4

      Code(s) basically require Manual J, and it's implied - or maybe even expressed explicitly - that that means done properly.

      1% to 0.4% just add a little bit to your heat loads. With the current climate situation, and worsening extremes, maybe you want that little bit. It's rare that it results in a capacity change in the equipment spec - you just happened to be on the cusp. Worst case, the house drops a few degrees from setpoint during extreme weather, but you're slightly better off at part-load conditions for the rest of the year.

      1. this_page_left_blank | | #6

        What he's asking for, and they're denying, still amply covers the 0.4% figures unless I'm reading it wrong. Manual J at 0.4% says 1.7 and 1.2, he's asking for 2 and 1.5. Bizarrely, they're saying the main floor is the one undersized, maybe because the air handler is sized for 3?

    2. this_page_left_blank | | #5

      " if there are any valid reasons form them using this 0.4%"

      No. And you have ample evidence they are willfully and negligently incompetent, in the form of their intentional bungling of the Manual-J to spit out numbers they had chosen in advance, followed up by their admission they will not stand behind their work if you insist on them following the industry standard guidelines. They're even refusing to install a system appropriate for their 0.4% numbers without the disclaimer.

  4. walta100 | | #7

    At this point the unit is installed. If this is what was bid and accepted $300 is a very low cost for the amount of work required to remove and reinstall the equipment.

    I would not accept the mismatched air handlers unless that combination of equipment is listed on the manufactures tables. My guess it is very unlikely to find a listing for 3T coil and 1.5T unit. The unlisted equipment is unlikely perform well at removing moisture from the air. You will find this listed in the engineering data as latent heat. Latent heat will not be very important if you live in a desert but it is the only thing that matters if you live in a swamp.


    1. this_page_left_blank | | #8

      My reading is that the 3 ton handler is the only thing already installed. I assumed it was there as part of an older system, and the $300 charge was for writing in the disclaimer.

      The client is asking for a 2 ton condenser, not 1.5. So the question is which sizing match is more important, air handler to condenser, or condenser to load? 3T to 2.5T to 1.5T, or 3T to 2T to 1.5T.

      1. Gwisejr | | #9

        Currently the HVAC vendor has already installed the Air Handlers in the house. 3 Ton Main, 2 Ton upstairs. This is a new construction. The ducting has also already been installed. No Heatpump compressors for either air handler is installed. Their own calculations show that the units should be 1.7 ton main and 1.2 ton second floor using teh 0.4% temp mean but their own correct manual J calculation using the 1% temp mean shows requirements of 1.5 ton for main and 1.1 ton for second level. They are installing Carrier 14 seer units and the smallest size available is 1.5 ton.

        I am installing solar panels and I want the smallest units that will do the job and as I understand it, for the main level, there is enough slack in the manual J tonnage recommendations that 1.5 ton is more than sufficient to cover for a 1.7 ton calculated recommendation. And if you factor in using the 1% mean instead of upping it to 0.4%, then it matches.

        I am building just north of Atlanta, GA in zone R3 on 5 acres in an agricultural area so I'm thinking it safe to go with the 1.5 ton solutions. I already asked them to install larger lines sets 5/8th so that I can upgrade to some VRF units in the near future. I would do it now but they just want too much.


  5. walta100 | | #10

    Did they put the ductwork and equipment in the attic? If yes you well may need the bigger unit upstairs.

    If the dirt the house is being built on is currently in your name it is your house you are in charge and you are free to fire anyone. If the only thing in your name is a piece of paper promising to deliver a house someday you are just along for the ride.


  6. Gwisejr | | #11

    So the main floor unit and ducting is in a conditioned crawlspace. The upstairs unit and ducting is in the hot attic although I did ask to have the ducting run so that I could pile up insulation on top to better control that along with the fact that the equipment area I will be enclosing and bringing into the conditions space in the near future. probably over winter spring time. The manual J calculations were made with those conditions.. meaning as they will exist right now before the attic modifications are made.

    The house is 2x4x16 center with Zip-R2 sheathing, galvalume tin roof double low-e windows, etc..

  7. Expert Member
    Dana Dorsett | | #12

    I'm not accustomed to the term "Mean Local Temperature" to indicate the outside design temperature. Is it commonly used this way in your area?

    The 0.4" and 1" outside design temperatures used in Manual-J are the 0.4th percentile temperature bin (only 35 hours out of 8760 hours in a year are warmer than that) and 1st percentile bin (only 87 hours out of 8760 are warmer) for the location, which have nothing to do with the MEAN or average temperatures for the location.

    Oversizing the equipment for the 1% design load by 1.2x is usually more than sufficient capacity to cover the worst heat wave. Oversizing by 2x or more generally leads to LOWER comfort. Air conditioning is most comfortable when it's actually blowing cool air, at modest cfm. At 2x oversizing it's only operating at a 50% duty cycle even at the 1% design temperature, and is never running nearly continuously. At 1.2x oversizing it will be running a 1/1.2= 83% duty cycle at design condition, and increases the duty cycle at outdoor temperatures above the 1% design temperature. It's actually more comfortable to be slightly undersized than 2x oversized, even though there will be a few hours per year when it doesn't quite keep up with the cooling load.

    With an oversized air handler & system it cycles between cold blast at higher (an noisier) than needed cfm followed by the lull- it cools the house to the set point temperature just fine. But human comfort isn't simply a temperature- intermittent high cfm cool blasts does not provide optimal comfort for the human occupants.

  8. Gwisejr | | #13


    Thanks for the clarification. I was using the term incorrectly. Its not a local thing here in Atlanta. Also, another correction, My builder did not want the additional $300, its just part of the form and they forgot to X it out.. I was contacted by the builder owner and they are looking into the discrepancy. He was not fully aware of what the hvac vendor did initially..

    thanks for all the input from the forum members.

  9. Gwisejr | | #14

    So the Saga continues..

    I spoke to the builder on Wed, they agreed that the HVAC sub messed up. Asked what I wanted to do. I Said that I would like them to correct the duct work and supply the calculations so that I can confirm, then let me get someone else to install correct units. Builder asked for second option stating that they may not be able to do that. I replied, that they correct evertthing and install correct Manual J calculated units.

    Got call from builder Thursday, HVAC sub does not want to unstall the 1.5 ton unit for main floor. Claims that they don't want to be responsible for us calling about house not cooling. Offered us $8200 for them to walk away and for us to find someone else to correct the ducting and install correct units. I told him that that amount seemed rather low. Even accounting for they the builder getting good discounts from a sub. I was expecting ~ $12k-16k for hvac for a two story 2500sqft home.

    So, can anyone tell me what a builder might ordinarily pay a hvac contractor for a complete system? Were talking basica 14 seer heat pumps.. I was told that they pay about $1640 per ton. That includes the ducting and all.

    Again that seems rather low. I just need to understand if the builder is trying to minimize his loss.


    PS this is in the Atlanta GA area..

    1. JC72 | | #16

      Yikes, sounds like the horse has left the barn. IMO I would start from scratch and have contact Energy Vanguard in Decatur. Allison Bales, a GBA contributor, is the owner and he may be able to direct you towards a better installer and/or design better system.

    2. user-2310254 | | #17

      I've worked with two HVAC contractors who know how to complete a Manual J and install an efficient and comfortable system.

      - Specialized Heating & Air Conditioning
      - Southern Home Performance

      Allison Bailes has recommended Canton Heating and Air in one of his posts.

    3. Expert Member
      AKOS TOTH | | #18

      There is nothing wrong with over sized ductwork. You generally want this as it reduces noise and blower power.

      The only spot there is an energy penalty is for any ducts in any unconditioned space (attic/ vented crawlspace) where you would get a slightly more loss as the air flows slower.

      I would just get the proper sized unit installed to the existing ducting. Just make sure the system is commissioned properly and all the rooms are getting the required flow rate.

      It might also make sense to pay a bit more for a higher SEER unit, you are in a place where there is a fair bit of AC use.

    4. joshdurston | | #21

      Keep the ductwork and take the $8200, and do it right. Even if you're out of pocket a bit.
      You can always balance down oversized ducts.
      It's easy to get 1.5tons (600cfm) of air thru ductwork sized for 3tons (1200cfm). You might even be able to use compact ducted mini split instead of a full sized air handler because of the reduction in required static pressure. These are quite a bit cheaper for some reason.

  10. Jon_R | | #15

    If it costs more to do it right, then you have the question of how much performance difference is there when AC units are ~50% over-sized. Contrary to popular opinion - not much. Even more true with some modulating equipment (which might be more efficient and make everyone happy).

    1. Expert Member
      BILL WICHERS | | #19

      The bigger issue with oversized AC units is poor dehumidification performance. While I’ve never actually measured it, I agree that there is likely not much difference in energy consumption between a right-sized and an oversized AC unit when only considering space cooling performance.

      Modulating equipment does tend to be much more flexible, but you still want to try to get the sizing right so that you’re operating in the most optimal efficiency range.


      1. Jon_R | | #20

        Poor dehumidification is mostly false. See Fig 7 where SHR only moved from .73 to .78 when running at 1/5 load. Most mini-splits do much worse than this with decreasing load. Yes, a 50% over-sized single speed is likely to perform BETTER than a perfectly sized mini-split.

  11. walta100 | | #22

    My wild guess is 12-16K per system is not a bad for a quality install with steel duct work. For a sloppy flex duct install maybe you could get both for 16K. My guess is the builder wants to pay the sub for the work completed and give you the balance of $8200.

    When I built my builder gave me a spreadsheet with a line for every subs bid one line was for his pay. I was free to get another bid if I wanted and we could discuss what bid was better and why.


  12. Gwisejr | | #23


    I was thinking the same thing that although he told me that the $8200 is what he paid total for all the work, that in fact he is still paying the sub for the ducting and probably keeping back some more.

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