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Load Calculation Dissagreement

jdclassen | Posted in General Questions on

I was performing a customer interview before performing an energy audit. All of the customers complaints clearly pointed to an oversized A/C system: High Humidity, unit turning on and off frequently, higher than expected bills, and thermostat down near 70 degrees.
4-month old home in Climate Zone 4 (non-marine) 2370 sqft Ranch on a slab, 11′ tall R-13 walls with well-installed Zip sheathing 1.5″ air gap under the all-brick exterior. R-44 Attic insulation (Flex duct under the insulation except in a couple of small spots.
I ran this through my Energy Auditing software (SnuggPro) as usual, and it came back with roughly 27000 btu cooling load. The system in the home is 48000 btu. I did a comparative check using (I’d love anyone’s opinion on that as a side note) and it came back very close; 25000 btu, to the modeling from the audit.
This morning I received a nasty phone call from the HVAC company that installed the unit along with about 400 other units in the subdivision, many of which are having similar issues. They sent me a copy of their Manual-J using Wrightsoft, and showing that the cooling load is 46000 btu. There is a difference in the design temperatures I used compared to theirs; They used 100 degrees outside instead of 96. And they had R-19 wall insulation which is actually incorrect, it is R-13.

I don’t mind being wrong, but something seems off here and I can’t put my finger on it.

Any thoughts would be appreciated.

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  1. freyr_design | | #1

    If they gave you the full manual j you should be able to see btu per assembly item and go from there. You can also put in their assumptions of the building to your software to see if there is a calculation difference. All software adhering to Acca manual j should provide the same outputs with the same inputs. I dont know if your software is a manual j calculation but manual j is not simply a btu loss to environment calculation and has safety factors and interior loads calcs. Try to input the data into coolcalc and see if you get something wildly different then the hvac contractor manual j as coolcalc is acca approved.

  2. Expert Member
    Akos | | #2

    It is funny how their sizing magically came out to 600sqft/ton. At least they are no longer using the 500sfqt/ton.

    The problem with any of these tools is you can make the output anything you want by fudging number (typical ones are design temp, air leakage and window values).

    Ducts and air handler in the attic do add a big unknow as depending on how well that is installed it can bump up the cooling load about 20% to 30%.

    Overall, I would say your cooling load is probably closer to reality. A quick check of thermostat runtime should back up your numbers.

    1. Expert Member
      DCcontrarian | | #6

      We should also keep in mind that the answer is never five tons!

  3. paul_wiedefeld | | #3

    Look at the infiltration number! And easy to check run time, either via bill or via observation. If they have hourly utility data, even better.

  4. Expert Member
    Michael Maines | | #4

    Also check their window sizes and performance values.

  5. jdclassen | | #5

    Well, the situation certainly escalated. I visited several neighbors who were having similar issues with moisture. I brought in an HVAC engineer and confirmed the oversizing. It was brought before the city inspector and it is looking like an entire subdivision of 386 homes will now be reviewed by the city to see how many are similarly oversized.
    The HVAC contractor is not too pleased, but the homeowners certainly are.....

  6. Expert Member
    DCcontrarian | | #7

    You can look up your design temperature here:

    Using 96 instead of 100 would reduce the sensible load by about 20%.

    My guess is they put their thumb on the scale a few other places. Twenty percent here, twenty percent there and pretty soon you're at four tons instead of two and a half.

    If 400 houses are really affected they may be looking at a class-action lawsuit. That's millions of dollars to fix.

  7. walta100 | | #8

    My guess is if you check each and every data point enter in their model J you will find many errors.

    Consider having the home owner get and run a large dehumidifier and setting it at 60%.
    That will lower the humidity and increase the sensible heat load and you have plenty of capacity for that. This is not the most energy efficient or elegant solution but it should make the home comfortable at a reasonable cost.

    It is beginning to sound like a legal battel and you do not sound very confident about your load calculation if so, you may want to avoid making yourself the expert witness.


    1. Expert Member
      MALCOLM TAYLOR | | #9


      From The OP's post #5 above:
      "I brought in an HVAC engineer and confirmed the oversizing."

  8. Expert Member
    DCcontrarian | | #10

    I'm going to take a shot in the dark:

    Is the HVAC company owned by private equity? I've been reading a lot about, and have been seeing locally, that private equity firms are buying up family-owned HVAC firms. And they're "optimizing" them. Which means pumping up the marketing budget, and taking all of the goodwill that has been built up over decades by providing fair and competent work and "monetizing" it, which means doing shoddy work and cheating their customers. It also means treating the employees badly.

    I won't do business any more with an HVAC company that isn't owner-operated.

  9. piperspace | | #11

    I would love to know what remedy was adopted in this case? What can be done if the cooling capacity is grossly oversized?

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