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  1. #1
    Join Date
    Jul 2012
    Posts
    9

    Calculating HVAC square footage

    When calculating the square footage of a home, do you consider only the net square footage or is it the gross square footage?

    In other words, if the footprint is say. . . 25' by 80', that would be 2,000 gross square feet.

    If you subtract out all the wall cavities, all the cabinets, and all the closets, which total say. . . 150 square feet, then you have a total of 1,850 net square feet.

    Which square footage do you use?

  2. #2
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
    Posts
    3,011
    we don't calculate square footage. we calculate net heat load surface area.
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  3. #3
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,241
    Load Estimate based on net square footage would be about 3% less than Gross SF for 4" thick walls for a portion of the total load.

    The window area is much more signifcant as a total load contributor due to U value in the range of 0.4 to 1 ( R = 2.5 to 1). Wall R value is ~ 11 to 20.
    NET load area for walls = Total Area - ( Window + Door Areas)
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  4. #4
    Join Date
    Jul 2012
    Posts
    9
    Okay, I understand very well that square footage is not the only consideration in determining the load. However, square footage is one of the factors to consider.

    My question is which square footage number, net or gross, is usually referred to in traditional calculations?

    Every hvac person I have ever spoken to over the years (maybe 9 or 10) has asked me "What is the square footage". My simple question is whether they are usually asking for gross or net square footage.

  5. #5
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
    1,375
    Gross. And they likely sized the equipment incorrectly this way.
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  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,727
    Guess you would have to ask them which sq ft they are asking for.

    A room by room load calc is done by the size of the room.
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  7. #7
    Join Date
    Jul 2012
    Posts
    9
    Okay, so that sounds to me as if you fellows are saying that the net square footage is the more correct starting number to use.

  8. #8
    Join Date
    Apr 2012
    Location
    winnipeg
    Posts
    1,330
    indeed
    it was working.... played with it.... now its broke.... whats the going hourly rate for HVAC repair

  9. #9
    Join Date
    Jul 2012
    Posts
    9
    The HVAC system in my house runs (blowing air) nonstop for at least 12-15 hours straight without a break per day from about the second or third week in June, thru July & August; and, from the beginning of December thru the end of February. The rest of the times on those days it cycles, but during the times I mentioned, it does nothing but run nonstop. And the worst thing about it is that even with all that runtime, the house is always cold in the winter, and always hot in the worst of the summer. I think running 12-15 hours without cycling is miserable. I hate having it blowing on me all the time, and even in the summer it is very dry. At night in the worst of the summer, even with the air running, I wake up in the middle of the night and wet my body down with water before I go and lay under a ceiling fan.

    The house has two 3-ton electric heat pump systems. . . one for upstairs and one for down. In the summer the downstairs is bearable but the upstairs is always hot. In the winter, both the up and downstairs are uncomfortably freezing cold, and the heat strips are running 100% of the 12-hour stretches I mentioned above. Over the last ten years that I've lived here, my electric bill in a house with only two people living in it is an average of $400-$500 per month, and we are ALWAYS either too hot in the summer, or too cold in the winter. It's not only miserable, but it's also embarassing to ever have people over and watch them either shiver or sweat & pant.

    Every HVAC person I ever have come here tells me something different. One says that my 3-ton systems are perfectly fine. He did a temperature-drop measurement of the air entering the air handler vs the air exiting the air handler, and swears all is well, and to just suck it up as far as being miserable is concerned. Another two companies told me that my systems are way undersized, and that I need two new 4-ton units. Another two companies told me that 4-ton units are oversized and that I need two 3.5-ton units. Everything I read on here says that the load calc is VERY precise and must be done by a professional. When I confront the HVAC people with the many differing sizing recommendations from all of them, they tend to just say something like "Well, you could go with 3-ton, or 3.5-ton, or 4-ton. . . It's really up to you".

  10. #10
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,289
    If all that one person did was check the temperature drop across the cooling coil, and nothing else, he was far from gathering a complete enough data set to offer an informed opinion of your system's performance. In my early days in this trade I did stuff like that because I did not know better, but at some point my knowledge base had to grow up.

    The ideal model for someone to come to your home and do nothing but evaluate your HVAC system's performance is just not there for many contractors. And when it's hot or freezing out, where's the blame? They're overworked and to spend time thoroughly evaluating your system's performance and then deliver detailed advice how to proceed toward a remedy is difficult when the work load is high.

    Does that excuse what your guy did, provided all he did was just measure temperature difference and nothing else? The "nothing else" is what I'd want to look at; a few other measurements that don't take long to obtain could reveal more info about the system's health than just a temperature split alone can.

    It is possible your systems are operating as they should but the house they are coupled to has severe heat gain and loss problems. Air leakage, both from ducts and the house enclosure itself, can account for a lot of penalty in not getting full delivery of the system's heating or cooling capacity to each room in the house. If the HVAC has not been properly serviced on a regular basis, it could be fouled with dirt, which also will reduce capacity. If you like every window where sun strikes it to never have curtains or blinds, that's another thing. The windows could be leaky and single pane...lots of maybes and ifs which perhaps may want you to consider the services of an energy auditor.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


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  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,241
    Posts # 9 sounds like you have GROSSly underperforming systems which get you a NET result of bank account drain.

    I would start with a blower door test and duct blaster test because it 'sounds like' you're heating and cooling the Outdoors. (unless you have a > 6,000 sq foot house with 1,400 square feet of glass)

    Call your local utility initially then an energy rater to get INDEPENDENT EVALUATIONS which give you a baseline to develop a foundation for a proper solution.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,372
    You need an energy audit. Sounds like thermal envelope and or duct leakage

  13. #13
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,397

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