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Community and Q&A

Oversize HVAC

D L | Posted in Energy Efficiency and Durability on

Hi all,

I am building a home to be as energy efficient as my budget allows, so it will be air tight with higher insulation etc. The home is a total of about 6000sf and is a ranch with basement in the midwest. I am now looking at my HVAC bids but noticed that some of the bids have over sized HVAC systems. The bids include a 5 ton ac 20seer unit, a 110k BTU 98% furnace, ERV fresh air system, and humidifier/dehumidifier.

I’ve read concerns around oversized systems with short cycling but would that really be an issue since the ac unit is 2 speed and the system includes an ERV which will dehumidify the home even if the AC unit doesn’t run long enough? If that is the case than comfort won’t be an issue with an oversize system correct?

I did get a bid from a green hvac vendor but the cost was 5k higher for a system that does not seem as high end (spec sheet) than the non-green bids. I do realize that the green hvac vendor is going to actually do a better manual j/p/d calculation and probably rightsize the system and ducts but the higher cost is harder to justify.

Also a question I have is, will the oversize system actually compensate better for abnormal usage patterns? We usually have a lot of parties in the summer so would a right size system be able to quickly cool down the home with the additional people as well as the doors opening and closing.

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  1. GBA Editor
    Martin Holladay | | #1

    There are several issues here.

    First, you wrote that you "noticed that some of the bids have oversized HVAC systems." You're probably right, but your comment raises the question, "How do you know that?" Have you commissioned and paid for an energy rater or energy consultant to perform an accurate heating load and cooling load calculation (using Manual J)?

    If you have, good for you.

    If you haven't, maybe what you are telling us is that some contractors estimated that your home has a higher heating or cooling load than other contractors.

    If these estimates differ, it's usually safe to assume that the contractors who estimate a high heating or cooling load are oversizing the equipment, and that the contractors who estimate a lower heating or cooling load are closer to the truth. So in theory you could use the lower estimates as your design guide. But this is a pretty rough way to design a heating and cooling system. Even the contractors with the lower estimates could be oversizing the equipment.

    So get a Manual J done by someone you trust -- and tell the consultant that you are planning to build a house that has a very low rate of air leakage and that will have above-code levels of insulation (if that's the case). Whoever does the Manual J calculation has to measure your windows and has to know your window specs and window orientations for each room, and has to know your actual insulation specs. You can't guess.

    Second: you're right that two-speed equipment is more forgiving of sizing errors than single-speed equipment.

    Third: you are wrong about one of your assumptions. You wrote that your "ERV ... will dehumidify the home even if the AC unit doesn't run long enough." In fact, when the weather is humid, running your ERV will add humidity to your home. An ERV is not a dehumidifier. The longer you run your ERV, the more humidity you will add to your home. The best you can say about an ERV is that it won't add humidity as fast as an HRV would. For more information on this issue, see HRV or ERV?

  2. D L | | #2

    Hi Martin,

    1. For some reason I thought I read somewhere that the rule of thumb for an energy star home was 1500-2000sf/ton but when I went back to look up the rule of thumb it was 1000sf/ton, I guess the 5 ton unit was not as oversized as I thought.

    3. I made a mistake in my posting, I meant to say the system includes an ERV and stand alone Dehumidifier that should dehumidify the home even if the AC unit doesn't run long enough. I am kind of new to all of this so I appreciate the hrv or erv link as it helped me understand how those two systems would add to the humidity when there is hot and humid weather.

    I guess what I am trying to understand is what I am losing by going with the non-green hvac vendors vs the green hvac vendor. From what I see the green hvac vendor is going to include the proper calculations and proper sizing of hvac and ducting system, does that typically translate to a higher cost? I could definitely use this money somewhere else in the home but this is the first energy efficient home I am building so I didn't want to make a mistake and regret it later.

  3. Nate G | | #3

    Do a quick-and-dirty calculation (15 mins) and find out for yourself:

    If you get a number similar to the numbers that the "non-green" fellows are offering, then that should help your decision-making. But if the non-green guys would put ductwork in the attic, that's a non-starter. Don't make that mistake.

  4. GBA Editor
    Martin Holladay | | #4

    When it comes to designing a heating and cooling system, I'm not sure what a "green" contractor or a "non-green" contractor is.

    The biggest mistake made by contractors is usually failure to perform an accurate room-by-room heating and cooling load calculation. The difference between a good contractor and a bad contractor in this regard has nothing to do with "greenness." What you want is a smart guy with a calculator and a computer and a sharp pencil. A geek is fine -- you don't need an environmentalist, just someone who is educated and accurate.

    The second most common mistake made by HVAC contractors is to design a bad duct system -- either by locating it in the wrong place (outside the building envelope), undersizing the ducts, failing to specify the use of mastic to seal duct seams, or failing to base the duct design on Manual D methods (which depend on accurate room-by-room calculations made by Manual J). It's possible that a "green" contractor (whatever that means) might be better than a non-green contractor in this department -- but again, you want an intelligent geek, not a romantic environmentalist, to perform the calculations and design.

    An old-school engineer who started out with a slide rule, and who has no interest in saving the planet, might be your best bet.

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