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Ductless sizing in the boondocks

Bill Finch | Posted in General Questions on

We’re converting a garage in 3A, and trying to properly size a ductless system. HVAC contractors in the area (there aren’t many) are fairly cavalier about sizing (Manual what?).

We estimated a 9,000 BTU would suffice …they’re certain that   1 ton or 1.5 ton Carrier would be necessary. Best we can do is rule-of-thumb-it at this stage. Help me if you can.

Nightime cooling and humidity control is the biggest concern. We’ll have 450 square feet of conditioned space for offices and a small bath. Insulation is to code, sealing is decent. There’s a large window on the southeast wall with big overhang and trees that prevent any real insolation durng the cooling season. There’s a patio door with high performance low-e glass on the northwest side that gets some sun very late in the day in June. No other openings. The floor is slab on grade.

The roof is metal and fairly reflective, an SRI above 50…this can’t possibly require 1.5 tons! But is .75 tons likely to be adequate? And does it matter if I go with the base 19 SEER or will the 25 SEER offer better performance?

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  1. Ethan Foley | | #1

    You could use an online load calculator that would get you fairly close if you take your time and do it right. It's not complicated but it does take time. Rules of thumb are pretty much useless. Try this one
    If humidity control is your concern, you'll definitely want it sized as closely to the load as possible so that it runs longer, giving it more time to remove humidity.

  2. Expert Member
    Dana Dorsett | | #2

    deleted double-post

  3. Expert Member
    Dana Dorsett | | #3

    I've seen paradoxical results with Coolcalc, but is pretty consistent.

    If you're going to rule-of-thumb it, for a code-min insulated house with code-min low-E windows a small house 1.5 tons per 1000 square feet is usually about right.

    For rule of thumb ballparking a small office start at a ton per 1ooo' then add up all the wattage of the equipment (computers, copiers, & lights, etc) that would be on during the day and multiply by 3.412BTU/hr per watt, and 400 BTU/hr per human. eg: Say there are 4 office workers, each with a 150 watt computer, and 200 watts of LED lighting for the whole place.

    For 450' at a ton /1000 comes in at 12,000 x 450/1000= 5400 BTU/hr

    4 people @ 400BTU/hr each adds another 1600 BTU/hr.

    Four 150 watt computers and 200 watts of lights is 800 watts, x 3.412= 2730 BTU/hr

    Add it up and you're at 10,030 BTU/hr

    If using an freebie online calculator, run the load numbers assuming zero occupants, then add in the body heat & plug loads as-above manually.

    When looking at the specs for ductless pull the AHRI submittal sheets and look at the maximum cooling, not the "rated" cooling, which is usually lower than the max. A 9000 BTU/hr rated Mitsubishi -FH09NA will deliver 12,000 BTU/hr of cooling, as does the 3/4 ton Fujitsu -9RLS3. The 6000 BTU/hr rated FH06NA is good for 9000 BTU/hr of cooling.

  4. Bill Finch | | #4

    Thanks, Dana, immensely, for cooking it up and boiling it down so we could make sense of it. The 9,000 btu Carrier model they would furnish is rated for 11,000 max cooling; the 12,000 is 13,000 max. I think the 9,000 btu will make us happy, but I'll follow up with the loadcalc and the coolcalc just to see what happens.

  5. Expert Member
    Dana Dorsett | | #5

    FWIW: I hate using rules of thumb, even though some rules of thumb are a lot more realistic than others. With minimal window area and a high SRI roof the rule of thumb I threw out there is likely to overshoot. If the freebie load calculators come in lower, use their numbers.

    Allison Bailes rants against the use rules of thumb regularly (here and elsewhere), but he has a Wrightsoft license and knows how to use it, and you don't. :-)

  6. Bill Finch | | #6

    THe loadcalc had some ambiguous questions, but seemed far more rational than the coolcalc, which did not seem to take into account factors such as window shading and operated with only the vaguest sense of what glass area actually was. MY loadcalc calculations came within a few hundred btu's of Dana's rule of thumb. THe cool calc recommendation was 50% higher than the loadcalc. With either program, it's hard to make a case for anything bigger than 9,000 btu.

  7. Bill Finch | | #7

    Yes, re: rules of thumb. I haven't been able to entice Mr. Bailes to drive by on his way to nowhere. If only consulting engineers grew on trees...but most of us really don't have a practical way to get an assessment done other than by the friendly local HVAC guys...who really are friendly until you ask too many questions (like, which model are you selling me? what's the SEER?). In this case, the company simply doesn't offer (or refused to consider) anything less than 9000 btu, and we're no better at installing it ourselves than we are at using Wrightsoft. So thumbs is what we got, and they may have saved us from buying a unit 3 times larger than appropriate (even if it may be 25 to 50% oversized).

    1. GBA Editor
      Martin Holladay | | #8

      If you haven't read it yet, you might want to check out this article: "Who Can Perform My Load Calculations?"

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