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Community and Q&A

HVAC conundrum

WBowling | Posted in Mechanicals on

Hi all,

I’m at the point of choosing an HVAC system for my house, and the process is easily the most frustrating experience of the building process thus far. The house is new construction in climate zone 4A, I have taken care to air seal well (OSB seams taped), I will have a closed crawlspace with R10 insulation, R21 dense pack fiberglass in the walls, and R40 loose cellulose on raised heel trusses. I have double pane, low e windows, with minimal exposed glass on the west and east sides; I do have some glass on the south, but I included a brow roof that should shade the south glass through the hottest part of the summer. The house is a true two stories with a relatively open floor plan, and there are minimal penetrations into the attic space. The actual floor space (i.e., not counting the room taken up by the exterior walls) is 1,782 square feet (891 square feet/floor).

My problem deals with sizing the units. I ran load calcs through Manual J software, and the numbers I’m getting just don’t jive with what I’m being quoted. My calculations suggest a bit under 2 tons capacity, but all my installers are recommending 3 to 4 tons. I’ve talked to three different folks now, and they’re all coming in with substantially higher recommendations than my calculations.

I have two related questions:

1) Does my load calc sound reasonable, or am I way off? I’m obviously not an HVAC professional, but I do know how to plug numbers into computer software. I realize you can’t provide me with an actual answer, I’m just hoping to be told whether my numbers sound completely off base given the experience of the community here at GBA.

2) I have been primarily considering minisplits, but I’m wondering if the community may have a better suggestion for my situation. I’m in a part of the world with relatively warm summers (plenty of days in the low-mid 90s with high humidity) and somewhat cool winters (normal temps in the low-mid 20s, with occasional drops below zero). I plan on using a wood stove for the bulk of my heating needs, but I am sometimes gone for up to 2 weeks during the winter (and thus need to provide a stable heat source when no one is at home to feed the fire). I’m installing efficient ceiling fans and casement windows, and plan on using these for cooling as much as possible, though our area’s humidity can sometimes hamper these efforts. Does anyone have any ideas as to a different system (i.e. not a heat pump) that will provide good efficiency under these conditions?

Thanks for any ideas,

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  1. GBA Editor
    Martin Holladay | | #1

    Your situation is not only common -- it's typical. Most HVAC contractors in the U.S. are unable to perform accurate load calculations, and routinely recommend equipment that is sized to handle 200% to 300% of the calculated load. It's a national scandal, and it's incredibly frustrating for educated homeowners who have to engage in conversations with ignorant HVAC contractors.

    If you read the articles posted on GBA, you'll find many stories by homeowners who are happy with the comfort provided by ductless minisplits or ducted minisplits. You might also be happy with the smallest possible furnace or conventional air-source heat pump, as long as you choose a modulating unit and design the ductwork well.

  2. brp_nh | | #2

    When overseeing our house build (zone 6, two stories, 1500 sq ft gross), our HERS rater (as part of Energy Star program) calculated our heating design load at 11.2k btu/hr. We used that number and decided on one mini-split for our first floor and it's worked out fine. Our installer was skeptical this would work and we had to sign the invoice stating the equipment sizing was our choice. I'm sure that if we had asked installers to decide on the equipment, they might have suggested 2-3+ mini splits.

    My point being that if you're confident in your calculations, I would go with that. If you're not confident, maybe hire an independent pro to do a Manual J (the money spent on this would probably be recouped in equipment savings). With an open floor plan and if you leave doors open, a ductless mini-split on each floor might be fine.

  3. WBowling | | #3

    Thank you all so much for the help. I've been really impressed with the willingness of every other trade to work with us in our quest for a pretty good house, but the HVAC folks have been frustrating.

    Thanks again for your help!

  4. user-6184358 | | #4

    You can try calling a title 24 Energy Calc company in California and see if they can run one of for you in your part of the country. It costs $200 to $250. That can backup you conclusions. I had to have it done to replace windows in my 1985 tract house.

  5. Expert Member
    Dana Dorsett | | #5

    A tight ~1800' code-min house would typically come in under 2 tons for cooling, and even 1.5 tons of better-class ductless mini-split would be able to manage the heating loads (but not necessarily the distribution) of a house like that, assuming a ~15-20F outside design temperature. A highly efficient shape such as a cube with a gable could even be within range of cold-climate1-ton (~16000 BTU/hr @ +17F) if the ventilation rates are low (or heat recovery ventilation).

    R40 in the attic is lower than the current IRC 2015 spec, but on a 2 story house the difference in heat load between an R40 attic and a code-min R49 attic is pretty small.

    I've given up on letting the HVAC contractors specify the equipment. Figure out what you need, tell them what you want, and that it's going to be put out to competitive bid. You can listen to what they have to say, but you don't have to take their advice. You're not crazy- the cooling load IS less than 2 tons, and the heat load would more than likely take less compressor tonnage than the peak cooling load.

    In terms of non-heat pump solutions that are reasonably efficient, what are your fuel options?

    How are you heating your domestic hot water? A condensing propane fired water heater puts out more heat than the house can use, and could be set up as a combi-system with baseboard heating as a backup for the wood-stove, though that's usually 2-3x as expensive as heating with mini-splits. If you're on the gas grid a condensing gas fired water heater might even come in cheaper than mini-splits (but won't air-condition.)

  6. bobhol | | #6

    I built a PGH 2 years ago 2000 sq feet in total in Zone6 7 Ontario Canada...1100 down and 900 up ...I put a mini split on both floors and used them both the first year with no problems ...this year I am only using the upper unit if the temperature falls below -10C and have had no issues with one unit keeping the house at 21C ...The first winter the 2 units cycled on and off a lot but using one unit only until it gets colder seems much better...regards Bob

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