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HVAC for a very small new and tight outbuilding

grmp945 | Posted in Green Products and Materials on

Hello all,

I am a homeowner in climate zone 2 currently building a very small outbuilding in my backyard. It measures just 12×14. I intend to use it as a shed / workshop. I will work and keep tools in this building so I’d like to control temperature and humidity. Also, the building will mostly be unoccupied – but I’d still like to keep it dehumidified all the time.

I am attempting to build using modern building science principles and techniques both for function as well as to better understand how to properly build (or spec) a home. I am trying to build it tight and insulate it well.

However, building tight and insulating it well creates a problem since it is just 168 square feet : how do I condition and dehumidify the building – especially in an economical way?

Being in CZ2, I am not really concerned about heating, but rather cooling and dehumidification during the summer and shoulder seasons. I have completed a load calculation using and arrived at a cooling load of only about 1,300 btu (unoccupied) to 2,600 (1 person). Although this isn’t a proper manual J load calculation, it should be fairly accurate because the building design is so small and simple.

With a cooling load of only 1,300 btu, how the heck do I cool and dehumidify this building? The cheapest option is a ‘window rattler’ – but those start at a whopping 5,000 btu. I am concerned that this option will not provide enough dehumidification in the summer, and definitely not work well in the shoulder seasons.

I have found a variable speed mini-split that will go down to 2,800 btu (and up to 9,000 btu) – but that is still oversized. Of course, the variable speed mini-split is also *much* more expensive than a window unit. A less expensive mini-split goes down to about 4,000 btu cooling.

Any recommendations on what to do for such a small cooling load?

The other issue is ventilation. I think I need only about 15CFM while it is occupied, and even less when it is vacant. This means even the smallest bath fan at 50cfm is way oversized. I don’t think it is possible to use a variable speed control on bath fans.

Any recommendations on how to add a small amount of ventilation while I am in the building? Should I even bother ventilating while the building is vacant?

Thanks for reading! Any help is much appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    If you're willing to spring for a mini-split, LG's LSU/LAN090HSV4 will throttle back to about 1000 BTU/hr, as will several other of their models.

    In climate zone 2 ventilation while vacant would add rather than remove moisture from the building, but may still be useful if there are any indoor air pollution sources, and not a huge cooling load.

    The supply-only 10cfm Panasonic FV-01WS2 would deliver close to ASHRAE 62.2 levels for a building that size with one occupant (0.03cfm per square foot, plus 7.5 cfm per occupant), and exceeds Building Science Corp's alternative recommendation for efficient buildings (0.01 cfm per square foot plus 7.5 cfm per occupant.)

    There is a 20/40 cfm version too, which is useful if you're using anything with solvents or flame or other fume sources while in use, or if ventilated only when occupied:

    1. Deleted | | #2


      1. grmp945 | | #3

        Kevin - I would appreciate if you started your own thread so as to keep mine on topic. Thanks.

  2. aunsafe2015 | | #4

    Small erv option: I own one and am reasonably happy with it.

    As for mini splits with super low minimum capacity, beware that some don't dehumidify well at the lower end of their range. My Mitsubishi turns down to 1,700 btu, but in it's low stages the supply air may be as warm as 64F meaning the coil isn't really cold enough to remove any moisture. You might want a small 30 pint standalone dehumidifier to go along with a mini split or window shaker.

  3. Trevor_Lambert | | #5

    Based on my limited experience, I think 15cfm is far too low for even one occupant. And what if you have two people in there? In my 2500' house, with two adults and a small child, the low setting of 50cfm cannot keep with the CO2 buildup. With such a small space, your CO2 levels are going to go above 1000ppm rather quickly; less than an hour, I would guess. What you're working on in there also matters, as it could be contributing pollutants.

    1. Expert Member
      Dana Dorsett | | #6

      >"... 15cfm is far too low for even one occupant. "

      But this building will have zero occupants. Nobody is going to sleeping or spending 8 hours at a time in there. Running even 15 cfm while unoccupied is extreme overkill, and with only intermittent use it's not likely to become a CO2 build-up issue. Things like gasoline vapor or pesticides would be of greater concern, if those sorts of things are stored in the shed.

      The 20-40cfm supply-0nly Panasonic FV-04WS2 controlled by an occupancy/vacancy sensor switch is probably the right way to go. When just stepping in to grab some tools the ventilation won't turn on. When set to 40cfm would it will deliver more than 2 complete air exchanges per hour in a building that size, and will turn-off automatically after the pre-set time-out after vacancy is detected.

  4. grmp945 | | #7

    Thanks so much for the informative replies. I had not considered that a mini split - even one that throttles down to 1K BTU - may not dehumidify sufficiently. Still, the LG units mentioned seem like the best option so far.

    I was hoping there might be some product aside from a mini split that could provide a very small amount of cooling. It seems like overkill to get a 9K - 12K variable speed mini split to normally deliver a ~1K cooling load. The only other option I've found is thermoelectric coolers, but the pricing on those is insane and is really not intended for my application anyway. One other (nonsensical) option I've though of is to not use any wall insulation in order to increase the cooling load!

    Since summer is ending, I may first try a standalone dehumidifier as suggested and see what kind of temperature swings I get going into next spring and summer.

    Regarding ventilation: Dana is correct - this little building will normally have zero occupants. There will be times when I spend several hours working in there though, so the Panasonic supply-only units controlled by an occupancy sensor or timer seem like an ideal solution. One question: should I assume that a positive pressure of about 20cfm will find an equal amount of leakage outdoors for a small and tight building? It seems like a lot of leakage to me for such a small building.

    Thanks again everyone :)

  5. walta100 | | #8

    With so little load you may want to look at one of the portable AC/heater/dehumidifier unit. Be sure to only consider units with 2 air hoses going outdoors.


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