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What kind of mini-split, if any, would you recommend to heat, cool, and dehumidify an outdoor metal storage container?

ValerieA | Posted in Energy Efficiency and Durability on

We have an outdoor metal container (45″ long x 8″ wide x 8″ high) in which we would like to store mostly photo slides, and fine art painted on wood. We would like to heat, cool, and dehumidify it as affordably as possible to reach about 50% humidity and 60 to 85 degrees temperature in Beacon, New York where it gets below freezing in winter and above ninety in summer. The container will be hooked up to the electric grid so we will have electricity.
How much electric power might we need a year for this? Would a minisplit unit be a good option and if so what brand(s) and with what power capacity? Would you recommend other options? We are not builders or electricians so any advice you can provide will be most welcome.

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  1. GBA Editor
    Martin Holladay | | #1

    The first step is to perform a heat loss and cooling load calculation using Manual J software (or a pencil and paper method). The R-value of a steel container is R-0.

    You might want to build a quickly built wood-frame building (one that includes insulation) instead of trying to heat and cool a steel container with R-0 walls and roof.

    One other problem: It's hard to get uniform temperatures in a 45-foot long steel container with R-0 walls and roof. If you put a ductless minisplit at one end, the far end of the container will still freeze (because the steel container is uninsulated).

    Fine art painted on wood? Those objets d'art deserve an insulated building.

  2. Dana1 | | #2

    With interior & exterior air films a cargo can is really good for about R1. That isn't much, but infinitely better than R0. If you install a couple continuous inches of rigid foil faced polyiso on the interior (including on the floor & doors, with the floor foam protected by 3/4" plywood), with half-inch plywood protecting the walls and ceiling it'll be good for R10-R11 at your ~+10F 99% outside design temperature. (Newburgh's 99% temperature bin: ) Lets' call it a U-factor of 0.10 BTU per hour per square foot per degree-F

    That's about 1600 square feet of surface area, and with an interior design temp of 60F ,and an outdoor design temp of +10F it's a 50F temperature difference, and a conducted heat loss of

    1600' x U0.10 x 50F= 8000 BTU/hr

    If the doors can't be gasketed well it won't be perfectly air tight, and there is going to be some additional load related to the infiltration, but unless it's really leaky it probably won't be more than 2000 BTU/hr. Call it roughly 10,000 BTU/hr. While that's within the range of a cold climate 3/4 ton mini-split, to have margin for those rare days when it drops to negative double digits you'll probably want at least a 1-ton. Either a Fujitsu -12RLS3H or a Mitsubishi -FH12NA would have that much capacity, but as Martin correctly points out, distribution from end to end would be a problem (even with insulation.) Use a mini-ducted version to make sure the heat gets to both ends of the cargo can, but due to the lower cold temp capacity of what's available in ducted mini-splits you'll need to bump the sizing up a bit. The 1.5 ton Fujitsu -18RLFCD mini-ducted mini split would still cover the load at -4F and would still be putting out considerable heat (amount not specified in the engineering tables) at -10F.

    Without insulation it's hopeless, of course.

    For the reducing the summertime cooling loads it will be worth painting at least the top of the cargo can with a high solar reflective index white paint or mop-on coating. Find an appropriate paint or coating on , which has third party testing data (both new & 3 year aged performance) on hundreds of products. Anything with an aged SRI of 80 or greater would be good. With a solar reflective top the cooling loads will be well within the capacity of any mini-split sized for the heating load.

    Without a source of interior humidity you would probably need to do active humidification to keep it as high as 50% RH most of the year. Unlike an occupied building with ventilation, there won't be much humidity injected into the container during the summer, and the cooling system will always remove some amount of moisture while running, reducing the interior RH, and during the heating season any outdoor air leakage will be removing moisture. The dew point of 50%RH / 60F air is about 41F, and the average outdoor dew points in your area will be below that from roughly mid October through the end of April. The only way to keep up with the drying from incidental leakage and the drying effects of air conditioning will be to add humidity. That has to be done pretty carefully and with good distribution to keep it at 50% RH without some spots hitting mold-threshold levels.

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