HVAC for a very small new and tight outbuilding
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 loadcalc.net 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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