The Micro Showcase is located in Washington DC, which suffers from sweltering summers. We needed real air-conditioning (not evaporative coolers).
Compared to a far cheaper traditional travel trailer, the only real advantage of a micro house is a) better insulation, b) better build quality and c) aesthetics. So keeping air conditioners entirely out of the window is a high priority. Leaving these aside, along with a number of other inferior options*, and a standard disclaimer about the importance of maximizing building insulation and air sealing before considering air conditioning, there are 3 basic possibilities:
- ‘ductless’ mini-split a/c systems, with the evaporator (outside) and air handler (inside) connected separately through the wall.
- ‘portable’ a/c units, which sit on the floor inside and vent outside through a hole in the wall (or the window-which would disqualify them).
- ‘through the wall’ a/c units, which look like window units but are specifically designed to be mounted on a frame in a wall, and can protrude into the tiny house, or outside.
|Ductless Mini Split A/C Unit||Portable A/C Unit||Through the Wall A/C Unit|
The main considerations in deciding between these 3 types of units include:
- Btu rating. With adequate insulation and air sealing no tiny home under 200 ft2 should need anything larger than 7000 Btus (see BTU calculator). Note there are some 6000 Btu and 7000 Btu mini split units by Fujitsu and Mitsubishi, though they appear to be packaged only with multizone (room) applications, or run on 240V. In the end it seems that there are currently no mini-split systems under 9000 btus, so they will typically be a bit oversized. Portables and through-the-wall units are commonly available at 5000-9000 Btus.
- Cool+heat: some, but not all mini-split, portable and through-the-wall a/c systems come with heating. Not having to pay for or wire an additional heating system is a great benefit. While some will choose to heat solely with a propane marine stove, it is most convenient to have a backup electric heat option to keep things from freezing when away from the house.
- High efficiency. SEER is the measure of energy efficiency, and mini-split systems currently range from 13-26 (the higher the better), typically significantly higher than the portable or through-the-wall units. Portable units keep the compressor inside the house, and draw air from the interior, so they tend to be rather inefficient (which many reviews attest to). Through-the-wall units do not have this problem. Note that there are a few portable ‘dual hose’ models that draw outside air in, instead of inside air, though almost all of these are 9000 Btus or greater.
- Indoor footprint: Given the tiny house size constraints, selecting narrow profile units is a priority. For mini-splits, the only indoor part is the air handler, which typically range from 7-12” wide and mount on the wall. Portable and through-the-wall units are quite bulky, but could be build into cabinetry. For through-the wall and portable units, it is important to note that while taking up more floor space during use, they require no exterior components, and can be easily detached and stored away during temperate months, unlike the mini-splits.
- Voltage: Note the majority of mini-split systems, and many through-the-wall units run on 240V. Microhouse builders typically go with 120 volt systems, as they do not require special wiring- check with your electrician on options.
- Multi-speed. Many units will have a ‘low’ setting that runs around 300-400 watts (as opposed to the 1200+ watts on high). At this low setting, future off-grid solar becomes slightly more realistic.
- Service availability: there seem to be many relatively unknown makers out there, apart from the well known brands- Fujitsu, Mitsubishi, Panasonic, Fedder, Friedrichs.
- Cost: compared to our alternatives, mini-splits are significantly more expensive both in terms of base price and installation costs (which typically requires an HVAC technician to charge the a/c lines, and occasional recharges). It is also not clear how well the exposed a/c lines hold up over long distance travel. Portable units require a small venting hole out the side or bottom of the house, while through the wall-units need to be framed inside a wall. Creating an opening for these is a minimal cost- even less so if built into a new construction.
So the takeaway:
For mini-splits: 9000 Btus may be overkill for a very small space, but they are an option. Look for the highest SEER, 115/120 volt heat pump models (that both heat/cool). Klimaire makes a 24 SEER model (KSIO009-H124) that qualifies, and there are always new models coming out. If you can use 220V, Fujitsu makes some of the highest efficiency mini-splits on the market, including the 9000 Btu, SEER 27.2 (model 9RLS2). Also, Mitsubishi now has a SEER 30 model, the Hyper-Heating Inverter (H2i) MSZ-FH, and LG makes the SEER 27.5 LA090HYV. (Fujitsu on display at the Showcase; also see blog post on mini-splits)
For portables: While there are a wide range of models available online and elsewhere, though some are much narrower than others, and none of the smaller models (8000 Btu’s and less) include heat (one exception here). But the fundamental inefficiencies of these units call us to consider only the dual-hose models, of which there are very few models under 9000 Btus (one exception by the relatively unknown manufacturer SPT here). Generally not recommended.
For through-the-wall units: Similar to their window-unit cousins, there is a wide array of units designed to be build into a side wall (the key difference being that these units do not have venting on the sides or top/bottom). Of the many options, this 8000 Btu Freidrich heat/cool model (UE08D11) fits the bill, gets high marks for performance and reliability, and can be installed flush with the exterior wall with an ‘architectural grate’. After many months of pondering, currently the through-the-wall approach is the preferred option for the Minim House. (Through the wall a/c unit on display at the Showcase)
*There are a range of other options that were investigated and discarded:
- Specialized a/c units that go from 1000-9000 Btus for cooling computer servers and such. See Kooltronic. The drawback is that none of the systems are residentially designed.
- RV units. It is surprisingly hard to find units smaller than 9000 Btu, and have the drawback of unsightly roof mounting.
- Marine units (such as those found here), but like weddings, unfortunately anything maritime seems to increase prices by 50%.
- Climateright 7000 and 2500 Btu tent and small enclosure air conditioners, which could work, and heat and cool, but with the disadvantages of ungainly (uninsulated!) large vent tubes, and the disconcerting caveat that the lowest outside temperature can be 36 degrees for operating heating.