HVAC system for a tiny house
I have posted a few times in the GBA Q&A and have gotten some excellent advice for my tiny house on wheels. I have read a few other tiny house posts by others on here, but there are not many.
Currently, I’m in the process of figuring out my HVAC setup for my tiny house. For reference, I will be in climate 3 hot/dry (southern California). My building envelope is vapor-open, with Roxul insulation. Intello plus will be my interior air barrier/ smart vapor retarder (maximum diffusion openness in summer).The goal is to be completely airtight, using high quality tapes and membranes.
I’m someone who is very allergic to mold spores, tree pollen, and sensitive to chemicals (I do not have MCS). With that being said, HVAC/proper ventilation is extremely important to me, as well as the longevity for this small house on wheels.
I’m breaking my HVAC setup into 4 components (feel free to chime in if I missed something)
I’m using the Lunos e2 ductless. https://foursevenfive.com/product-category/ventilation/lunos-e2-2/
-Range hood ~200 CFM
-Bathroom Fan ~100 CFM
3) Heating/Air Condition
-Planning on using a space (electric heater). Not sure about AC
4) Hepa Filter
-Winix 5300-2 https://www.amazon.com/Winix-5300-2-Purifier-PlasmaWave-Reducing/dp/B01D8DAYBA/ref=sr_1_1?ie=UTF8&qid=1494183528&sr=8-1&keywords=Winix+PlasmaWave+5300+True+HEPA+Air+Purifier
Some of the questions I have:
1) What air conditioning set up do you recommend? I have read about the many benefits of mini-splits, but also, they are very expensive and require professional installation. If I chose not to go mini-split, should I be worries about condensation build up/ air quality concerns with a wall or standing ac unit? Note, that AC will probably be running 6-7 months out of the year.
2) Considering I will be in warm climate, what do you recommend for heating? Being in a 28×8 space, I am lead to believe that an electric space heater will suffice.
3) I will 2×4 studs bathroom ceiling (loft above). I’m leaning towards exposed studs. Any concerns/issues with exposed studs? Planning on using a low VOC water repellent stain.
4) What would you recommend for a bathroom fan? I’ve read that ceiling fans are best but I also have read some pros of walls fans. With my bathroom setup, a wall fan would be more practical, but a bit concerned about it handling moisture as well as a ceiling fan. I’ve been told the Panasonic whisper line is good.
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In such a small house, you won't need much ventilation. Use your bathroom exhaust fan and range hood sparingly -- only when necessary.
A window-mounted (through-the-wall) air conditioner will be fine. Here is a link to an article with more information: Window-Mounted Air Conditioners Save Energy.
You wrote, "I will [use] 2x4 studs [for the] bathroom ceiling (loft above). I’m leaning towards exposed studs. Any concerns/issues with exposed studs?"
Studs are used to frame walls. I think the word you are looking for is "joists," although it is conceivable that you are talking about rafters.
Assuming that you are talking about joists, there is nothing wrong with leaving joists exposed, assuming that there is conditioned space above (your loft) and conditioned space below (your bathroom).
A half-ton point terminal heat pump (PTHP) that both heats & cools would be more useful than an AC-only unit. eg:
(There are others- the key is one using the heat pump for heating, not just resistance heating.)
They run ~USD$700 + another $100-200 for a through-wall sleeve & grille.
Martin - Sounds good. I do plan on having my windows open most of the time when showering as well. If I did choose to go with a ceiling fan, do you have any suggestions on how to make that work with a 2x4 exposed ceiling?
And my mistake. I knew they are called joists.. just too much going on in my brain at the time of the post.
Regarding a window/wall mounted ac, my concern is what you mentioned in the comments of your article: "window-mounted air conditioners are noisy, ugly, and wet." Wet, being the key term here. With the cooling being done inside the wall cavity, I have to think there could be some potential issues. Is there any specific type of unit I should look for if I choose a wall unit or perhaps any other alternatives other than a mini split?
Dana- Thanks for the suggestion. What's the benefit of this over a mini-split? Also, can you elaborate when you say "more useful" than an ac unit? How so? What if I have a separate AC and heat unit.
I really won't be needing much heat, so I want something very simple and cost effective.
Q. "If I did choose to go with a ceiling fan, do you have any suggestions on how to make that work with a 2x4 exposed ceiling?"
A. Are you talking about a ceiling fan or a bathroom exhaust fan? A ceiling fan is exposed (with visible blades). A bathroom exhaust fan has a grille. If you are talking about a bathroom exhaust fan, it would usually be installed between the ceiling joists. You'll need a duct to the exterior. You can enclose the duct and the fan housing with either drywall or boards.
Q. "Window-mounted air conditioners are noisy, ugly, and wet."
A. You are quoting Curt Kinder, not me. All air conditioners have to handle condensation. Install tubing to handle the condensation -- you'll have to do that no matter what type of air conditioner you install. Ugly is in the eye of the beholder -- if you want A.C., you'll need some type of equipment. And it is certainly true that air conditioners aren't silent. If you want silent, turn off the air conditioner and open the window.
Note that Kinder lives/works in Florida, a climate with outdoor dew points lingering in the 70s F for weeks on end, with infinitely higher risks of condensation in the wall than in dry SoCal where outdoor dew points averaging well south of 50F , rarely peaking north of 60F. Insulating the metal wall sleeve with a half-inch of rigid foil or plastic faced EPS or polyiso sealed in place with can-foam reduces YOUR risk (and probably Curt's) effectively to zero.
In your climate only rarely would any part of the sleeve reach the outdoor dew point even without insulation, whereas in Florida there will be plenty of times when that can happen. Refrigerant line sets of split systems are WELL below the outdoor dew points in FL, but are rarely insulated with more than 1/2" walled neoprene foam.
Sounds good. I'm not too worried about the noise of the AC, actually.
Dana, I'd like to avoid rigid foam completely, but that is great advice. Do you know if there are any options where I will not have to have the AC in the wall or window? How do you feel about portable/standalone air conditioners?
You don't need to insulate the sleeve with foam in your climate- the dew points are too low to accumulate enough moisture in a vapor-open construction type wall. (That suggestion is really for those who might need to worry about it, in far more humid climates than yours.)
In a tiny house floor space is precious. A portable AC unit would be able to cool the place but it's less than ideal: It eats up at least a couple square feet of floor space, can't heat the place the other 5-6 months of the year, is usually noisier than a PTHP, and it would need to be stored during the months it is not in use.
A half-ton PTHP sticks out into the room about as much as a slimmer-profile mini-split head, is far cheaper up front and easier to install than a mini-split. It's noisier than a mini-split and doesn't modulate, but now that they come with scroll or rotary compressors (verify that before you buy) they are typically quieter than most window AC units. Most newer hotels/motels use PTHPs for heating & cooling the rooms, and with the blower speeds on low they're pretty innocuous sound-wise, though still somewhat louder than a refrigerator. The Amana PTHPs are/were advertised to be the quietest in the industry, but it's a competitive market, with moving targets on that front.
PTHPs are designed as a real piece of HVAC equipment, to be installed in a somewhat permanent fashion, with a substantial service life and high reliability. I wish the same were true for window units (some are, some aren't), and portable AC units, most of which seem to be designed for lowest initial cost, and very short warranty periods. I'm not sure if there is a whole lot of difference between vendors, the Gree ETAC07HP230V20AA seems to have a slightly better warranty than the Amana PTH073G35AXXX or LG LP073HDUC, and there are several others out there in the same ~$700 price & efficiency point. Like any piece of HVAC equipment, having multiple local vendors & service companies to support it can be more important than a couple hundred USD in upfront cost.
Dana, so in regards to AC only, what makes PTHP superior to a wall ac unit? My budget for an AC unit is $300. Do you have any products to recommend in this range?