Help me plan cooling my new house/rooms with mini splits

6 years ago

I'm in the middle of building a house... framing almost done. I'm in northern california and would like to use mini splits for air conditioning. I have in floor radiant heat and also a wood fire place for heat so i have no need for heat. Was told mini splits are a good energy efficient way to add air conditioning as a big central air system is not in the plans/budget.

Heres the floor plan:

My thinking was one three zone on the right side (three bedrooms, each approx ± 14'x12')

Then maybe a dual zone for the great room and master? or a separate one for each?

Going with mini splits allows me to do it in stages (budget) but i need to run power now. I've been told its efficient for cooling but not the best for heating.

I just looked into some 3 zone ones and they seem to only come with 16' of hose. am i able to do longer runs?

where is a good supplier to look at units?

any recommendations as to go with ceiling vents (look better but another hole in the envelope) the big ass wall blowers, or something else?

here's a simplified version of my floor plan too:

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ahreno
Original Author

6 years ago

2700 sqft. it's real hot in summer but cools down at night to the mid 50s usually. we live in a small cabin currently with no a/c and it's fine. I don't see a real a/c as an affordable option to do right now so.im kind of stuck.

I have a concrete slab for my floor so hopefully that helps keep it.cool

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Comments (8)

PRO
Austin Air Companie
6 years ago
Each room that you want comfortable will need it's own cooling head, located within the room.

Because of this problem mini splits wind up being more expensive for larger floor plans with a multiple of rooms.

I don't think your climate is an overly hot climate, but comfort isn't a one size fits all.

The more equipment a location has, the higher the expense will be to maintain it.

As an example: it also comes down to use: how do you plan to use the structure. Looking at your floor plan the master bath / master closet / master toilet area present problems: air conditioning doesn't travel thru walls.

If you are sharing the master suite with another person, this can heat things up.

Because toilet area is tucked away in a room by itself in the master suite, if you don't cool this small room, you will most certainly regret it. Not to mention the closet area and the bath room of the master. Because the toilet room is an inner room of the house will likely present challenges to hanging the cooling head on the wall and again when the unit breaks down.

What is the total square footage of this home?
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Yes, they can be had in cooling-only. I think that the heat pumps are a little more expensive than cooling only, but not much. I can not comment on the relative efficiency. The heat pumps are mechanically more complex so they have more failure modes. I have three multi outdoor units for my seven room house. The reversal valve crapped out on one of mine just before the one-year warranty date. (Note to you AC guys, it stuck in heat mode and would reverse if I tapped on the valve with the butt end of a screwdriver.) I do not think that is a big consideration, but you might want to discuss that. What is the advantage of heat pumps to you? Maybe a lot and maybe none. Do you heat with oil, gas or propane? You can compare your heating costs with your current fuel and electricity rates and decide if you want to heat with heat pumps in relatively mild, but cool, weather. There is a cut-off temp where they fail to perform well when it gets colder. You can zone your heat when you are using them, adding more economy, heating only the rooms are sitting in to comfortable temp and leaving your main t-stat set lower. (This gets a little complicated if the t-stat is in a room that you are heating with the mini split.) Tip: when heating with high wall or ceiling units, you will probably want to turn up the blower manually higher than its programmed speed to keep the heat near the floor and not accumulate near the ceiling. On the other hand, your ceiling paddle fan will take care of this as well if you have them. One thing you might want to keep in mind. I can say that the Mitsu (and perhaps all brand) multi are less efficient than the 1:1 units and they do not do as well as the single units with heating at low temperatures. For the former, I think it stems from the fact that the compressors only throttle back to a certain percentage of max. For example, a single unit might be rated at 9000 BTU. If you only need a little cooling, and assume that they throttle back to 10%, you can efficiently get 900 btu out of them running constantly with no on/off. Your tri unit is probably 22,000 btu and can throttle back to 2200 btu with the same assumption so your compressor will be flicking on and off more often which yields poorer humidity control and not as even cooling. Please, keep in mind the relative nature of my comments. These things all work really well compared to window units and compared to central forced air. Be sure to consider all of your alternatives for the installation. In addition to wall-hung, you can have floor mounted, short ducts to hidden units and ceiling-mounted. That all costs more though. You have probably seen the pictures of installations with the plumbing straight through the walls to the outside of the house. Mine are run inside the walls to underneath the house. Again it might cost you a little more, but talk about it with your contractor. You should look at the MItsubishi, Sanyo, Toshiba and perhaps, other web sites to get some more information about their equipment alternatives. Your contractor should be willing to discuss all of this at length with you since you are making a big investment. I hope they can evaluate your cut off point for economical heating with the heat pumps vs. your hydronic heat. They should be able to estimate for cooling only vs heat pumps. The price difference might be trivial, however.
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@fsq4cw - how about a ballpark price just for the air-to-water heat pump? Just out of my own curiosity. Note that due to @mtvhike having a fairly large solar array and the specifics of his tariff scheme, I'm not sure that any significant capital cost to reduce his electricity usage will pay off well. This shows one of the oddities about specifics of solar tariffs. As I understand the way it works is this: his base tariff for electricity is 9.5 cents/kwh. Any solar he generates offsets the electricity he consumes (calculated annually?). BUT: any solar generated in excess of consumption is only paid at the wholesale rate, which is dramatically lower than the retail rate (call it 1 cent / kwh). Right now - without full year data - he expects to consume somewhat more than his panels generate. The upshot is that his savings from reduced consumption are limited (You could analyse this differently but I find it easier to just take what he already has as the base case, I think the conclusions are the same). Let's say, for example, he consumes 4,000 kwh annually more than he generates, for a bit less than $400 (netted out annually). If he saves more energy than that, every 'extra' kwh saved only gets him 1c / kwh. At least unless or until he adds some significant extra load, like buying an EV or putting an aluminium smelter in his garage or something. So: I'd say it makes sense to consider a minisplit primarily for the air conditioning comfort and if it also offsets some of his straight resistance heating, great, there's some modest savings there (within a 'limit' of however much his consumption currently exceeds his solar generation). Spending much additional for the 1c/kwh savings just won't make much sense financially. This is pretty simplistic as right now he has historical data for consumption and the solar array is relatively new so annual generation only estimated, and of course from year to year both may vary. And at any rate, a good argument to not rush into anything with high upfront expenses or to prioritise other projects for the time being.
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I don't have an answer for you. But we live in Denver. Had a swamp cooler for too many years and now have minisplits. Central AC wasn't an option for us as we have no ductwork. You have nicely put together the pros and cons of each solution. There is no one right answer. The only thing to do is to prioritize what is most important to you. If a cold bedroom is #1 then that says mini-split. If no wall unit and resale are more important to you then that says central AC. Truth be told whomever buys your house after you can easily switch to another solution if they are unhappy with your choice. I can say that we cold soak our bedroom with our minisplit and then turn it off at night on most days so as not to have any noise or blowing air. We have a ceiling cassette in the bedroom and the duct that would blow right on our heads is blocked off. We do have the wall units in three rooms. The family room/kitchen, the master bath and a second bedroom that is used as an office. Our main greatroom also has a ceiling cassette. Whichever you choose you will be sooooo happy to get rid of the swamp cooler. With Denver's up and down temperatures in the spring and fall it is much easier to just turn the minisplits on and off. Also they have a heat capability so provide a back up system to our infloor radiant heat should we need it. We've only used it that way once, when our boiler was being replaced. But it is nice to have a back up if something else were to go out on our heating system.
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mike_home
6 years ago
Is radiant floor heat a done deal? If not have you considered a forced hot air system with AC?

Radiant floor heating is very nice but very expensive. For probably less money you could install a top of the line heating and cooling system and be very comfortable the whole year. You then wouldn't have to deal with installing and maintaining mini splits, and experiencing uneven temperatures in the summer.
ahreno
Original Author
6 years ago
radiant is in already. not too expensive. I did it all myself.sp far. about $1200 in pipe and $2500 in water heater.
PRO
Austin Air Companie
6 years ago
2700 square feet is rather large for mini splits. It's not saying it can't be done, but you'll wind up being 'equipment poor' in short order because maintenance will be much higher with more equipment and when equipment starts breaking down it will be even worse. This could be as soon as within 10 years or sooner.

And you really don't have a choice with 'most' mini split configurations and the configuration of the home you show. You will most likely need a mini split cooling head in nearly every room and rooms that you don't put any cooling heads in will likely be considerably warmer.

If it were me I would put in a ducted forced air system with vents in every room and call it a day.
ionized_gw
6 years ago
If window units will keep you comfortable, then mini splits will as well, but with less noise and higher efficiency. Although window units are getting better all the time, mini splits will probably do better humidity control too.

Mini splits vs. ducted systems is more of a continuum than it used to be. There are mini split ducted units that can handle more than one room. The difference, I think, is that the static duct pressure is much lower so the design approach is totally different.

In a well insulated and constructed home, heat and humidity move between rooms a lot faster than through the exterior walls. That's why zoning does little good in economy measure in that kind of home. It will help you with any uneven temperature problems.

Complexity, as Austin pointed out, increases with the number of units so that is a negative.

Keep in mind that with more than one compressor, you throttle back to a lower output than with one large one running three. That keeps you out of the cycling mode longer and keeps humidity under control better if you are not running all three units indoors.

I've posted a lot about mini splits on this forum even some this week. Take a look.
marcaevans
6 years ago
I just finished building in Bay Area. I used Mitsubishi mini split for heating and cooling so I could offset with solar. Mitsubishi has multi-zone units that are nice (I used 4 out of 5 zones. House is just under 3k sq ft / 2 floors.

Mitsubishi also offers a more American style head unit that looks like a furnace and connects like a regular ducted forced air unit. I have two of those.

Don’t let anyone do it without a manual J calc. Every HVAC guys seems to say it’s not needed, just use 1 ton for every 500 sq ft. My manual J came out to need only 2 tons for 3000 sq ft!!! We ended up being conservative and putting in 3.5 ton Mitsubishi unit b/c variable speed anyway.
ionized_gw
6 years ago
I neglected to mention that the ducted mini splits and ceiling cassettes are lower efficiency than the high wall models.

Help me plan cooling my new house/rooms with mini splits

ahreno
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Help me plan cooling my new house/rooms with mini splits

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