Best HVAC option for a slab on grade home
Our 1,400 sqft home is being designed as a slab on grade. We had planned to have one mini split for the home with ceiling fans in each room. Our concern is this would not be adequate for the bedrooms. Our options after talking with the contractor seem to be…
1. Install the main living mini split but also install one per bedroom. This adds cost and the contractor isn’t fond of the idea as unlike a regular air system it’s not filtering the air throughout the home.
2. Have the trusses on the bedroom side of the home built as attic trusses creating a small room in the attic. The room would be finished and conditioned like the rest of the house and from that room could easily run ducts down to the two bedrooms and out the side to the connected vaulted ceiling in the living room making all spaces conditioned. Because we plan to use Intello for our vapor barrier, we’re not sure if adding attic trusses over only a portion of the home would complicate things and possibly lead to insulation and air barrier mistakes? The contractors in our area are not yet doing much in the way of building science so while they are willing to work with us, they are learning too so I don’t want to make things overly difficult.
3. Build the home with a conditioned mini basement rather than a slab to allow for a regular HVAC system to run the length of the house. The reason we chose a slab originally is because our area is very wet (not far from wetlands). Even for the slab, we are digging down as little as possible and then going up with the frost wall and bringing in fill around it. Neighbors all have basements but have said that their sump pumps run 24/7.
I have attached a picture of what the finished home will look similar to. These options we’re talking about would only be for the main living space side of the home. The smaller dogtrot/bonus space is approximately 24×24 and would have one mini split for ease.
Do you feel one of these options is better than another? Or maybe there’s something else we haven’t thought of? Like exposed ducts?
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Replies
I think your best bet is option 1 or something close to it.
On alot of the jobs I worked on at an architecture firm that was often teaching contractors high performance, the learning curve tends to add mistakes and significant time to the first big "high performance" job they do. If I were in your position I would not risk that project being your house. So any complication of the air barrier eliminates option 2 in my mind (although an *insulated and airsealed* attic under trusses seems like a great way to go here).
Option 3 seems like alot of work and money to put in more conditioned space only dedicated to mechanicals. Not only that, but you're opening the door to potential water issues. Keep the water out if you can.
Now option 1 sounds good, but maybe there's a way to eliminate putting one per room - perhaps soffiting down the ceiling of closets if the bedrooms and putting small ceiling mounted air handlers / cassettes for one split outdoor unit? If you choose to do wall-hung non-ducted mini split units, follow the plethora of advice on here about not facing them directly towards beds, etc.
Thank you! Is the not facing them towards beds because of air quality issues?
Yes, and one minisplit per room seems pretty overkill. They can usually condition abt 1,100sf each if I recall correctly. So 2-3 bedrooms on one minisplit should be fine, provided the bedrooms have similar orientation - if one bedroom is getting much more sun exposure, they will not get conditioned evenly.
Mind you I am not an HVAC expert. Hopefully you can get someone like Martin or Allison in here to verify.
Sounds like you're still in the planning stages. What about if you make the walls a foot taller and then drop the ceiling a foot to give room for mechanicals? Then put a ducted mini-split in the space. In a slab-on-grade house you're going to need space to run all sorts of stuff and without a dropped ceiling you're going to have a zillion holes in your air barrier because everyone is going to want to run through the attic.
Thank you, that’s a really good point about the air barrier.
Note that a single mini split technically fails to meet the definition of central heatings requirement to provide heat each room. Even without code enforcement the insurance and financing have been known to refuse to play along.
Have you looked at the “concealed ducted mini splits” that fit in a dropped ceiling?
Consider pouring a thin “rat slab” and have a 4-foot conditioned crawlspace for the mechanicals.
This get the living space up off the ground so when that hundred-year rain storm comes and drops a month’s worth of rain in one day the house stays dry and you replace what was in the crawlspace.
To my eye the photo looks like a pole barn. If the plan is the end up with an energy efficient home sharpen your pencil and do the math but I think you will find absolutely every thing that make a pole building cost less to build becomes an expensive problem when you try to make it be tight and well insulated in the end, it will cost more to start with the pole barn.
Walta
Thank you Walta, it is a standard home build, but we are using corrugated metal siding for the exterior over a coravent rainscreen. I can see how it looks similar to a pole barn building (this will be the main house on our farm so that’s kind of the look we’re hoping for).
We talked with the builder again today and it sounds like the rat slab/mini conditioned basement you suggested may be the best route. They would still build up and bring in fill around the exterior like they were going to for the slab, but a few positives is that we would not have to pay to bring in nearly 5’ of fill to build the interior up with quality fill/soil for the slab. It also allows storm shelter and visibility of leaks if a plumbing issues were to happen. Thank you everyone for your thoughts.
Another option: if your house is small and tight then the output of one mini split will meet your heating and cooling loads. A ductless head is relatively cheap and efficient. The challenge then becomes distribution—how to keep temperatures even in all spaces. A mechanical ventilation system (which you will probably need anyway) that serves all rooms will actually tend to eliminate temperature differences between rooms and allow a point source heating/cooling device to serve a whole house. This is pretty standard in passive house construction. Basically the erv makes a compartmentalized space behave as though it is one volume.
This is very interesting but I’ll admit I’m not fully following. Do you have links by chance to examples of what you’re referring to? By mechanical ventilation system that serves all rooms, are you talking about the HRV?
Yes, an erv or hrv. A typical ventilation plan might have each bedroom supplied say at 24 cfm, kitchen exhaust at 36, bathrooms exhaust at 24 and other rooms supply/exhaust as desired. This circulation will eliminate or greatly reduce temperature differences between rooms on a given floor; things are not so simple between floors because of the stack effect, so it’s usually necessary to have one mini split downstairs and another upstairs.
Zehnder sells excellent ERVs and HRVs. Check out their website. I’ve installld a number them. Btw, they are selling the last of their previous generation ca350 units for I think $1700. Very reliable, no bells and whistles. You might get all the parts for $5000 or so that way. Installation is not difficult if you read their manual and plan ahead for duct paths.
There are other manufacturers as well, and you can certainly shave cost but Zehnder’s system of components—registers, ducts, manifold, grills—makes it all super simple.
I think this approach (balanced zoned ventilation and a simple mini split for heating and cooling) checks the boxes for hvac in low load houses. The two together (Mitsubishi mini split and zehnder erv) probably come in at $25-30k in most markets?
I haven't read all the other replies so please excuse me if I repeat others.
First, are the bedrooms used much during the day? If they're mostly used at night then you may want to consider whether during the cooling season you need much additional cooling to those rooms.
Second, depending on the details of your plan, if you have a 9' plate height you can often have an 8' ceiling (or even 7'6" in a pinch) for spaces like bathrooms, hallways and utility rooms. This will often yield enough space to run ducts completely within the conditioned space. One or two strategically located access panels is a good idea.
I'm attaching a picture of a design where I did that. Most of the footprint has vaulted ceilings but over the 2 bathrooms, utility space and hallway I brought the ceiling down a foot. The trick to making it work is that from this dropped ceiling I can bring ducts directly to one wall of each bedroom and the living room. Return ducts come from the kitchen and baths.
We have completed several slab on grade projects recently and have successfully adopted a version of your second opinion. Attic trusses seem like a viable option on paper but in practice, they are one of most challenging designs to work with.
Our version of the attic truss design essentially eliminates the integrated floor/bottom chord that is typically part of the standard AT design. The head room will likely be reduced considerably as a collar tie is still required in the truss. We orient this arrangement over a hallway and use an access panel system that allows us to gain complete access for filter changes and even eventual equipment replacement. The benefit of creating this space over a hall is that access is never restricted and with an access panel system like Bauco II https://bauco.com/ aesthetics aren’t compromised either.
I have included some photos from recent projects including one where we designed this concept over a 1.5 story home with a hybrid scissor truss. I have also made several videos highlighting this strategy which are available on my Instagram channel if you’re interested.
https://www.instagram.com/sommerbrothers/profilecard/?igsh=MW9hMWQxbTNiNmdtNQ==