One or two ducted minisplits
In the article, https://www.greenbuildingadvisor.com/article/just-two-minisplits-heat-and-cool-the-whole-house , martin advises using two minisplits for a two story house, the second unit just to keep the second floor cool. However, I want to hide a ducted minisplit under the stairs of a great room (with loft), with almost no ductwork. Given that the second unit is just to cool the upper floor, would it make more sense in this context to run a duct with an automated damper straight up through the wall to the second floor using the same ducted minisplit head? So when it’s heating it’s blowing under the stairs, and when cooling it’s blowing above the stairs. This would potentially save both money and space compared to the two minisplit solution. I was unable to find any information on such a configuration using minisplits and was wondering if anyone here had tried something similar. I realize the vertical run would be kind of long for a ducted minisplit (about 15 feet) and the damper would probably leak a little, but the run should be straight and the damper would just leak into conditioned space.
thanks!
Carson
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Carson,
I'll give this a bump. Have you completed a Manual J?
I did do a manual J with coolcalc, heating and cooling loads for the great room were calculated at around 20k so I'm planning on a 24k indoor unit.
Ducted unit is no different than any standard HVAC system. You can use a single unit for serving two stories but the ducting and distribution needs to be designed properly.
Most ducted units are lower pressure, you end up having to run larger ducts but not a big issue. Also you generally should only use flex for feeding rooms for sound isolation.
Generally for mixed climate 2 story setup you want your supply registers near the floor by your windows and return register both on the main floor and most important one up high near the ceiling of the 2nd floor.
It is also good to split the main and 2nd supplies into two trunks with dampers you can adjust seasonally. If you want to get fancy, you can go with some zoning but that can get complicated quickly.
If you can't get registers near the floor on the main living space, one compromise is to put in a strip of floor heat around the perimeter. This doesn't need to put out much heat, only roughly the losses of your windows, so most of your heat will still come from the ducted unit, but will keep the floor warmer and prevent cold air sitting there.
Thank you Akos,
I can easily supply the floor near the living space but getting out near the windows would require multiple ~20' runs, and I have about 30' of linear windows to cover, and the return near the ceiling would be nearly 24'. From what I've read on here, this is perhaps too much for one of these low static concealed duct units to handle? How would a ductless minisplit handle these sorts of issues for a great room, or would it not? I like the idea of using radiant strips around the perimeter, I'm already going to be putting in spot radiant heating for the bedrooms as we are in a heating dominated climate (zone 5b). This is making me think I may need to revisit just using hydronic radiant for heating and one minisplit on the second floor for cooling.
Long runs are not an issue, the problem is too many bends. As long as you don't go nuts on bends, a 20' run is doable even with a low static unit. Going with the mid static (ie Fujitsu AOU24RGLX outdoor with ARU24RGLX head), these do 0.8" pretty close to typical house air handler. You can put a lot of ducting on it without issues. The ducting still needs to be designed, don't skimp on this and pay somebody that knows what they are doing. Don't trust the HVAC installer.
The 24' ceiling height is not a problem (loss through straight ducts is small). You generally want the return near the celing of the 2nd floor. Without this return air will tend to stratify and you'll end up with warm bedrooms in the summer.
As for the window, getting a couple of supplies along the length and maybe one at either end of the main room should do the trick.
If you would have asked me 5 years ago, I would have said go with the hydronic floor heat +wallmount AC (my own home built this way). Now, I think there are better places to spend the approximately $20k the setup would cost.
Spot resistance heat is cheap and gets you the warm floor where you want them. Provided the main heat is handled by the heat pump, the additional energy cost for that would small. Even if a operating cost of gas fueled hydronic is cheaper, the install cost kills your savings, just not worth it anymore.
Great info, thank you! Labor here is very expensive so I’m just trying to educate myself on the options available. Electric radiant is something I can DIY so perhaps that will be the best route for the perimeter issue. Would a ductless minisplit really be able to warm the windows in a great room or would I have the same issue?
It isn't really about warming the windows but mixing the air near the windows. Because even the best windows make terrible walls, there is a fair bit of heat loss there and the cold air tends to flow down along the window and settle at floor level. If you have some mixing, than this shouldn't happen.
You can do this with a wall mount but you have to be careful with the location and height. Most do a decent job of directing airflow and should do a good job mixing. A single central wallmount in an open well insulated place does a decent job of heating about 1000sqft. The edges would still be colder though, so bit of floor heat there never hurts.
A reasonable in-between could be a hyperheat wall/floor mount in the main space and a smaller low static ducted unit upstairs, each on its own outdoor unit. This would reduce your ducting labor a bit.
One thing that might be worth it is to get some better windows there. For fixed windows, there is a relatively small up-charge for triple pane. Generally in colder climate these are worth it just for the additional comfort.
"One thing that might be worth it is to get some better windows there. For fixed windows, there is a relatively small up-charge for triple pane. Generally in colder climate these are worth it just for the additional comfort."
my window rep was pretty sure that double pane with a low-e coating would still be fairly comfortable to sit next to. Is that not correct? I think the upcharge for going to triple pane windows was around 5k for all of them. I could probably put a strip of electric radiant near the windows for about $600, so that's probably much cheaper.
I'm in Zone 5 and I have both dual pane low-e and triple pane low-e at home.
There is significant difference in comfort between the two, not saying that the dual panes are cold, just that the triple pane is much better especially on those clear cold nights.
Not sure if it is worth the $5k extra (or how that compares to the cost of the windows), but maybe selectively putting it at the lower section where you'll be standing/sitting would be worth it.
Also watch with larger windows and orientation, very easy to create a hothouse. I have some largish west facing triple panes with low SHG coating and it still doubles my AC consumption in the afternoons when the sun shines in. So make sure you have shading designed into the house.
"Also watch with larger windows and orientation, very easy to create a hothouse. I have some largish west facing triple panes with low SHG coating and it still doubles my AC consumption in the afternoons when the sun shines in. So make sure you have shading designed into the house."
That is a concern as I am going to have a large wall of windows facing northwest as a "view wall". It does have a 4' overhang though and summer temperatures here (zone 5b) typically aren't that bad. I'm hoping I can add external solar shades after construction if necessary. My window rep recommended against mixing window types because they may look off from the interior/exterior.
There is no free lunch here.
If you want large windows you'll have to pay for in energy use. A decent 0.2U factor window with 0.2SHGC, in my area (2F design temp), you are looking at a heat loss of 20btu/sqft so a 25'x30' triangular window you are around 7500btu. As long as you get about that much heat there, the window should feel comfortable. I would still be leaning towards triple pane, but not necessary.
In the summer time with 45 deg NW orientation you are looking at an afternoon peak of around 500BTU/sqft or 15 ton peak. A bit of interior thermal mass will delay the heat, but it will cook in there. 4' overhang doesn't provide much shading with tall windows. You need either trees, pergola, exterior shutters/slats or a commercial AC unit.
Probably cheaper to figure out how to limit the western exposure while maintaining the view. For example extending the roof and having a covered deck in that area can provide some real shading for the windows.
EDIT: Made a mistake with metric conversions. The heat gain should be around 100btu/sqft or 3 ton.
Of those options for cooling, I'm thinking external motorized shades on the bottom row of windows makes the most sense. A quick check on homedepot tells me it would cost about $400-700 for 2-3 shades. Heating the area along the windows at 7500 with radiant would be out of the question, that would take about 500sqft of radiant and a lot of energy. Hopefully the minisplit would help get enough heat over there though with some minor supplemental heat from radiant.
With modern coated windows, once the heat is inside, you own it. Interior shades do reduce the heat load but I think it is around 50%. Exterior shading is what really works, design it into the house.
Insulated shades do make windows more comfortable in the winter, you just have to watch your interior RH as you can end up with condensation on the interior surface.
Delivering heat to the windows with the mini split is the most efficient, thus my earlier recommendation of a couple of registers along the bottom there. That with a bit of spot floor heat should be a very comfortable setup.
Would a ductless minisplit with a sensor be able to direct hot air across the room instead of running ducts all over? That’s what the marketing seems to imply.
Regarding radiant and labor costs, and I know this is another oddball idea on here that shouldn’t be attempted, but I was thinking about it and kind of surprised I could find no mention on the internet of just using the air sealed joists as a giant duct to heat the floor as radiant and not even worry about running ducts into the house? Basically an air driven hypocaust.
There is some stuff out there for slabs:
https://www.ecohome.net/guides/3368/solar-air-heated-radiant-floor-kit-diy-problem-soils-expansive-clay-leed-passive-house-zne/
I'm pretty sure it is against code to just run inside the joist bays without ducting, to get decent coverage with ducting, it would be a lot of work.
yikes that system does look a lot more complicated than just running pex. Just to go along with my original thinking though, if one were already putting in rigid foam on the bottom of the joists (like as recommended here: https://www.greenbuildingadvisor.com/question/best-way-to-install-rigid-foam-insulation-below-floor-joists-from-crawl-space), then "in theory" you've already done most of the work, and minor leakage into the crawlspace you need to condition anyway may not be bad. Or maybe not.
>"That is a concern as I am going to have a large wall of windows facing northwest as a "view wall". "
The solar gains of NW facing windows isn't much in most zone 5 locations, but almost any single low-E double pane will still be an overall net heat gainer (even in winter). But it's a HUGE heat loss and comfort issue once the sun goes down in winter, and raises the PEAK heat load even if it slightly reduces overall heating energy use.
A "view wall" may look great in the glossy architectural magazine pictures, but it's a truly lousy way to build a house for comfort in a zone 5 climate.
That's a good point, but fortunately I'm less likely to be sitting next to the windows at night anyway. cellular shades may help a bit there too.
Carson,
I would ask an experienced installer to look at your project. I use a Daikin ducted unit to service my top floor (about 1,000 square feet). It replaced a old forced air system with a big fan but works fine.
Hi Steve, I appreciate that and I am trying to contact both local hvac and third party designers (all very busy it seems). Given the huge expense and desire to go electric, I’m trying to educate myself on at least a route to aim for. I would blame this site for that:)
At my Vermont house the view is to the North and I could not resist big windows there. But I got pleated shades with the side track that makes them almost air tight. As a result I don't lose nearly so much heat from the windows on long winter nights.
that sounds like a great idea! I just looked it up and a DOE page said it could reduce heat loss by over 40%.