Ductless minisplits have a lot going for them. These high-performance air-source heat pumps operate efficiently at much lower outdoor temperatures than standard heat pumps, and they don’t suffer the same energy losses due to leaky ducts. A tight, well-insulated house may need only one or two wall-mounted heads to maintain comfort indoor conditions, in summer and winter.
It’s the “wall-mounted” part, however, that not everyone warms up to.
That is the case with Jerry Liebler’s wife — or “boss,” as Jerry introduces her in a recent Q&A post at Green Building Advisor.
Liebler is convinced a Mitsubishi Hyper Heating system would meet his heating and cooling needs. But the “boss dislikes the looks of minisplit indoor units.”
Liebler’s proposed solution is to place the head in a closet along with a small air handler and an outlet duct through the floor.
“A ‘shelf’ would run horizontally around the minisplit and the outlet duct of the air handler,” he writes. “With the closet door closed there would, in effect, be a ‘plenum’ above the shelf, pressurized by the air handler.”
Liebler thinks the air handler’s motor would overcome the friction losses of the duct work. Ducts through the closet floor would be connected to conventional ducts to distribute heated or cooled air.
“Has anyone done something similar?” he asks. “See any problems?”
That’s the topic for this Q&A Spotlight.
Have you thought of a ducted minisplit?
“You don’t have to invent (and cobble together) your own ducted minisplit unit,” Holladay writes.
Yes, Liebler says, but none of them is capable of heating and cooling more than a couple of rooms, and “none offers the cold weather heating of the [Mitsubishi] Hyper Heat units.” (Mitsubishi says those units will operate at 13 degrees below zero.)
“That’s right,” adds Dana Dorsett. “The very low temperature units are only compatible with certain heads, none of which are mini-duct cassettes.
“I’ve never seen anybody hacking a standard head with with a separator, but I’d think that a minisplit would have a hard time managing the coil temperature with the outside air flow influences imparted by an air handler driving air through the coil,” Dorsett adds. “The control algorithms for the minisplit’s control are optimized for its own blower pulling and delivering air from a low-impedance equal-pressure air path.”
Using the basement as a return plenum
Jin Kazama suggests that Liebler install the minisplit as intended in a main room of the house and then design a “simple recirculating air system with a simple fan or multiple fans that mix all of the building air together.”
In Liebler’s view, his proposal is about as simple as it could get — and his wife won’t have to look at the minisplit head. “With a duct branch to each room and each room having a return grille into the basement, in effect the air handler will move the house air past the minisplit so the minisplit’s temperature sensors etc. will see the average conditions of the whole house,” he says.
“What I’m proposing is just what you suggested: a simple ducted air recirculation system, to which I’ve added the ability to hide the minisplit by closing a closet door. I think this system will work much better than any attempt to use ventilation air to equalize temperatures.”
Sorry, it just won’t fly
Keith Gustafson remains unconvinced. “I do not think your solution will work well,” he says. “The minisplit relies on throwing the air and has a controller that tries to make assumptions about what is going on in the room.
“I suggest you build the inside unit into a false soffit or a floor-to-ceiling bookcase to hide its looks. Room to room, install a ventilation system in the closet your propose,” Gustafson says.
Kazama is sticking with his suggestion to buy a ducted minisplit, even if the efficiency is lower than the ductless configuration Liebler wants to use.
As for Liebler? “I’d be a rich man if I had a single dollar for every time I’ve been told something won’t work and proceeded anyway, almost always getting the exact results I expected,” he replies. “It will work &* well… The performance penalty, especially when it’s really cold, of the ducted systems is simply unacceptable.”
Our expert’s opinion
We asked GBA technical director Peter Yost to weigh in. Here’s what he had to say:
Nothing like going to the source: I spoke with David Hazel, Regional Manager–Channel Development with Mitsubishi Electric. “No way,” he said unequivocally when I described Jerry Liebler’s proposed hidden and ducted installation. “Any type of restricted, ducted, or pressurized installation such as this will void the warranty.
“Show me the detail,” he continued, “but I will tell you right now, you need at least 3 in. of clearance above the indoor unit so that there is no restriction of air flow into the top of the unit and there is plenty of room to access the unit from the top.”
I mentioned this issue of the minisplit’s “unsightly” indoor unit heads to GBA Advisor and Passive House architect Steve Baczek. “I have had plenty of clients object to the heads and propose this: Design a flush-mount shelf with the appropriate clearances (see attached detail). Stick this indoor head pocket in all sorts of neat places: the backside of an adjacent closet, kitchen soffit, above a pantry door, stairwell…”
Steve used the Mitsubishi spec sheet to get the dimensions right for the closet shelf. This seems like the best of all possible worlds.
I doubled back with David Hazel of Mitsubishi on Steve’s proposed detail and he had these comments:
- Mitsubishi recommends that this type of “pocket” installation include the unit projecting 2 to 3 inches out from the face of the wall, just to be sure that when the indoor unit is in heating mode (when the louvers are angled down for delivery) all of the conditioned air is delivered unobstructed into the room. This is not an issue during cooling since the louvers are set either horizontal or even slight tilted up for cooling.
- Just how much the unit actually needs to project will be based on the model; different indoor units have slightly different louvers and that can affect the needed projection.
- Be sure that the pocket is indeed isolated (like a microwave oven shelf) so that the indoor unit is truly and entirely in the space being conditioned.