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Q&A Spotlight

Putting the Duct Back in Ductless

A would-be HVAC designer wonders if a ductless minisplit head can be hidden in a closet and connected to conventional ductwork

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A ductless minisplit head isn't everyone's cup of tea, at least not aesthetically. One reader wonders if he can still get the benefits of a ductless system if he hides the head and makes his own ducts.
Image Credit: Fujitsu
A ductless minisplit head isn't everyone's cup of tea, at least not aesthetically. One reader wonders if he can still get the benefits of a ductless system if he hides the head and makes his own ducts.
Image Credit: Fujitsu
Bob Alsop, a GBA reader, wanted to disguise the appearance of a ductless minisplit unit installed above the kitchen sink. He came up with the solution shown in the photo. Alsop reports, "We had our cabinet guy hide it in a cabinet with an upswing louvered door that hides it well, and can be opened when you need full volume. It doesn't seem to block the air flow."
Image Credit: Bob Alsop
Working with Mitsubishi specs, GBA Advisor Steve Baczek suggests this as a way to hiding a ductless minisplit head.
Image Credit: Steve Baczek

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?

As GBA senior editor Martin Holladay points out, many manufacturers (including LG, Fujitsu, and Mitsubishi) make ducted minisplit systems.

“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.

3 Comments

  1. Hobbit _ | | #1

    options
    Back when I was heat-pump shopping there was a *rumor* that
    Mitsubishi was going to have a central ducted vertical air-handler
    to mate up with their minisplit-oriented outdoor units. They
    don't seem to have gotten this quite together yet for the
    purely residential market but there *are* some ducted indoor
    units in the "city multi" line of stuff, in the PVFY series
    from about 1 ton and up, but your typical residential HVAC installer
    would probably look at you like you had two heads if you brought
    up the subject.

    Daikin, on the other hand, had the existing "inverter ducted"
    offering and despite being a rebranded Goodman indoor fancoil,
    appears to be a much more well-developed and integrated system
    aimed at the residential furnace-replacement market. That's
    exactly why I wound up with the system I have now.

    Mitsu and the others *really* need to get off their butts and
    compete for real in this market, because the easier it becomes to
    change out a forced-air furnace the more people will want to do it.

    _H*

  2. Aj Builder, Upstate NY Zone 6a | | #2

    more Daikin/Goodman info please
    Hobbit, do tell more about your exact install including some model numbers....

    Very interested....

  3. Dale Comeau | | #3

    Hire a Fujitsu Professional
    We are able, with the Fujitsu product, to address this concern in two ways that allow you to have an ultra high efficiency heat pump rated at 11.3 to 12.2 HSPF, depending on size you choose. You have the best of both worlds; hidden equipment and ductless efficiency.

    Solution one would be to mount Fujitsu's ARU air handler in the closet and use their Auto Louvre Grill http://portal.fujitsugeneral.com/doc_repository/454/Auto_Louver_Grille_Kit_Halcyon.pdf to make a clean, controlled entry into the room. Return air would be handled through a filter grill in the closet door. I would apply this setup only if the Auto Louvre is able to direct air into the main living area of the home. Being able to have an open and long throw area is very important.

    Alternately we would design a ducted system. http://portal.fujitsugeneral.com/doc_repository/454/RLFCD-RLFCC%20Brochure.pdf We have done many such design installs and have had absolute success. You of course don't have the fan power to manage a fully ducted plenum system so duct design is very critical. Done right, you'd think you had true central And the cost, efficiency and comfort is superior. In the case of a larger dwelling we simply utilize two or more air handlers. Now we have a fully zoned system!

    As far as output, efficiency at low temperatures and performance, you may have to make a disclaimer at times. Sometimes when given the option, a customer will opt to use their baseboard electric for two weeks in February to supplement rather than install that second system. Give them the info and let them decide.

    Oh yes: beware of Mitsubishi's claims of operation at super low temperatures. Yes they will work and yes the output is reduced. Get your professional installer to show you the engineering data to see what happens at -10 or -20 degC You may be surprised. If you are looking for a true heat pump heating system for all seasons, Mitsubishi can do it but Fujitsu does it better. In the frozen north it is very critical to understand where you will encounter your specific balance point.

    Be reasonable though. The days of designing a system for the just in case scenario are past. In you already have electric or wood then you are covered. If you have oil and want it gone you have more to think about.

    Dale

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