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Ducting a ductless minisplit

user-1137156 | Posted in Energy Efficiency and Durability on

I have read of the problems with the distribution of hot air and cool air in well sealed and insulated houses, and I have a “boss” who dislikes the looks of minisplit indoor units. But we don’t need more heat or cooling than a Mitsubishi Hyper Heat can provide.

I’m thinking of locating a minisplit’s indoor unit in a closet with a small air handler (like Electro Industries HE-N-00-21). The closet would have a return air duct under the air handler and an outlet duct through the floor under the minisplit. A “shelf” would run horizontally around the minisplit and the outlet duct of the air handler

With the closet door closed, there would, in effect, be a “plenum” above the shelf, pressurized by the air handler. The air handler’s ECM motor would overcome the friction losses of the ductwork. The ducts through the closet floor are connected to a conventional forced air heating/cooling duct network.

Has anyone done something similar? See any problems?

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  1. GBA Editor
    Martin Holladay | | #1

    You don't have to invent (and cobble together) your own ducted minisplit unit.

    Many minisplit manufacturers (including LG, Fujitsu, and Mitsubishi) make ducted units.

  2. user-1137156 | | #2

    None of the ducted mini splits handle more than a couple of rooms and none offers the cold weather heating of the Hyper Heat units.

  3. Expert Member
    Dana Dorsett | | #3

    That's right- the very low temp 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 mini-split would have a hard time managing the coil temp with the outside air flow influences imparted by an air handler driving air through the coil. The control algorithms for the mini-split's control are optimized for it's own blower pulling & delivering air from a low-impedance equal-pressure air path.

    Now that Daikin has acquired Goodman they are marrying their outdoor mini-split units with variable speed air handlers, but they only have a specified output down to -20C. IIRC they have automatic resistance-heating coils in the air handlers for when the heat pump isn't keeping up, but I believe it turns off the heat pump when the heating coils are active.

  4. mackstann | | #4

    I found this combo with a quick Google search:

    Mitsubishi PUZ-HA30NHA2 30,000 BTU outdoor condenser (with Hyper Heat)
    + Mitsubishi PEAD-A30AA4 30,000 BTU ducted indoor air handler

    Listed at $6853.60 on

  5. user-1137156 | | #5

    Good point about the coil temperature! Undoubtedly the fan speed of the mini split is controlled as part of it's function. How about, simply eliminating the "shelf" and ducting the air handler output to above the mini split head. The full flow of the air handler would flow through the closet but most would bypass the mini split. The mini split would control how much it wanted to "process".

  6. Expert Member
    Dana Dorsett | | #6

    Nick: A real problem with Mitsubishi is the breadth & depth of their offerings, and not all heads work with all outdoor units to the same specs. I'd definitely want Mitsubishi sign-off on it before going ahead and installing a PEAD-xxxx mini-duct on a unit that needs to run at -25C. (Even contractors blow the compatibility and full-range specs issues on these all the time.) That said, that pairing is rated 32,000BTU/hr out @ -15C in the general catalog. See page 25, which is a good sign:

    The same cassette mated with the PUZ-HA30NHA4 (rather than PUZ-HA30NHA2, one number different) is only rated at 23,000BTU/hr @ +17F with no cool temperature rating (see p.21).

    I'd want to see the extended range specs on any of 'em.

    Jerry: What is the outside design temp & heat load we're working with here?

  7. user-1137156 | | #7

    Heat load is 12,000 BTU/hr @ -10f..

  8. jinmtvt | | #8

    Jerry, why not just use the mini split as intented in a main room,
    and design a simple recirculating air system with a simple fan or multiple fans
    that mixes all of the building air together , you could even catch air from a HRV in there same time.

    If your wife can't accept to live with the look of a mini-split on a wall, you might need to consider
    a change of wife ?? ( the newer minis look good )

    You will probably have hard time working with the automatic functions and temperature detecting functions of the Mits/Fujis/Daikins high efficiency models in a small closet.

    that said, i've just installed 4 9RLS2H in my house and i love them,
    can't wait to try them out in the winter! ( rated 9KBTU of heat at -25c i believe !! )

  9. user-1137156 | | #9

    After being married for over 50 years changing wives is Not a option! With the air handler and one sided ducting (using the basement as a 'return" air plenum) I have about the simplest, full coverage, air recirculating system, one fan with ECM, 600 CFM or more total flow. With a duct branch to each room and each room having a return grill into the basement In effect, the air handler will move the house past the mini split so the mini split's temperature sensors etc. will see the average conditions of the whole house. With the closet door open the remote control of the mini split can set it's mode and set point. The air handler itself is controlled by a centrally located thermostat, set at the same temperature as the mini split.. As to the ventilation system, it can be of minimum size, exhaust from the two full bathrooms and simply dump it's fresh air into the basement. 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 mini split by closing a closet door. I think this system will work much better than any attempt to use ventilation air to equalize temperatures.

  10. gusfhb | | #10

    I do not think your solution will work well. 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 you propose

  11. jinmtvt | | #11

    Jerry if you wish to do as you described or a similar method,
    please get a "ducted" split unit.
    Mitsu and Fuji have some i believe, even if the effefiency is lower, will be much easier to work.

    All of the -25C ( super heat and H models ) rely on complex software and sensors, and will NOT work in a closet or having air from basement thrown in front of them.

    The only way they will work is if you set the waypoint out of possible range and the fan to high( fixed)
    so they never reach the lower operating speeds ...thus getting a very bad efficiency.

    50 years!!! congrats :)
    i'm close to 15 now, but my wife has no or almost no decisional involvement in our house mechanical setups :p ( must be a 50years of marriage thing . .maybe :p )

  12. user-1137156 | | #12

    Keith & Jim
    You both are so pessimistic! I'd be a rich man if I had a single $ for every time I've been told something won't work and proceeded any way almost always getting the exact results I expected! IT WILL WORK & WELL! All that mysterious software does is determine how much heat to move based on temperature readings. If it sees colder air, it'll add more heat, in heating mode or do less cooling in cooling mode. It has NO knowledge of and cannot care where the colder air came from. Sure it's a variable capacity unit so it works less and less hard as it sees temperatures closer to it's goal. It has to be smart enough to handle the blast of cold air when an outside door is opened in a small room after it's tapered down to no heat output. The air handler starting will give it the same kind of shock.. All that fancy software has to still work if the unit is in a drafty spot with rapid changes of temperature due to whatever. The performance penalty, especially when it's real y cold, of the ducted systems is simply UNACCEPTABLE! I also will have another heat source, a masonry heater. and this approach spreads that heat to the distant places as well.

  13. jinmtvt | | #13

    Well then on with it! keep us updated on performance and problems ... :)

  14. gusfhb | | #14

    Well, if'n ya know everythin' why is ya askin' questions?

    Maybe it will work, or maybe you will have paid thousands of dollars to look at a blinking error light, or not have acceptable performance.

    It is risk and reward.

    The risk is moderate to high that your system will not function as you hope. There is no reward above some of the mentioned alternatives. The unit can be effectively camouflaged without putting it in a closet

    I have many 'science projects' going on in my house. None of them will fail to heat the house.

    One of the bigger reasons not to do it is that no one is going to be able to help you if it fail's to function.

  15. user-1137156 | | #15

    "why is ya askin' questions?"
    The questions were:
    Has anyone done something similar?" Not answered so still maybe.
    and See any problems? Which Dana's answer led to a change of the "design" . What has followed is some pessimism without apparent reasons. To the pessimists I ask what specification, of the mini split, I'm violating? If I'm not violating any specification, I expect the equipment manufacturer(s) to support their products.

  16. user-965403 | | #16

    I guess I feel the same way as your wife (as well as my wife) that mini-splits are less than "beautiful" when hanging on the wall. We had our cabinet guy hide it in a cabinet (see above the sink) with an upswing louvered door that hides it well, and can be opened when you need full volume. It works out pretty well in both heat and cool modes.
    Doesn't seem to block the air flow.

  17. user-1137156 | | #17

    How do you control the mini split? The standard infrared remote will be blocked by a "cabinet". How do you clean the filters? The control issue can, possibly, be resolved with a wired control which is advertised as available for the RLS2H series. Is air circulation still reasonable?

  18. user-1137156 | | #18

    The air handler I started looking at is WAY overpriced! I found a much more appealing unit in the Goodman MBVC1200 AA1 at less than 1/3 the cost.. This air handler is wired to and controlled by a 2 stage heating/cooling thermostat. It may be possible to also use the same thermostat to control a RLS2H by using it' optional control components.

  19. user-965403 | | #19

    The grillwork does not block the signal from the remote. Only thing you need to do is stop the "sweep" of the outlet fins so that it does not blow the output into the cabinet. Filters are no problem because everything is accessed from the front. The top picture does not show it, but the cabinet front flips up on hinges for air flow and access. Air distribution is not changed, except for the sweep of the fins, but that doesn't seem to make much of a difference.

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