Putting the Duct Back in Ductless Mini-Splits
A would-be HVAC designer wonders if a ductless mini-split head can be hidden in a closet and connected to conventional ductwork.
This post by Scott Gibson first appeared on Green Building Advisor.
A ductless mini-split 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.
Ductless mini-splits have a lot going for them. These high-performance air-source heat pumps operate efficiently in much lower 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, summer and winter.
It's the "wall-mounted" part, however, that not everyone warms up to. As is the case with Jerry Liebler's wife, as Jerry introduced 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 his wife “dislikes the looks of mini-split 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 mini-split 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."
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Liebler thinks the air handler's motor would overcome the friction losses of the ductwork. 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 mini-split?
"You don't have to invent (and cobble together) your own ducted mini-split unit," Holladay writes.
Yes, Liebler says, but none of them are capable of heating and cooling more than a couple of rooms, and "none offers the cold-weather heating of the Hyper Heat units." (Mitsubishi says those units will operate at 13ºF below zero.)
"That's right," adds Dana Dorsett. "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 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," Dorsett adds. "The control algorithms for the mini-split'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 Liebler install the mini-split 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 mixes 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 mini-split head.
"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 [air] past the mini-split so the mini-split'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 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."
Sorry, it just won't fly
Keith Gustafson remains unconvinced.
"I do not think your solution will work well," he says. "The mini-split 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 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," Gustafson says.
Kazama is sticking with his suggestion to buy a ducted mini-split, 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; the performance penalty, especially when it's really cold, of the ducted systems is simply unacceptable."
Our expert's opinion
Peter Yost, vice president of technical services at BuildingGreen and technical director at Green Building Advisor, had this to say:
Nothing like going to the source. I spoke with David Hazel, regional manager – channel development, with Mitsubishi Electric.
“No way,” says David, 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.”
I also happened to mention this issue of mini-split “unsightly” indoor unit heads to Green Building 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 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… The idea here is that while the head is still completely visible, it is much less obtrusive in a flush mount.”
Steve used the Mitsubishi spec sheet Hazel suggested to get the dimensions right for the closet shelf. Seems like the best of all possible worlds?
I doubled back with David Hazel of Mitsubishi on Steve’s detail and he had these comments:
- Mitsubishi recommends that this type of “pocket” installation include the unit projecting 2–3" 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 slightly 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.
(2013, October 3). Putting the Duct Back in Ductless Mini-Splits. Retrieved from https://www.buildinggreen.com/news-article/putting-duct-back-ductless-mini-splits