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Can a MiniSplit Live Happily in the Attic?

To solve an air conditioning problem, a contractor proposes to put two units in an unconditioned attic

Posted on Mar 19 2018 by Scott Gibson

A GBAGreenBuildingAdvisor.com reader named Inverter0815 lives in a two-story colonial in New Jersey that's hard to keep comfortable in summer. In order to get the three upstairs bedrooms down to a relatively comfortable 75° in July and August, Inverter must set the thermostat on his single-zone 2 1/2-ton air conditioner to 67°.

An HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor has taken a look at the house and has a recommendation, Inverter explains in this Q&A post: install two Mitsubishi SEZ model ducted minisplits in the attic. One will cool the master bedroom, the other will cool the two bedrooms where Inverter's children sleep.

There's one snag. The attic at Inverter's house is unconditioned.

"What got me thinking is that Mitsubishi recommends installing the SEZ units in a dropped ceiling or closet," Inverter writes. "Can these be installed in an unconditioned attic as well? The [temperature] gets up to 130° on a sunny day in the summer, so I am a little worried. Would a traditional air handler be a better idea?"

That's the underlying question for this Q&A Spotlight.

The units seem oversized

What strikes Chris King first, though, is the capacity of the minisplits that the contractor has recommended. At 20,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /hour, King says, they seem "incredibly overpowered" for cooling just three bedrooms.

"I also live in N.J. and use a 18,000 Btu/h unit for 1,500 square feet of 1960s poorly insulated home," King says. "Seems like your biggest issue is that your current system is single-zone."

Each of the units the contractor recommends, however, can be dialed back to 3,800 Btu per hour, Inverter says, and the output of the outdoor condenser can be lowered to 6,000 Btu/h. That means that if both units are running, each can operate at between 3,800 and 12,000 Btu/h.

Another option, he adds, would be to install a single SEZ model with a capacity of 18,000 Btu/h — but a single unit would not allow the room temperatures to be controlled separately.

That said, it's the proposed location of the units rather than their capacity that has prompted Inverter's question. And on this count, Yupster and others are unequivocal in counseling against putting the minisplits in an uninsulated attic.

"This is a terrible idea," he says.

Insulation and air-sealing may be to blame

There is one more preliminary before we get to that, says Yupster.

"It sounds like you have an existing forced-air system in the basement. If this is the case, the likely cause of your uncomfortable rooms is a combination of things," Yupster writes. "Poor insulation and poor air sealing will make any room uncomfortable regardless of how much energy you pump into it."

Air sealing is a particular concern, and that's something that Inverter can solve fairly easily. With air leaks reduced, Yupster suggests calling in an HVAC contractor who can measure air flow through the ductwork to the upstairs bedrooms and correct any imbalances in the system. To do that, Inverter will have to find the right contractor.

"The key is getting someone who is willing to do measurements, static pressure, flow at the register, temperature change through the system, etc." Yupster says. "I design these systems every day and measure and commission as many systems as I can."

Plus, Yupster adds, correcting any problems with Inverter's existing heating and cooling system may prove far less expensive than installing new equipment.

"As far as your ductwork goes, often you can't get appropriate air flow upstairs because something in the system is too restrictive," he writes. "Sometimes it's a high-efficiency filter, sometimes it's a cooling coil with a high pressure drop, sometimes it's undersized ductwork. All of these can be remedied at significantly less cost (usually) than a whole new system."

Air-sealing the house would reduce the tendency of indoor air to stratify, and thereby lower cooling loads upstairs, he adds. If the air ducts are too small, reducing the flow of air through them is one way of making them the right size, and air-sealing and insulation would allow Inverter to do that.

There are other options

David McNeely points to some potential problems with the ducted minisplit option, and, like other readers, suggests that Inverter have a look at some alternatives.

A ducted unit will require return air in the form of a 400-square-inch grille, he says, and if Inverter wants a MERV filter in the system he should plan on a grille of about 20 inches by 25 inches. In addition, he'll need service access, plus another 12 inches on the side where controls are located.

"So that's a lot of square footage out of a ceiling, and don't think you can stick it in a closet or bathroom ceiling without cutting a major hole in the closet door or wall, because a 400 cfm draw requires it to come from the whole upstairs," he says.

For these and other reasons, McNeely, Yupster and Dana Dorsett all suggest that Inverter look at some alternatives, such as wall-mounted ductless minisplits or ceiling cassettes.

Dorsett, for example, says three half-ton heads (6,000 Btu/hour each) would do it with one installed in each of the three bedrooms. Or, he adds, install one unit of 6,000 to 9,000 Btu/hour in the master bedroom, and a shared 9,000 Btu/hour ducted unit in a closet between the other two bedrooms.

Walls that might be used for a ductless minisplit are interior walls, Inverter replies, and there's a problem with what wall-mounted units look like. "I am aware that nothing performs as well and as efficiently as a wall unit, but it really is not an option," he says. "There is no closet between the bedrooms that can be used, so that's not an option either."

Cost may be a factor

Nor is Inverter a fan of ceiling-mounted cassette minisplits. He thinks the 2-foot-square unit would look "massive" in one of the smaller bedrooms, and he doubts he would be able to buy three cassettes and an outdoor compressor for less than $20,000.

Not so, Dorsett says. He's certain that Inverter could get a three-zone compressor and three cassettes for "well under" $20,000, possibly even less than $10,000 if he put the job out to bid. Buying the hardware on line is about $6,000, he said, and a competitive bid shouldn't be any more than twice that, even in an expensive market like New Jersey. Units with a smaller capacity would be even less.

Yes, adds Walter Ahlgrim, with enough equipment in the attic it would be possible to keep the second floor of the house cool — but don't start there.

"In my opinion, the contractor’s solution is the one that puts the most cash in the contractor’s pocket," says Ahlgrim. "It is also the solution with the highest operating costs. Putting equipment and or ductwork in any unconditioned space is not just a bad idea, it is a stupid, dumb, and lazy idea. The real solution starts with an energy auditEnergy audit that also includes inspections and tests to assess moisture flow, combustion safety, thermal comfort, indoor air quality, and durability. including blower door testing with [infrared] photography. This will help you identify and fix the air leaks and get you get the correct insulation installed. If you still have a problem, get a Manual J calculation to tell you if need more equipment or better ductwork."

Air-sealing and insulation will cost less than buying new equipment, he adds. "The real payoff will come from savings on your electric bills for years to come," he says. "Adding more equipment will only raise your monthly bills."

Our expert's opinion

Peter Yost, GBA's technical director, had this to say:

I think Yupster and Ahlgrim have this right: This is an enclosure/ducting problem looking for a mechanical system solution. Inverter did not respond to Yupster or Ahlgrim with any information about building enclosure performance, and we don’t know enough about existing ducting to know if an approach such as Aeroseal could completely change the air delivery problem to the second floor.

Inverter should be looking for:

  • An HVAC contractor who can do a Manual J load calculation for his current home and system size and who can give him more information on his duct system.
  • An auditor certified by the Building Performance Institute who can do a performance assessment of his enclosure. Inverter should use those results to prioritize enclosure improvements before installing any additional cooling capacity.


Tags: , ,

Image Credits:

  1. Mitsubishi Electric

1.
Mar 21, 2018 9:29 AM ET

Edited Mar 21, 2018 9:30 AM ET.

mini-split replacement question?
by Antonio Oliver

Are the minisplit manufacturers of ducted minis now recommending their systems for replacing legacy, in-place ducted A/C systems? Without regard to in-place ducting?

Similarly, are any manufacturers selling "regular" ducted heat pump systems with inverters? And COPs that compete with the minisplit systems?


2.
Mar 21, 2018 11:06 AM ET

Short answers, yes, and yes.
by Dana Dorsett

The Carrier Infinity Greenspeed, Bryant Evolution Extreme, and Trane XB20i models are all modulating inverter drive heat pump systems.

Mitsubishi's MVZ series ( https://meus.mylinkdrive.com/files/M-Series_MVZ_Multi-Position_Air_Handl... ) and PVA series (http://www.mitsubishicomfort.com/taxonomy/term/43163 ) air handlers are suitable for retrofitting to pre-existing ducts, as are Daikin's ARUF series (http://www.daikinlatam.com/download/SS-DARUF.pdf ) and LG's ARNU vertical ducted series (http://www.lg-vrf.com/vertical-ahu.aspx/ARNU123NJA4 ).

I'm sure there are others.


3.
Mar 21, 2018 11:38 AM ET

We have dealt with this
by Judson Aley

We have dealt with this problem several times, equipment and duct work in un-conditioned attics. This situation exists all over CT. The duct work is always a disaster of bent, crimped and stepped on flex so we remove all the flex, air seal the floor/2nd floor ceiling, put in a plywood floor if there was none, vent & insulate the rafters with Roxul or fiberglass spanned with 1-2” foil or white vinyl faced polyiso board, tape the joints, re-install duct work using as much hard pipe as possible and correctly installed flex. Its labor intensive and costly (but probably not more than all the new equipment). It works, the heating, and cooling becomes more uniform and it cuts their utility bills. Plus, the home owners love how neat and clean their attic look when its all done. All the material you need for this is readily available at the local lumber yard and is simple to install. The white vinyl faced polyiso has to be special ordered by the pallet-also, it is fire rated so some owners want it for that reason.


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