There’s good reason for this global popularity: They’re generally easier to install than ducted air systems, use less energy to operate, and allow users to set individual temperature “zones” within one system. In recent years, American homeowners have become more aware of their benefits, says Herb Woerpel, an editor with ACHR News.

"Due to advancements in technology and efficiency and the swelling consumer awareness, ductless equipment continues to be one of the fastest-growing sectors throughout the HVACR industry," he says in this article.

Despite the current interest in ductless air conditioning and heating, the vast majority of the North American residential air conditioning market is dominated by ducted systems and room air conditioners. In fact, only 9% of all HVAC systems sold in the U.S. are ductless, according to Chisult Insight.

Industry experts attribute the widespread use of ducted and room air systems to ingrained construction practices among North American builders. In addition, many heating and cooling contractors have large investments in the tools and labor needed to install ducted systems, but aren’t equipped for ductless installation.

“Manufacturers who sell air conditioning in the U.S. sell to distributors, who sell to contractors, and contractors who have been doing this for many years are used to that kind of [ducted] product,” says Allan Dziwoki, division vice president and general manager of Panasonic Appliances Air Conditioning North America. “And that’s what they go to the distributor and buy, and that’s just the way it is. And builders are also used to that style of air conditioning in homes.”

However, ductless market share is expected to rise in coming years. Navigant Research predicts that annual U.S. revenue from ductless systems will increase to more than $9 billion by 2020, up from $3.9 billion in 2013. Dziwoki says that Panasonic sees market share for ductless growing by a factor of 15% to 20% each year. So now may be a good time for home builders to take a look at the costs and benefits of this often-overlooked approach to indoor air comfort. (Click here for a roundup of eight ductless mini-split systems for new homes.)

How It Works
In a ductless mini-split system, each room has its own individual air handler connected to a separate outdoor system, which may either be a heat pump or a dedicated air conditioner. These outdoor units are generally smaller and contain smaller blowers than ducted central heat pumps, as they are designed to serve single spaces.

Both single and multi-zone outdoor units are available, and up to eight indoor units generally connect to one outdoor multi-zone unit. The conduit between the indoor and outdoor unit contains the power cable, refrigerant tubing, and suction tubing, along with a condensate drain. This allows ductless systems to avoid the energy losses associated with forcing air through ducts, which can account for more than 30% of the energy used to operate a ducted air system, according to the DOE. Many of these losses occur when ducts leak or pass through unheated spaces, such as attics or crawlspaces.

While conventional central air systems typically serve all the rooms in a home, even if not all of those rooms are occupied, a ductless system’s air handlers can be individually controlled or turned off altogether if no one is in the room. This helps save energy and increase comfort and control, as homeowners can set different temperature zones within their homes.

According to the DOE, modern ductless mini-split systems can operate at heating efficiencies as high as 12 HSPF and cooling efficiencies up to 30 SEER. Most ductless systems can operate in temperatures from 115 to -15 degrees Fahrenheit, with some loss of operating efficiency in below-zero temperatures. Colder climates may need a backup HVAC system in temperatures outside the mini-split’s operating range, such as a separate gas furnace.

Most builders who use mini-split ductless heat pumps find they are best suited as a whole-home solution for small homes with open floor plans or as a supplemental temperature-zoning feature for large, well-insulated homes.

“In the United States, the Northeast drove the bulk of adoption at the beginning because of the number of older homes that were built without HVAC made them perfectly suited for duct-free systems,” says Dale Fields, director of residential and light commercial sales with LG Appliances. “More recently there has been an uptick of construction in Southwest, which is driving the adoption as there is a greater emphasis on energy efficiency in new construction.”

Small, highly efficient homes are the application of choice for Atmosphere Design-Build, a Grass Valley, Calif.-based design-build firm known for its net-zero-energy homes built to Passive House principles. Many of the firm’s house projects use a single ductless mini-split for home temperature control and a balanced heat-recovery ventilation system for air filtration and whole-home temperature distribution.

“Like any mechanical equipment, the ductless mini-split is fine-tuned to whatever the building’s need is,” says co-founder Mela Breen, principal designer of Atmosphere Design-Build. “We typically use them in projects that are very low load… We definitely go after creating a high-performance assembly first-- windows, walls--and then the mechanical equipment is just a component that helps with that.”

Some ductless models allow users to set specific heating and cooling schedules for different days and times, whether via proprietary remote control or a smart device connection. Traci Shortt, a homeowner in New England, says her Panasonic mini-split system is more responsive than ducted systems.

“With the mini-splits, it’s completely programmable. You can set it up for when you’re away or for certain times of day to come on, for a couple of hours before you come home,” she says. “But it works so quickly that honestly we barely bother programming it, because if we don’t turn it on, within less than an hour it’ll cool the house down to a comfortable level.”

Challenges and Solutions
Despite their many benefits, ductless systems do have some drawbacks, starting with costs. Ductless units typically cost about 30% more than central air systems, not including ductwork, based on cooling capacity, according to the DOE. On top of that, if the indoor units are oversized for the space they are going to occupy, they will simply waste energy, counteracting any cost- and energy-saving benefit.

“They’re high cost if you’re just putting them in every room of a house that needs a lot of heating and cooling,” Breen says. “If you have a unit in every room of a 4,000 square foot house, then that’s probably not the best approach for that piece of equipment. If you only need one or two units, they’re cheaper than a traditional HVAC unit.”

Breen also notes that ductless mini-split systems cannot perform air filtration in the same way a ducted system can–a particular issue in California. “It’s super smoky from the wildfires,” she says. “[And] they don’t have same kind of air filtration capacity as a ducted system that you can put filters on. So that’s a tradeoff as well.”

And no amount of interior design flexibility will satisfy homeowners that dislike the look of the indoor components in their living space. “On the design end I don’t love the way they look,” Breen says. “There’s always a tradeoff in every decision you make.”

Many major manufacturers, including Mitsubishi, Lennox, and Daikin, offer alternative indoor components for mini-split systems. In addition to the traditional air handler, designers, builders, and buyers can choose a floor-mounted unit, a cassette-style opening mounted to the ceiling, or a slim-duct unit if the minimal ducting required is feasible.

Alternately, buyers may be willing to sacrifice aesthetics for energy sayings. “They’re basically the cheapest way to heat and cool a house for the kind of building that we do. So, that often is a driver in the decision, where [clients] are like, ‘Okay, I’ll get used to the look of it.’ And then they have insanely low energy bills,” says Breen.

Expanding Education
A lack of awareness about ductless systems is a major barrier to their implementation, and builders and homeowners sometimes find it difficult to find a local ductless professional.

In an effort to fill this knowledge gap, major manufacturers offer a number of opportunities for HVAC trade professionals to obtain training and expertise in repairs and installation for their product lines. “We do see that product knowledge is a gap, and so Lennox has taken steps to take that on as an initiative,” says Brittani Youman, product marketing manager at Lennox.

Both Lennox and Rheem offer online training courses in ductless systems, as well as on-site training and consultations, whether in regional centers or on the jobsite itself. “Access to product training is one of the challenges we hear about consistently. Contractors are eager to receive more training, and we know making the time to train is mission critical,” says David Sanders, product manager of mini-splits at Rheem.

In addition to its own training centers and online courses, LG has partnered with select vocational schools, including Tarrant County Community College in Fort Worth, Tex., to develop a curriculum that includes duct-free system training.

“On the whole, contractors are becoming more and more comfortable with the technology, but education does continue to pose a substantial obstacle as many vocational schools still do not incorporate duct-free technology into their curriculum,” says Fields. “It is for this reason we are focusing on supporting education and training opportunities for our contractors.”

The Future of Mini-Split Adoption
In recent years, manufacturers have begun offering uber-efficient ductless mini-split units, with SEER ratings as high as 42 for Carrier’s newest single-zone mini-split system. These technological advancements, along with growing consumer awareness and demand for energy-efficient homes, have served to propel the sector’s growth.

“Since they were first introduced in the U.S., HVACR mini-split systems have evolved exponentially,” says Woerpel. “Today’s systems offer greater system capacity, longer pipe lengths, larger height separation, wider operation ranges, loftier efficiency levels, the advent of inverter-driven compressor technology, the onset of intelligent controls, and more.”

At the same time, the mini-split market share is growing in North America, with a 16% increase in the number of mini splits sold each year on average. According to JP Morgan, at current growth rates, ductless systems could represent 15% of the overall North American HVAC industry within the next several years.

“Currently, mini-splits account for 30% of the total heat pump market,” says Youman. “As the years progress we expect that to steadily increase, especially as awareness is growing across North America.”

Manufacturers will undoubtedly continue to improve the performance of ductless mini-split products based on consumer desires and responses in the market. “The owners of the more expensive homes want more comfort options, and of course they want the energy efficiency,” says Eric Griffin, national sales manager at Panasonic. “I think you’ll see the units getting a little bit larger, but you’re also going to see them being more and more efficient because of the compressor technology gets better each year and the motor technology gets better each year.”