Ductless mini-splits

Published: February 12, 2014


Ductless mini-splits (DMS) are electrically-powered HVAC products that provide both space heating and cooling by extracting latent heat in air from one location and transferring it to another. DMS systems utilize a vapor compression cycle to move heat from indoors to outdoors in the cooling season and from outdoors to indoors during the heating season. During the heating season, refrigerant is compressed in the outdoor unit and is sent indoors as hot gas. As the refrigerant condenses, it releases heat and is ultimately piped back outdoors to repeat the process. The reverse of this process is used during the cooling season.

Ductless mini-split inside unit

A typical DMS system consists of an outdoor unit, indoor ports, and piping for the refrigerant. For cold climate applications such as Wisconsin, a cold climate ductless mini-split (ccDMS) would be required to adequately provide heating in sub-freezing conditions. These systems are inverter-driven and have the ability to run at variable speeds allowing them to work efficiently in very low temperatures.

The ductless mini-split technology has become the norm in Asian and Western European markets. Technological advancements, like the inverter-driven cold-climate DMS, have come out of these markets, allowing them to be more efficient and more cost effective.

We want to acknowledge our appreciation for the efforts of Tim Lindstrom, Nick Funk, and Dan DelVescovo (students at UW-Madison) for their involvement in the research of this technology



Implementation costs

$3500- $4500 installed costs (for a single port system)

Potential energy savings

20-40% of cooling load when compared to a conventional central air conditional system

Market barrier(s)

lack of technical knowledge

Ductless mini-splits have applications in both residential and commercial settings.

Outdoor unit of the ductless mini-split

Optimal residential DMS applications include new, small single-family homes; single-family homes where electric resistance is the primary heating option; homes lacking central air conditioning systems; and older or new homes without access to natural gas. In Wisconsin, DMS may be particularly well-suited for those homes that have hydronic or radiant heating and therefore do not have ducts; adding DMS may be a more palatable option in comparison to retrofitting the home with ducts for central air conditioning or adding window or sleeve air conditioning units. Also, DMS can be installed in multifamily buildings for unit-level heating and cooling.

DMS systems can be applied in the commercial sector if the operation is small and meets requirements that make energy savings from DMS attractive, such as medium to low internal heat gains, regular occupancy, and wintertime heating load. Typical office layout is another reason why DMS is applicable in small commercial settings. In most small commercial spaces, a single port system can provide 90% of the heating load compared to 45-80% in a residential application, especially in open-office layouts.


Energy savings: Compared to central air conditioning which has a typical seasonal energy efficiency ratio (SEER) of around 13, DMS systems can have a SEER of 26, which saves the consumer operational costs. Compared to electric resistance heat, the DMS can also offer savings of about 30-40% in heating costs.

Flexibility: Because there are no ducts, the mini-splits can be placed in the house in a variety of locations, per the preference of the homeowner or building owner. The indoor part of the system can be wall-mounted, placed in a drop-ceiling or be placed on the floor.

Easy to install: Because there are no ducts to install, the DMS is relatively easy to install, only requiring a small hole to run the conduit from the inside to the outdoor units. The conduit can be run at a variety of distances, so if the room to be cooled or heated is in the front of the house, the outdoor unit can be placed discreetly in the back of the house.

Zoned heating and cooling: Unlike central air conditioners or most central heating systems for which the temperature is set for the entire house's space conditioning needs, the DMS system can provide zoned modulation of temperature. If a household or small commercial facility has multiple indoor units, the spaces can be managed such that an indoor unit only runs when that space is being occupied. This allows for greater control of the building's conditioning needs.

Reduction in duct energy losses: For those buildings that have ducts in unconditioned spaces, like a crawlspace or attic, energy losses from ducts through that space could be substantial. DMS systems avoid potential losses through ducts.

Challenges and market barriers

Cost: Cost can be a deterrent when homeowners think about installing a cooling system. DMS systems are more expensive than window air conditioning units, but can be cost competitive for homes that would have to add ducts if central air conditioning would be installed. Also, for homes with many rooms to be conditioned, the need for multiple indoor units is increased to maintain an even temperature, which increases costs.

Lack of knowledge: While the technology is not complicated, its newness is a barrier for both HVAC contractors and homeowners. The systems are now becoming more well-known for installers, which may allay some of these initial challenges. For homeowners, particularly in the Midwest where the DMS systems are not well-known, the lack of knowledge of the systems in general pose a challenge for market penetration.

Statewide energy savings

While ductless mini-splits can provide heating for cold weather climates, they will likely be seen in Wisconsin as an alternative way to providing cooling to houses that are currently heated through hydronic or radiant heat with no ducts. These homes either currently do not have mechanical cooling or may use individual window air conditioners for space cooling. Ductless mini-splits offer a more efficient way to provide space cooling for those homes unable to add ducting for traditional air conditioners.

While cooling energy comprises of a small amount (~3%) of residential electricity load in Wisconsin, there is a possibility of increasing this total amount with increased installations of ductless mini-splits, since these homes may be adding an additional electricity load where there are none.

More research is needed to understand the potential market penetration of ductless mini-splits for both cooling and heating in Wisconsin.

Financial incentives

There are currently no financial incentives in the state of Wisconsin for ductless mini-splits.

Ductless Mini-Split Air Conditioners from the Department of Energy
summary This Department of Energy webpage provides a general overview of what ductless mini-splits are and in what applications these systems might be appropriate. The webpage outlines advantages and disadvantages of DMS in comparison to other cooling and heating options. The webpage primarily focuses on the cooling needs that DMS systems can meet.
citation “Ductless Mini-Split Air Conditioners” Department of Energy.
link http://energy.gov/energysaver/articles/ductless-mini-split-air-conditioners, accessed June 18, 2014
note This Mitsubishi dealers website also offers a comparison between window a/c units and mini-splits: http://mitsubishiacdealers.com/info/mini-split-or-window-unit

Ductless vs. Central Air Conditioner
summary This website from Mitsubishi provides an overview of the choice between central air conditioners and ductless mini-splits, provided from the perspective of the manufacturer. The webpage offers benefits and disadvantages of the mini-split and provides links for more information.
citation “Ductless vs. Central Air Conditioner” Mitsubishi Electric Heating and Cooling.
link http://mitsubishiacdealers.com/info/ductless-or-central-air, accessed June 18, 2014

Ductless Mini-Split Heat Pump Comfort Evaluation
summary This report sponsored by the Department of Energy examined ductless mini-splits through a field test of two homes in Austin, Texas. Temperature and relative humidity was monitored in four rooms in each home, before and after the home's central heating and cooling system was retrofitted with DMS allowing researchers to put comfort-related observations in context. A sample size of two, however, cannot be extrapolated to a larger population.
citation Roth, et al. “Ductless Mini-Split Heat Pump Comfort Evaluation” Department of Energy, Buildings Technologies Program. March 2013.
link http://apps1.eere.energy.gov/buildings/publications/pdfs/building_americ...

Ductless Heat Pump Engineering Analysis: Single-Family and Manufactured Homes with Electric Forced-Air Furnaces
summary This study was based on field research completed in the Pacific Northwest to understand performance of DMS systems in single-family and mobile homes. The researchers used detailed field monitoring to distinguish performance impacts attributable to how occupants used the heating and cooling system from the impacts related to the efficiency and performance of the technical system. This field study only looked at sites where DMS systems were installed to displace existing electrical heating systems which are more common in the Northwest United States. The analysis demonstrated significant electric savings (~5,000 kWh/year) for those DMS that were installed in homes previously heated through a forced-air electric furnace.
citation Baylon, et al. “Ductless Heat Pump Engineering Analysis: Single-Family and Manufactured Homes with Electric Forced-Air Furnaces” Bonneville Power Administration's Energy Efficiency Emerging Technologies Initiative. December 2012.
link http://www.bpa.gov/energy/n/emerging_technology/pdf/DHP_FAF_Dec_12.pdf

Ductless Heat Pump Engineering Analysis: Multifamily and Small Commercial Buildings
summary This study examines DMS in multifamily residential buildings and small commercial buildings. For this field research, the multifamily component included 12 submetered units and 188 units that were analyzed using a utility bill analysis. Ten small commercial buildings were evaluated. The multifamily buildings showed less than favorable savings from installation of the DMS but six of the small commercial spaces, which had office and retail use patterns, demonstrated significant opportunities for savings.
citation Larson, et al. “Ductless Heat Pump Engineering Analysis: Multifamily and Small Commercial Buildings” Bonneville Power Administration's Energy Efficiency Emerging Technologies Initiative. December 2012.
link http://www.bpa.gov/energy/n/emerging_technology/pdf/DHPx_Multifamily%20_...

Final Summary Report for the Ductless Heat Pump Impact and Process Evaluation
summary This report summarizes an evaluation of the Northwest Ductless Heat Pump Pilot Project that ran from October 2008 through December 2009. The evaluation includes an analysis of fived components of technical performance and market acceptance: market progress evaluation, laboratory testing, field monitoring, billing analysis, and cost analysis and non-energy benefits.
citation "Final Summary Report for the Ductless Heat Pump Impact and Process Evaluation" Prepared by Ecotope for the Northwest Energy Efficiency Alliance . February, 2014.

Northeast/Mid-Atlantic Air-Source Heat Pump Market Strategies Report
summary This report assesses opportunities and identifies market barriers to the adoption of air-source heat pumps (ASHP) systems. It provides recommendations for program,
industry and policy strategies to realize the energy and cost savings and avoided GHG emissions from home ASHP systems. The report focuses on ASHP's new high performing technologies such as ductless mini-splits that, in comparison to older, conventional ASHPs, can perform at high levels of efficiency even at very cold outdoor temperatures. The market potential for such systems is significant to reduce heating energy consumption and costs, especially for those homes that currently use electricity, oil, or propane for heating.
citation “Northeast/Mid-Atlantic Air-Source Heat Pump Market Strategies Report” Northeast Energy Efficiency Partnerships. January 2014.
link http://www.neep.org/Assets/uploads/files/market-strategies/emerging-tech/ASHP/ASHP%20Report_FINAL%20(2).pdf