High-performance ductless heat pumps are an efficient alternative to traditional ducted heat pump systems, gas or oil furnaces, and electric resistance heating systems (Figure 1). Advances in technology in recent years have increased heat pump performance to the point that units are now available with heating efficiencies above 13 HSPF and cooling efficiencies above SEER 30.

Ductless heat pumps have been used in Asia and Europe since the 1970s and they comprise 80% to 90% of the residential HVAC market there. They have been used in U.S. commercial buildings since the 1980s and are now rapidly growing in the U.S. residential market.

Ductless vs. Traditional Ducted Heat Pumps

Ductless heat pump systems use the same basic heat pump technology as ducted systems, but the distribution of heating and cooling within the home is different. With a traditional ducted “central air” system, one centrally located air handler blows conditioned air through a system of ducts that deliver the air to individual rooms through registers (vents) in the floor, ceiling, or walls. The central air handler is usually located in an attic, crawlspace, basement, garage, or closet. With a ductless system, on the other hand, there is no central air handler or ductwork. Instead, small indoor units are installed inside the space they are heating and cooling. These indoor units are essentially small air handlers having a small fan that blows air directly into the space. Indoor units are often referred to as “heads” or as “fan coils.” The indoor units are usually hung on a wall or installed in a ceiling (Figures 2 and 3).

Ductless configurations allow better zoning and reduced fan energy consumption. Less fan energy is required because the units do not have to push air through ductwork. Ductless units allow zoned heating and cooling because each indoor unit operates independently of the others (Figure 4). In this way, different areas of the home can be set to different temperatures if desired. Some units can even be turned off while other units continue to heat or cool.

A major advantage of ductless systems when compared to ducted systems has to do with the potential heating and cooling losses through ductwork and the air handler. If a ducted system has the air handler and/or ducts located outside the conditioned space (for example, in a vented attic, vented crawlspace, or garage), heating and cooling losses will occur. It has been shown that losses related to duct systems can account for more than 30% of heating and cooling energy consumption if ducts are located in an unconditioned area. These losses occur through air leaks in the air handler and ducts, as well as through conduction of heat through the walls of the duct or air handler. With a ductless system, all of these losses are eliminated because the indoor units are located within the conditioned space. Ducted systems can eliminate these losses as well, however, simply by ensuring the air handler and all ducts are located within the thermal boundary (insulated walls, floors, and ceiling) of the home so that any heated or cooled air that leaks from  the system simply goes into the conditioned space and no real losses occur. However, space constraints can make it difficult to locate all ductwork and the air handler inside.

Ductless systems also tend to differ from traditional ducted systems in terms of performance. In general, ductless systems tend to have higher cooling efficiencies, provide poorer dehumidification in cooling mode, and provide less air filtration. There are also currently (as of 2023) many more ductless models than traditional ducted models on the market that can provide reliable heating at very cold temperatures (referred to as “cold-climate heat pumps”). Some models can operate at an outdoor temperature of -15°F for heating and up to 115°F for cooling, eliminating the need for backup heat sources in many locations. It is important to note that all of these characteristics are simply general trends, and none of these aspects are inherent to ductless or ducted technology. Market trends change continually, and individual models should be compared with respect to all of these performance areas.

Ductless heat pumps generally have inverter-controlled, variable-speed compressors. Ducted systems can also have variable-speed compressors, but single-speed compressors are more common in these more traditional units. The variable speed inverter technology adjusts the compressor speed, allowing the system to adapt more smoothly to shifts in demand with less temperature variation and lower energy use. When maximum capacity is not needed, the compressor revolution and power decreases, increasing energy efficiency. In contrast, conventional single-stage air-conditioning and heating systems must stop and start repetitively. The inverter compressor allows the system to ramp down to as little as 10% of rated capacity (Figure 5). Ductless heat pumps and advanced ducted units also have linear expansion valves rather than open/close valves, and multi-speed rather than single-speed fans to continuously match the heating or cooling load. The capacity on a variable speed heat pump can have a wide range; for example, one ductless model reports a capacity range of 3,100 to 24,000 Btu/hr in heating mode and 3,800 to 14,500 Btu/hr in cooling mode.

Heat pumps and air conditioners have traditionally been relied upon for dehumidification as well as cooling. When providing cooling under typical conditions, these systems will automatically remove a certain amount of moisture from the air. A standard way to quantify how well a unit dehumidifies is called the Sensible Heat Ratio (SHR). SHR could theoretically range from 0 to 1, with lower values indicating better dehumidification performance. Heat pumps and air-conditioners have traditionally had SHRs of around 0.75. This has historically proven to provide sufficient dehumidification when the unit is properly sized and humidity conditions are not abnormal. Newer, more efficient units tend to have higher SHRs (worse dehumidification). This is particularly true for ductless systems. In homes where humidity may be an issue, a separate dehumidification system may be needed. See the Climate tab for more information.

For more information on refrigerant-based heating and cooling systems, see Traditional Split Heat Pumps and Mechanical Air-Conditioning.

Ductless System Configurations

The simplest ductless heat pumps consist of a single outdoor unit (containing the compressor, expansion valve, and coil) and an indoor fan coil (a “head”) (Figure 6).  These one-to-one systems are often called “mini-splits”. The outdoor unit is mounted on an exterior wall or on a concrete, stone, or plastic composite pad outside the house; refrigerant tubing and control wiring connect the inside and outside units through a small hole in the wall.

Ductless heat pumps can be designed to provide heating and cooling to just one space such as an addition, a room that cannot easily be reached by ductwork, or an area that tends to get too cold in winter or overheat in summer.  These systems would typically have only one indoor head (mini-split system). Ductless systems can serve an entire house as well. This typically requires more than one indoor head (“multi-split” system). It is generally not recommended to connect more than three indoor units to one outdoor unit, but some models allow as many as eight indoor units to connect to one outdoor unit.

Ductless heat pumps work well for use in small, very efficient homes with open floor plans or in larger well-insulated homes where zoned heating and cooling are desired. They also work well in additions. They can also be a beneficial part of an energy-efficient renovation where a less-efficient central heating system is kept in place for supplemental heating or cooling and one or more ductless heat pumps are installed in the primary living areas. Since interior space is not needed for bulky ductwork or air handlers, ductless systems are very convenient for retrofit applications.

Zoning can easily be accomplished by using multiple heads. Each head is individually controlled by its own wireless thermostat, which also communicates with the outdoor unit (Figure 7).

The three-zone ductless multi-split system in Figure 7 conditions the two bedrooms upstairs and the living room downstairs. While the system shown in this scenario could include additional ductless heads to condition the kitchen, the office, and the bathroom, these additional units would provide more capacity than is needed to condition the small spaces and the extra units would add considerably to the overall cost of the system. Another option, available from several manufacturers, is a ducted mini-split air handler with short ducts to provide conditioned air to several nearby rooms at once, such as bedrooms, bathrooms, offices, or storage rooms. The air handler is typically a horizontal unit, although some manufacturers make vertical units. The air handler has a larger blower motor to move air through the supply ducts and it also has a ducted return.

The ducted air handler is connected by refrigerant tubing to the outdoor unit along with any other ducted or ductless air handlers that are part of the system and, as with all of the indoor units, it must be verified that it matches the outdoor unit using the AHRI equipment matching system.

One outdoor unit may be able to accommodate a single ductless head, a single ducted air handler, multiple ductless heads, multiple ducted air handlers, or any combination of these (Figure 8). Verify capacity with the manufacturer for the models you are considering.

Some larger, commercial-scale heat pumps are variable refrigerant flow (VRF) systems. Variable refrigerant flow refers to the system's ability to control the amount of refrigerant flowing to each indoor fan coil. With VRF technology, one outdoor unit can be connected to different indoor fan coils (heads) that are heating or cooling in different zones. Some high-end systems can even provide heating to one zone while simultaneously providing cooling to another because they have an additional refrigerant line to each internal unit and a controller which directs them to pull heat from rooms calling for cooling and send it to rooms calling for heating rather than rejecting it to the outdoors. By moving the refrigerant from one zone to another, the system allows for some heat recovery. The multiple heads can be of differing capacities and configurations, providing for additional individualized comfort control.

Sizing and Selecting a Ductless Heat Pump

Correctly sizing the outdoor unit and each indoor unit (head) to the space loads is imperative for efficient and comfortable operation. Correctly locating the heads is also important for air delivery to the desired location. Oversized or incorrectly located air handlers can result in short cycling, which wastes energy and does not provide proper temperature or humidity control. A system that is too large will be more expensive to buy and operate.

Calculate the heating and cooling load for the home or space to be conditioned using the ACCA Manual J Residential Load Calculation. This calculation should be performed for individual zones (one per indoor unit) as well as for the entire house.

Properly size the equipment for the design heating or cooling load following the sizing guidelines in ACCA Manual S: Residential Equipment Selection. When determining equipment sizing per ACCA Manual S, use the original equipment manufacturer (OEM)’s expanded performance table to obtain performance data at design conditions, rather than using the performance data on the AHRI certificate, which lists heating and cooling capacity and SEER2 and EER2 cooling efficiencies at factory conditions of 95°F outdoor, 80°F indoor, and 67°F wet bulb.

If you are participating in an energy-efficiency program, select cooling equipment that complies with the requirements for your climate zone. To determine your climate zone, see the International Energy Conservation Code (IECC) 2021 climate zone map on the Climate tab.

Properly match the indoor and outdoor components of the heat pump system as demonstrated by a certificate from the Air Conditioning, Heating and Refrigeration Institute (AHRI). AHRI assigns a certification number and efficiency ratings to specific combinations of equipment (outdoor unit, indoor unit, indoor coil, fan type, etc.) that have been tested by the manufacturer according to AHRI test procedures using AHRI-specified test conditions (AHRI 210/240-2023). If an AHRI certificate is not available, a copy of the catalog data provided by the original equipment manufacturer (OEM) should be attached to the system indicating an acceptable combination and performance data. 

Install the heat pump in accordance with the manufacturer’s instructions and relevant standards including ACCA Standard 5: HVAC Quality Installation Specification and ACCA’s Technician's Guide for Quality Installations and ACCA Standard 9: HVAC Quality Installation Verification Protocols.

How to Select and Install Ductless Heat Pumps

1. Choose a system configuration based on homeowner needs. This includes determining whether to use ducted, ductless, or combination systems, how many zones and indoor heads are needed, the boundaries of each zone, and the location of each head.
2. Calculate heating and cooling loads for each zone and for the entire home using ACCA Manual J. This is especially important if you have done significant air sealing and insulating, which will reduce your heating and cooling load.
3. Size and select equipment using ACCA Manual S. When determining equipment sizing per ACCA Manual S, use the original equipment manufacturer (OEM)’s expanded performance table to obtain performance data at design conditions, rather than using the performance data on the AHRI certificate. For example, if the home is kept at 75°F and the outdoor design temperature for the location is 100°F, locate those columns on the performance table and match them to the design load.
4. Choose the highest performing model project costs will allow, to meet the design heating and cooling load of the project. Take note of dehumidification performance as indicated by the sensible heat ratio (SHR) of the unit (lower SHR indicates better dehumidification during cooling). Note cooling efficiency (SEER2), heating efficiency (HSPF2), noise criteria, air filtration (higher MERV filters provide better filtration), and cold weather performance. If you are participating in an energy-efficiency program, select equipment that complies with the requirements for your climate zone.
5. Determine whether a whole-house dehumidifier will be needed. If the selected heat pump has a high SHR, then additional dehumidification may be needed, particularly if located in a humid environment (Moisture Region A in the maps on the Climate Tab).
6. Confirm that the indoor and outdoor components of the heat pump system match, as demonstrated by a certificate from the Air Conditioning, Heating and Refrigeration Institute (AHRI) or a copy of the catalog data provided by the original equipment manufacturer indicating an acceptable combination.
7. During construction, keep the copper refrigerant tubing charged with dry nitrogen and sealed with solder to keep moisture out of the lines.
8. After connecting the indoor unit and the outdoor unit, vacuum the lines to 500 microns to remove air pockets.
9. Follow the manufacturer’s recommendations for refrigerant charging. Too much or too little refrigerant can reduce the efficiency of the equipment and lead to premature component failures. Use the charging method recommended by the manufacturer. There are three methods for charging: the subcooling method (typically for units with a thermal expansion valve), the superheat method (typically for units with a fixed orifice), or the weigh-in method (using the refrigerant weight amount listed on the data plate on the outdoor unit). Verify that you are using the correct method for the specific heat pump model to be installed. Refrigerant charging must be done by an EPA certified technician.
10. Make sure the condensate line and drain pans are correctly installed.
11. Test the system in all modes to ensure proper operation.