Ductless air conditioning systems, also known as "mini-split" systems, are an excellent way to cool and heat a space. They are available in many different configurations that can accommodate different applications. In the simplest form a ductless system consists of an indoor head unit, an outdoor condensing unit and a line set (piping). The indoor unit houses an evaporator coil and a fan (blower). The outdoor unit houses the compressor (or inverter) a condenser fan and an expansion device. The line set connects the two units and is usually concealed in a line set cover. This setup is ideal for an individual room or in many cases multiple connecting rooms. Single head ductless systems can even cool an entire floor if it is an "open concept" space. Ductless systems can run multiple indoor units from one outdoor condenser. The line sets can be run through a basement or an attic to access different sides of the building.
Ductless systems are extremely efficient. Some systems are available in up to 23 SEER. Simply put, ductless conditions only the space it's in. You can condition only the areas you are in as you deem necessary. You don't have to run the whole system, as with central air, if you're only using part of the building.
Ductless systems are visible in the space. However, there are options. Most units are very attractive to begin with. Generally, they are white in color and are mounted close to the ceiling. They tend to blend in and aren't the eyesore many people fear they might be. LG makes a system called ArtCool that uses a 24x24 square indoor unit. The unit only sticks out from the wall a few inches and houses an interchangeable photo of your choice. When the unit is turned on, small flaps open on all four sides to reveal the blower vents. When the unit is off it looks just like a picture frame. Until recently these were only available in a one to one configuration, but now we are able to use multiple indoor units supplied by one outdoor condenser. Ductless systems are also available in a concealed configuration that is unseen in the space. These systems are more labor intensive and unfortunately don't provide as much airflow as a standard unit. They can be very useful as part of a-multi unit system where they must be hidden for aesthetics.
The only downside to ductless is drainage. If the indoor unit is on an exterior wall, the condensate from the evaporator coil can be drained directly outside. If the unit has to go on an interior wall or in a ceiling space, the condensate must be pumped elsewhere. This is common and can be dealt with, it just requires additional material and labor.
Ductless provides incredibly efficient heat. Most systems will run at 100% capacity down to 17 degrees F. Mitsubishi's Hyper Heat system will deliver 100% capacity down to 5 degrees F. Below 5 degrees, a Hyper Heat system will actually reduce capacity to continue to provide heat without reducing efficiency.