A split air conditioning system refers to having the condenser outside and a separate indoor refrigeration unit. This is in contrast to a “unitary” system where everything is in a single package (like a typical window unit). Central air conditioning systems are an example of a split system1.
A mini-split system is one where small indoor units are fed by an outside condenser. These are also ductless, because they only cool the area where they are installed – cool air blows directly from these units into the room. Each indoor cassette gets refrigerant through a small line. This flows to the indoor unit where a fan then blows through a cooling coil. Condensation is removed with a small water line that runs in parallel to the refrigerant line to the outside. They have several advantages and a few disadvantages over “ducted” or central air systems.
- They don’t need ducts. In new construction, a mini-split system avoids the expense of installing ductwork throughout the house. This gives advantages in construction and space savings. Routing ductwork is a major design consideration that may restrict ceiling heights, where stairways can be located and a host of other considerations.
- They are more hygienic. A concern about bacteria (Legionnaires disease in particular) and mold buildup in ductwork, as well as the possibility of ducts moving dust through the system makes maintenance more of a chore. The mini-split system cassettes can have more efficient filters in the smaller units which are easier to access.
- They can be zoned. Unlike central air systems which require closing a vent (and even that is only a partial solution), each indoor cassette can be independently controlled. The controls have become quite sophisticated – beside the traditional thermostat (one for each room) they can be controlled by timer and even over the Internet if desired.
- Quick response – because they only handle air for a small area (usually a single room) the indoor units can respond more quickly than a central air system to changes in temperature, both up and down. This gives a smoother experience without the swings commonly seen with ducted systems.
- Cost savings. Central air systems waste energy cooling the ducts the air travels through and they usually cool the entire house – even rooms that are unoccupied. Ductless mini-splits more closely match demand. This gives considerable energy savings over time. Zoning also adds to these savings.
- Installation is easier because only a 3-inch hole is required to penetrate the outside wall and refrigerant lines can be run through attic space or even outside the house. Installation is more difficult behind walls to where the cassette is located, but still easier than running ductwork to the same location. Some homeowners install the units themselves2. The recommendation is to get an analysis from a certified HVAC technician before attempting a do-it-yourself job to make sure the capacity of the system and placement of indoor cassettes meets standards.
- More secure. In burglaries, a window or through-wall unit can be an entry point for a burglar. They simply kick out the air conditioner and go in the hole. With mini-splits (and central air) this isn’t available to them.
- Heat pump function. Like central air, these systems also have the capability to switch over to a heating function, essentially reversing the process of cooling. They do not, however, give a great deal of heat compared to traditional furnaces. They can lower heating bills and are a useful option as fuel prices rise.
- Initial outlay is higher in most circumstances. Since houses in the US typically have ducts installed already (if not for central air, then for forced air heating) the initial cost of a mini-split system is about 30% higher than central air3. Further, a ductless mini-split system has higher maintenance costs which may overcome the savings gained through zoning. However, on new installations, both systems are on par. This is because installing ductwork is very expensive and adds significantly to the cost of a central air system.
- No fresh air. Mini-splits recirculate room air. While central air systems do this as well, other types of air conditioners (window and through wall) draw in fresh air. Some manufacturers use this as a positive feature because they advertise an air purification benefit. The opposite view is that fresh air is worthwhile to prevent “stale air.”
- Limits. Mini-split systems have up to four interior units. If this isn’t enough to properly cool a home, another outdoor unit has to be installed. There are also limits on how far away an indoor unit can be from the exterior compressor. Lines have to carry refrigerant both ways and pump pressure along with heat loss/gain restricts distances. This can be a problem if the outdoor unit is hidden in the back of a house but the home needs an inside unit at the front.
- Aesthetics. In the US, the clear preference is to have openings flush with the wall. Consumers may feel the mounted cassettes are ugly or obtrusive. There are concealed units, but these are usually more expensive. There is also the matter of running the necessary lines to the indoor units. These should be behind the wall surface, but in some refits this isn’t possible and they show.
- Cooling capacity. Ductless mini-split systems are limited in the amount of air they can move. The smaller (and less noticeable) the indoor units are, the smaller the fan inside has to be. The trend is toward high-velocity fans to overcome this, but there is a tradeoff with power consumption and the ability to direct air flow to those areas that need it most. Combined, these problems make it difficult to cool large open areas.
- Noise. Because individual cassettes have a blower fan in the room, the noise the fan makes is louder than central air. They are much less noisy than unitary systems.
According to the US Department of Energy, a ductless mini-split system is the better choice for small footprint living spaces (apartments or condominiums) and in new construction where ducts will not be used for forced air heating4. The trend in the US toward mini-splits over ducted systems seems to bear this out. Many Americans see them as new, untested technology, but these systems have been popular in Europe and Asia for many years. Another driver is the energy savings realized over time through zoning.