Ducted air systems are used in the majority of comfort cooling applications in North America. In contrast, ductless refrigerant systems in Asia, and ductless water systems in Europe dominate their respective HVAC markets. These practices reflect the historical evolution of air conditioning in each market.
Use of the coal-fired basement furnace evolved in the in the late 19th and early 20th century in North America to keep coal dust and storage away from occupied spaces. Coal was used for coal-fired boilers or coal-fired furnaces. Older buildings in the Northeastern part of the US used hydronic heat distributed via radiators. District steam in New York City and other major cities gave rise to ‘cast-iron’ and the pipefitting trade. With the invention of air conditioning, radiators were replaced with two to four pipe fan coil systems and existing pipe chases were upgraded to handle chilled water as well as hydronic heat. This expertise and influence for water-based systems still exists today in the Northeast US.
Different factors influenced construction practices with the western expansion of the US. Both wood and land was plentiful. Demand for detached multi-room single-family homes grew at a rapid pace. It became common practice to duct heated air from the basement furnace to all the rooms – first by gravity and then forced air. Wood construction was common and the space between the wall studs and floor/ceiling joists became the ductwork. After WWII, contractors invested heavily in duct-fabrication to satisfy the new construction demand for housing and quick-build commercial applications. When the market for central air conditioning began during the 1960’s and 70’s, it was easy to add an “A” coil (evaporator) to the top of the furnace and add a condensing unit outside. Because residential and commercial practices influence each other, the use of a common duct for both heating and cooling carried over to all commercial building types as well. DX (direct expansion) Rooftop VAV (variable air volume) exploded in the 80’s as the demand for cost-efficient and fast installation increased dramatically.
Europe and Asia’s HVAC evolution was driven by different construction practices and constraints. Europe’s path was/is similar to that of the Northeast US. Older buildings (and there are a lot of them in Europe) allowed no provision for ductwork – masonry construction had no hollow walls. These buildings were heated using piped radiation systems. Contractors developed a strong pipe-fitting culture and engineers developed tremendous knowledge of designing water-based systems. Hence, non-ducted water-based systems are still preferred today.
Asian apartments were/are predominantly one or two room. Through the 1950’s, many were heated by a single kerosene heater. Again, construction was/is poured concrete or concrete blocks with no hollow walls. Central heating using hydronic radiators were rare. The window unit boom in the 1960’s and 70’s provided cooling and safer electric-strip heating in one device to satisfy the needs of these small living quarters that had no provision for piping or ductwork. In the late 1970’s and 1980’s, the evaporator/heating and compressor/condenser were separated into indoor and outdoor components– hence, mini-split. This was not only a quieter alternative to the ‘window-shaker’, but gave the owner back his/her outdoor view. The next logical evolution was multiple DX fan coils from one condensing unit for commercial and multi-family applications. VRF split systems with multiple indoor units were introduced for multi room applications.
Ducted or Non-ducted?
As the above history lesson shows, the driver for ducted or non-ducted has nothing to do with efficiency. The driver has to do with simple evolution of HVAC in relation to cultural building market drivers.
When deciding which system to use, each has advantages and disadvantages dependent upon application, climate, building construction, and other factors. Multi-family housing, dormitories, and hotel applications all are construction-types where a common air return is not permitted. This is the logical market for non-ducted products. VRF provides an alternative to hydronic fan coil systems, water-source heat pumps, and PTAC’s. Existing masonry buildings – where ducting may be too expensive to add – also is a sensible solution for non-ducted products. Several building types require a mix of system types. Health care applications are an excellent example of a mixed-use system approach (e.g., fan coils for patient rooms, ducted for common areas).
For other commercial applications – e.g., schools, education, offices, retail/restaurants, theaters, casinos, factories – a high performance ducted system makes sense from a total life-cycle cost standpoint. Ducted systems offer lower installed cost, better efficiency (e.g., cooling with compressor-free outside air) and code compliant ventilation within the same system.
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