Ductless HVAC Systems
Ductless HVAC systems, like rooftop systems, have been widely promoted by some for their low first costs and their ability to provide a quick and easy solution to building HVAC problems. And like rooftop units, they have been widely criticized by maintenance personnel as being difficult to maintain. Ductless HVAC systems, like all other HVAC systems, have their applications and their limitations. Properly applied, they can provide flexibility and improve comfort in a cost-effective way. Improperly applied, they can cause maintenance headaches, contribute to IAQ problems and cause comfort problems for building occupants.
Ductless HVAC systems come in a number of different configurations. Most are two- or four-pipe fan coil units that can provide both heating and cooling. More recently, units have been introduced that are essentially ductless heat pumps, with one portion of the unit mounted in the conditioned space, and the other mounted outside the facility.
The systems offer several advantages that go beyond low first costs. With no central air handler, less mechanical equipment space is required. Lower ceilings can be used since the units require no ductwork. With individual thermostats controlling each unit, spaces can be easily zoned. With all operating components, such as the fan, the thermostat, and the heating and cooling coils located in a single unit, modifications can be readily made to the HVAC system to match changes made in space configuration or use. For these reasons, split ductless systems are frequently found in educational and healthcare facilities, computer rooms, lobbies and building entryways.
The biggest drawback of ductless systems is their inability to supply the occupied space with fresh air. Some fan coil system designs did include a small outdoor air intake, but ongoing problems with the freezing of coils led many maintenance departments to block off these intakes. Split ductless systems do not supply the conditioned space with fresh air. Fresh air to these areas must be provided by a separate system.
Maintenance requirements are an important issue with ductless systems. Because the systems are so simple in their design, and the components so relatively small, maintenance requirements can easily be overlooked. But in spite of their simple design, ductless systems require regular maintenance. Filters must be changed. Condensate pans must be checked and cleaned. Fans and belts must be inspected. Controls must be tested. The problem for many maintenance managers is that the cost of performing the required maintenance may offset any cost advantage offered by the system. But overlooking the maintenance requirements will decrease system performance, shorten equipment life and adversely impact indoor air quality.
Nevertheless, there are good applications for ductless HVAC systems. Because each ductless unit is controlled by its own thermostat, conventional fan-coil-based ductless systems provide an unlimited level of zone control, ideal for applications where individual room control is required. Split ductless systems are also suitable for meeting the HVAC needs of spaces having unusually high cooling loads, such as rooms housing telecommunications or computer equipment.
