High Velocity Air Handlers, (Hi Vee Units)
One might think that the name describes the way these units operate, but a more appropriate name would be high pressure units. A much higher air pressure is found in these systems and that is why this type of heating and cooling unit can use smaller ducts than traditional low velocity, low pressure duct systems. The original units were developed for what appeared to be a niche market at the time where every inch of space saved in a residential unit meant more money for the developers. With the Hi Vee units a 2" duct can deliver the same amount of air a traditional duct twice the size. All branch ducts in Hi Vee systems are flexible and very quick to install and in an age where skilled trades are earning big dollars it is important to speed up the install and reduce labor costs. With Hi Vee systems it is possible to feel the air flow up to 20 feet away from the outlet. Each 2" outlet that can be installed in a ceiling, floor or wall will deliver a little over 30 Cubic Feet of Air per Minute (CFM) and the air requirements for a particular room may be much higher so multiple outlets are required. If there was a single concern about Hi Vee units it would be that they are noisier than traditional systems. Even with the new variable speed motors that speed up or slow down based upon pressure in the duct there is a noticeable air noise.
The market for Hi Vee units has expanded due to the flexibility of the system and is now used in all HVAC applications from commercial and industrial to residential. The fan cabinet can be matched with hot water heating coils or electric resistance heaters. The cooling system can be a conventional split system with air cooled condenser to a chilled water coil from a central system. The air handler itself can be mounted horizontal, vertical or even upside down in a counter-flow position. Zone control can even be added to the system with motorized dampers and any of the many programmable thermostats can be used with the system. Humidification can be added to the system if there is sufficient space in the mechanical room and the only difference in this option is that the bypass duct for the humidifier must be the same size as a single branch outlet (2") and the outlet from the start of the supply duct is recommended to be at least 24" from the air handler to reduce turbulence in the duct. The bypass of the humidifier must also have a damper that must be closed during cooling operation to prevent freeze up problems. The Hi Vee systems will also accept options such as freeze stats or anti ice controls, high efficiency filters and ultra violet air purifiers.
If there was a limitation to these units it would be their capacity for add on cooling. Some units ate limited to a maximum of 3 tons of add on cooling and a few can deliver sufficient air flow for up to 5 tons. This is more than enough for the average house built to new construction standards for insulation. Multiple units can be added for larger capacity needs. This type of system has virtually replaced the central system using chillers and large boilers in the condominium market giving each owner control of their own system, energy costs and maintenance. It promotes conservation over central systems that serve an entire building and should reduce the condo fees for common elements service and maintenance that would be required on central systems. The manufacturer of a system is chosen by the builder in new construction of condos, but if buying a single or a few units for your single family dwelling or office, be sure to select a unit with a proven track record and one that your service contractor will have ready access to parts should you experience a breakdown.