Many owners of older homes that are without forced-air heating, ventilation, and air conditioning systems would like to add central air conditioning or heating ducts, but are cannot due to their high cost and the difficulty associated with the installation process. Installation of ductwork may require removing large sections of walls, floors or ceilings, or adding unsightly chases. With high velocity heating and cooling systems home owners can reduce or even eliminate these alterations by using small diameter ducts that can be more easily installed through floor, ceiling, and wall cavities. Manufacturers of these high velocity systems assert that they operate quietly and improve dehumidification, air mixing, and energy efficiency over standard air-delivery duct systems.
High velocity heating and cooling systems use a special fan coil and air handling unit that generates high pressure air forced through small diameter ducts. The main supply trunk can be a rectangular or round duct that supplies air to flexible, insulated, 2″ diameter plastic feeder ducts. Air passes through sound attenuating tubing at the end of a duct run before entering the room through a plastic collar fitting. Supply air is delivered into the room at 440 to 1200 cubic feet per minute, CFM.
These systems are also called Small Duct High Velocity (SDHV) systems which use a traditional outdoor condensing unit for air conditioning and heat pump systems. As an option, a water coil can be mounted in the air handling unit for boiler heating or even chilled water cooling, even though the later is less common. Installers may also add electric heating coils to the air handler which can provide a supplemental heating capability to a high velocity air-conditioning system. High velocity systems can satisfy cooling capacities between 18,000 and 60,000 BTUH, 1 ½ to 5 tons, and heating capacities between 24,000 and 143,000 BTUH.
Small Duct High Velocity, SDHV, systems can be installed in any type of home, whether it is a remodel or new home construction. Most SDHV systems remove approximately 30% more moisture in the cooling mode than conventional systems. This results in cooler, dryer air to create lower humidity levels in the home that create a more comfortable environment while the temperature may be higher.
Savings and System Benefits
Manufacturers of high velocity units claim that dehumidification with high-velocity air distribution is better than for conventional forced-air systems. In the air conditioning season, lower humidity levels can allow occupants to feel comfortable with the thermostat set at higher temperature, resulting in reduced energy costs. In both heating and cooling seasons, delivering air at higher velocity through smaller supply lines mixes the room air more effectively, reducing temperature stratification (separate layers of conditioned and unconditioned air) and eliminating the need to place duct outlets below windows or along exterior walls to avoid hot or cold spots.
In existing homes, the 2″ supply lines can eliminate the need for structural alterations. This can be desirable in historic homes where existing features, like plaster moldings, would be difficult to change or duplicate. For this reason, high-velocity systems can be the most cost-effective option for retrofits, although they may be more expensive than conventional ductwork in new construction.
A Word about Proper Unit Sizing for Dehumidification
An air conditioner’s ability to remove moisture increases when the equipment runs for longer periods of time. At the beginning of every cycle in hot moist climates, the air conditioner puts moisture into the house as water is evaporated off the inside coil. The air coming off of the cooling coil is actually saturated; meaning the air’s relative humidity is 100%. Since a smaller air conditioner runs longer to keep the house at the desired temperature, it removes more moisture than a larger unit would be able to achieve.
A 5-ton unit running for five minutes would remove 1.4 pounds of water, while a 2.5-ton air conditioner, in the same house, running for ten minutes would remove 1.7 pounds of moisture. The amount of moisture removed is not only a function of how long the air conditioner runs, but also it’s Sensible Heat Ratio (SHR) which describes the percentage of the total capacity delivered as lower house temperature.
A low Sensible Heat Ratio will result in more moisture removal. For hot wet climates where moisture removal is important, such as in the South andSoutheastern United States, air flow across the coil should be reduced slightly to increase the SHR and the air conditioner condensing unit and indoor coil combination should be chosen to have a low SHR. Your heating and air conditioning contractor will have to select the appropriate indoor coil if you don’t use the outdoor unit that is matched to the indoor (evaporator) coil. If the indoor and outdoor coils are not matched, he cannot use the manufacturer’s published SHR for the coil.
High velocity air conditioning systems offer an energy efficient alternative to traditional air condition air handler configurations that will also improve the comfort of your home. Even though the cost may be slightly higher, you can save on disruption and messy demolition of walls and ceilings.