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Installing High-Velocity HVAC

Installing High-Velocity HVAC

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    Continuously blowing outlets provide heating and cooling and can be installed high or low on the wall, as well as in the ceiling or floor.

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    Placement isn't as important as providing an accurate number of outlets per room and avoiding blockage.

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    The outlet grid design imposes laminar air flow, correcting noisy vents.

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    The system's air handler, or blower unit, can be installed in a horizontal, hi-boy, or counterflow attitude and located to maximize usable floor space. A conventional system isn't as lenient in its placement.

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    An operating temperature of 120° to 140°F permits use of a standard water heater to supply the fancoil unit. Slower passage of air over a larger than conventional coil surface enables higher coil-to-air heat transfer. Because of the lower temperatures, all ductwork must be completely insulated to retain operating efficiency.

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    A single-diameter, minimally sized plenum stores and distributes pressurized air to the branch ducts at an equal rate. A conventional system relies on progressive step-downs in the size of the main trunk to maintain a relatively equal volume of airflow at each vent, making installation far more labor intensive.

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    Airtight ductwork is absolutely essential to the function of a high-velocity system. The author seals every joint in the ductwork with a clear, SMACNA-rated tape. Careful and attentive application ensures an airtight joint and makes leak testing unnecessary.

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    Branch ducts must be no shorter than 10 feet long and no longer than 25 feet. The flexi-duct can be coiled back upon itself to provide the minimum duct length, with no reduction in performance.

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    The author uses a standard blast gate to regulate fresh, outdoor makeup air entering the return plenum at a rate of about 10% of the total volume. Return air is drawn through a HEPA filter and conditioned before being returned to the living space.

For years, I was a staunch radiant heating installer who picked up a few air-conditioning jobs to fill in the slow summer months in New Hampshire. But a growing demand from my customers for year-round indoor climate control led me to look for a system that would accomplish heating, ventilation, and air conditioning in one package. I wasn't willing to make the compromises imposed by conventional systems, in which heating and cooling needs are practically in conflict -- warm air rises and cool air settles, leaving no optimal place to locate the vents to satisfy both functions. I wanted a system that would work equally well in a chicken coop, a standard home, and a multimillion-dollar house. My search led me to the high-velocity system, a technology refined in the mid-1990s to its current reliable and efficient performance. Over the past six years, my crew and I have installed more than 300 high-velocity systems in both new and existing homes.

High Velocity vs. Conventional Forced Air

The most obvious difference between a high-velocity system and a conventional forced-air system concerns their basic operating cycles. In a conventional system, the air handler and boiler or condenser cycles on and off as the thermostat signals for heating or cooling, depending on the season. In a high-velocity system, on the other hand, the heating and cooling equipment cycles on and off as needed, but the air handler runs continuously to maintain pressure within the ductwork. This pressurized air is delivered through inconspicuous room outlets at speeds high enough to create a slipstream effect that keeps the room air thoroughly mixed. This eliminates the stratification, or hot and cold spots, that can develop between operating cycles in a conventional forced-air system. The air temperature maintained by high-velocity conditioning varies by only about 1°F between the floor and ceiling of the average home. The constantly running blower is said to use about the same amount of energy as a 100-watt light bulb.

Clean and quiet. Blower units are powered by energy-efficient 1/4- to 3/4-hp motors that generate little direct noise. A constant supply of fresh, outdoor air drawn into the return duct and pulled through high-grade HEPA filtration offers an exceptional degree of indoor air quality. Ambient dust is gradually eliminated from the home's interior once the high-velocity system is activated and never has a chance to settle in the ductwork due to the constant airflow.

Simple installation. High-velocity systems are also remarkably easy to install. Most of that ease comes from the simple ductwork; insulated, flexible ducts branch off a uniformly sized plenum, or main trunk. The branch ducts are small enough to fit within conventional framing tolerances, eliminating the need for most framing modifications. The plenum may be installed in the basement, crawlspace, or attic and usually consists of a single-diameter run. Because velocity is developed at the outlets, bends and elbows in the ductwork have relatively little effect on the airflow -- a key difference from conventional forced-air systems, in which every elbow must be factored in and deducted from the total allowable run.

System Sizing

The company we work with, Energy Saving Products (888/652-2219, www.hi-velocity.com), manufactures the Hi-Velocity Soft-Aire system and offers five different air handlers of increasing capacity. Each of the available air handler models is rated to supply a minimum and a maximum number of outlets. For example, the HV-30 unit supplies a minimum of 8 outlets and a maximum of 14; the HV-140 supplies a minimum 40 outlets and a maximum of 50.

As with any heating or cooling system, the first order of business is to calculate the heat loss and gain for the whole house and for each room in the conditioned envelope. With that information in hand, we select the appropriate air handler from the chart provided by the manufacturer, for the calculated load.

Room outlets. To calculate the number of room outlets for a heating- only system, we divide the whole-house Btuh heating load by the minimum number of outlets for the selected air handler. This provides the number of Btuh delivered by each outlet. Once we have this information, finding the right number of outlets for any given room is basically a matter of dividing the room load by the per-outlet figure and rounding the result up to the nearest whole number.

In a dual system — one that will handle both heating and cooling — the number of outlets calculated for one function may differ from the number for the other. In our area, such a discrepancy usually occurs because more outlets are required for cooling. That’s not a problem, though — we simply default to the cooling figure, which has no adverse effect on the heating side.