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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.

Heat-Transfer Efficiency

High-velocity heating systems are typically run at the same 150-180°F temperature range as conventional systems. In some homes, though, we use a standard water heater as the heat source, rather than a conventional boiler, so the system can provide domestic hot water as well as heat. In that case, we lower the operating temperature to 140°F or even 120°F and install a tempering valve to prevent scalding.

Although Energy Saving Products recommends using a water heater as a heat source only for spaces under 1,400 square feet, we’ve used this approach successfully in well-insulated larger spaces as well. But if you’re going to use a water heater, it’s important to take that into account during the design phase, because the lower temperatures mean fewer Btuhs per outlet, so more outlets will be needed.

On the cooling side. The rule of thumb for conventional air-conditioning systems is that it takes 400 cfm of forced air per ton of load to properly evaporate the refrigerant and provide effective heat transfer. In a high-velocity system, though, the same level of cooling can be achieved at a flow of 250 cfm, thanks to the higher static pressure in the ductwork. In effect, the air is packed more tightly, so fewer cubic feet have to pass through the cooling coil to remove an equivalent amount of heat. Because the air stays in contact with the coil for longer than it does in a conventional system, high-velocity systems remove more humidity from the air, leading to greater comfort even at higher operating temperatures.

Condensing unit. We can use just about any maker’s condenser with the air handler, which arrives ready for both heating and cooling with no additional relays required. This allows us to tailor each system to the specific needs of our customers. Some want quieter units, some want higher efficiency; others don’t require as much cooling and simply want to keep costs down.

The unit of measure used to define the efficiency of different air-conditioning units is the SEER, or seasonal electrical energy rating. Commonly available units are rated at 10, while the highestperforming units rate as high as 18. (These ratings don’t represent the total theoretical range, but rather what the market supplies. And while the most efficient units are rated as high as 18, in practice you can only reach level 14 on a split, fieldinstalled system.) A higher SEER value becomes more important in warmer climates with high electrical costs.

Minimizing system noise is another important consideration. The higher the SEER number, the bigger the unit becomes and the slower the fan has to turn, resulting in much quieter operation. An insulating blanket on the compressor also helps to dampen noise. To balance noise and comfort, we usually recommend a unit rated at 12 SEER in our southern New Hampshire area.

Duct Runs

In a high-velocity system, the complicated supply-side trunk system is replaced by a simple plenum where pressurized air is gathered, stored, and distributed equally to all outlets. In most of our installations, that main run is a constant diameter throughout — typically 8-inchdiameter, 28-gauge metal duct.

The system’s reliance on constant air pressure, rather than air volume, mandates absolutely airtight ductwork. We carefully seal every joint and plenum-tobranch connection with duct tape that’s been rated by SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association). We find that a visual inspection is really all it takes to confirm joint tightness. The core duct is metal, wrapped in an insulating sleeve with a vapor-proof cover. We also completely tape the vapor barrier to prevent condensation from forming on the ductwork.

In high-velocity’s infancy, one of the problems noted was a tendency of some outlets to whistle in service. To prevent noise generation at the outlets, experimentation has proven that the 2-inchdiameter branch ducts must be a minimum of 10 feet long. When the distance from plenum to outlet is less than 10 feet, the flexi-duct can simply be coiled back upon itself to provide the minimum 10-foot length.

To prevent distortion and reduced air flow, we limit bends in the flexi-duct to a 6-inch radius. However, branch runs must also be no longer than 25 feet. Note that the Btuh output of branch runs between 15 and 25 feet long is reduced. During the planning stage, it’s necessary to deduct 10% from the output of a 15-foot branch run, 20% from a 20-foot run, and 35% from a 25-foot run. This restriction plays an important part in the layout of the plenum, and it may also necessitate increasing the number of vent outlets in a particular area to provide enough heating or cooling.

Minimal structural changes are called for to accommodate branch ducts because the flexible duct is small enough to fit within conventional wall framing and can easily wrap around obstacles.

Return air. Unlike the supply side, the return side of a high-velocity system uses conventional rectangular ductwork. A sizing chart directs the system design and must be followed to the letter. The pressurizing supply side and the depressurizing return side must be balanced within a 10% plus-or-minus variance. An oversized return duct will prevent the blower from producing enough static pressure to support the highvelocity effect.

Proper sizing of the return ducts and grilles also helps to prevent noise. We find that sizing the return grilles generously will keep things quiet. If a particular fancoil has a recommended return air size of 113 square inches, for example, we prefer to install a grille that is 10% to 20% larger — 124 to 135 square inches — to prevent velocity noise.

Makeup air. In a conditioned building envelope, air displacement follows two routes: exfiltration and infiltration. Conditioned air escapes, or exfiltrates, by every available path — through seams, gaps, open doors and windows, kitchen and bath exhaust ducts, etc. To replace the escaping air, fresh air must infiltrate the envelope by similar means. Infiltrating air is usually cold in the winter and is contaminated by airborne dirt and pollutants.

To create a controlled system, we introduce an exterior-mounted passive air inlet into the return duct. This makeup air is drawn through the HEPA filters, across the cooling or heating coil, and distributed throughout the house. We adjust the inlet opening to account for roughly 10% of the return air supply, which is controlled by a simple blast gate. The high-velocity system produces a slightly positive indoor pressure, effectively blocking infiltration drafts.

Room Outlets

Room outlets can be located on the floor or ceiling, on interior or exterior walls, and even in kickspaces. But careful placement of the outlets is important for homeowner comfort.

Under the influence. Each room outlet has a “zone of influence” that’s about 8 inches in diameter and can be felt at a distance of 4 to 5 feet from the outlet. These streams of moving air keep the room air circulating to prevent stratification, but they shouldn’t be allowed to play directly against the home’s occupants. Moving air has a chilling effect (that’s why fans keep you cool and weather forecasters like to talk about the wind-chill factor), so a stream of cool air can seem uncomfortably cold, while a stream of heated air may not seem warm enough.

For the sake of comfort, outlets should be located in room corners, out of main traffic patterns. We avoid installing kickspace outlets below a sink or range, or in any other area where they’re likely to blow on someone’s feet. We also avoid areas likely to be obstructed by furniture, drapes, rugs, plants, or other objects. With proper planning, the occupants will have no sensation of air movement. Broad outlet distribution is preferable, but not essential, for proper function. We’ve clustered three outlets within an existing abandoned floor duct with complete success.

Thorp Thomas owns and operates Heatkits, Inc., in Exeter, N.H.