Vertical configuration unit ventilators

September 12, 2000
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Most everyone is familiar with classroom unit ventilators. We in the hvac industry know them as a product class; others remember them from their school days as the steel box under the window that regurgitated noisy heat on winter days.

They made a great warm seat, and produced a satisfying “boom” as swinging legs crashed into the front service panels.

Most students never knew that unit not only provided heat but also introduced outside air for ventilation. Over the last 100 years, ventilation requirements per student have fluctuated from 0 to 30 cfm, down to 10 cfm in the 1920s, dropping to 5 cfm as a result of the 1970s energy crunch and back to today’s 15 cfm per student.

Today, standard under-the-window unit ventilators are still used in both new construction and renovation. However, they are being joined by a new generation of vertical configuration unit ventilators (vcuv).

Like standard under-window units, vcuv’s are installed on the outside wall of the classroom. Older schools relied on expanses of windows running the length of the outside wall to allow for natural light. This also provided a location for a standard under-the-window ventilator.

Smaller footprint

Today’s schools are more often built with solid, well-insulated exterior walls with one or two escape windows and rely on efficient electric lighting. The floor-to-window exterior walls provide an excellent location for the installation of vcuv’s.

Existing schools of the older design are often energy-updated by removing old, inefficient windows and replacing them with insulated panels, which make them good candidates for vcuv’s.

The new vcuv’s offer advantages over their predecessors.

With the addition of computers, TVs, VCRs, and more students, classroom floor space is at a premium. The standard unit ventilator takes up 16 sq ft of floor space, and with self-contained cooling, as much as 34 sq ft. They also create between 56 to 103 sq ft of dead wall-window area above the unit. The new vcuv with a/c takes up 4.5 sq ft of floor area and uses 21 sq ft of available wall-window area.

In addition, the vcuv provides improved air distribution. Conditioned supply air leaves the unit at about 7 ft above the floor through front double-deflective grilles and side grilles. The airstream passes above seated students so as not to subject them to the discomfort of a wind chill effect from the moving air.

Return air is drawn back to the unit closer to the floor, providing complete circulation and reducing stratification. The effectiveness of the distribution is easily seen in classrooms by the movement of mobiles, flags, or artwork hanging in the four corners of the room.

While direct distribution is cost effective and works well, some designers prefer to use a limited supply duct system. Typically, flex duct running from a unit plenum above a dropped ceiling to four ceiling supply diffusers is used, or a single duct with registers concealed in a soffit running above the windows the length of the outside wall.

Three types

Vcuv manufacturers offer equipment lines that can be roughly broken down into three types:

Terminal equipment is connected to the building’s central boiler and/or chiller, and acts as a fresh air mixing and conditioned air-distribution system within the classroom. While these are the lowest-cost vcuv’s, they are the highest-priced systems due to the cost of the central equipment and labor involved in the piping systems.

Vcuv’s can be ordered with combination coils for two-pipe systems or heating-cooling coils for four-pipe systems.

Hybrid vcuv’s are matched to the central boiler for heating, except they have self-contained or remote condensing units for cooling. These types of vcuv’s are typically seen in renovation projects. The existing boiler is reconditioned; at the same time, the unit ventilators are replaced and upgraded to add air conditioning using a vcuv with self-contained cooling or a remote condensing unit.

Many new schools, particularly in northern climates, are designed from a “wish list” that includes mechanical cooling. However, when that list meets the budget, cooling becomes the first casualty. In these cases, a vcuv with evaporator package can be specified. An outdoor condensing unit and high-voltage service can then be installed at a later date when additional money becomes available.

Stand-alone equipment reduces the initial installation cost by eliminating the need for boilers, chillers, boiler rooms, and the associated piping. They rely on electric heat, heat pumps, or a gas furnace to supply heat on a classroom-by-classroom basis.

When air conditioning is requested, it is either a self-contained DX system or a remote, outdoor condensing unit.

The gas furnace option has been a relatively recent addition. Gas-fired vcuv’s must meet all of the requirements of the National Life Safety Code (NFPA-101) and National Fuel Gas Code (NFPA-54), and should be specifically designed for classroom applications.

Installing a residential gas furnace in a classroom is not recommended, and typically is not supported by the furnace maker as an acceptable application.

The low operating cost and comparatively low installation cost of gas-heat vcuv’s have proven popular with schools that cannot afford the high cost of replacing a failed boiler and rotted or plugged asbestos-covered pipes. They are also an excellent choice for room additions when a boiler and chiller cannot support additional space.

In addition, should a component fail, stand-alone systems have the advantage of shutting down only the affected classroom and not the entire school.

When looks count

While vcuv’s are more aesthetically acceptable than outdoor, wall-hung products, some architects and designers dislike the exterior grilles required by unit ventilators, particularly the large grilles required by self-contained cooling units.

These can be hidden to some extent by matching the color of the grille with the surrounding brick or exterior surface.

An alternative is to use terminal units with a central boiler and chiller, or the hybrid units with remote condensing units. These units can be installed in the corner of an inside wall backing up to a hallway, and with a partition wall separating it from a unit in the corner of the neighboring classroom.

In this location, a single duct dropped from the roof can provide ventilation air to both units, while a chase to the roof will carry condensate and liquid lines to a pair of roof-mounted condensing units.

For central systems, the hot and chilled water supply and return piping are run in the ceiling of the hallway, reducing piping cost and installation labor.

Maintenance and repair are also major concerns for most schools. The indoor location, reliance on off-the-shelf parts, and reduced repairs from vandalism make the vcuv an appropriate choice. Service contractors appreciate the ease of maintenance and repair when compared to servicing roof or outdoor wall-mounted equipment during rain, snowstorms, or 100°F heat.

School district maintenance directors appreciate the reduced roof leaks often associated with roof-penetrating equipment, or damage to roof systems caused by maintenance personnel walking on the roof.

Finally, the indoor location reduces the opportunity of equipment becoming the target of vandalism to which outdoor wall-hung or rooftop equipment is often subjected.

Operation of vcuv’s can be as simple as a standard wall thermostat, with manual selection of heating or cooling, or a more sophisticated setback thermostat with a timer or occupancy sensor to determine occupied or unoccupied periods.

Sophisticated microprocessor controls are available with 10-year battery back-up, power-outage-proof programming, automatic changeover from heating to cooling, economizer cooling cycles, pre- and post-conditioning cycles, air quality monitoring, mixed air, discharge air and outside air sensors, filter-cleaning schedules, and even school calendar scheduling.

Most manufacturers also offer factory installation of controls from third-party controls companies that may be specified to interact with existing building management systems.

Although vcuv’s are not the final answer to school environmental conditioning, they do provide the designer with another option for a cost-effective means to meet the current ventilation requirements.

Unfortunately, their use will leave a generation of students who will not know the pleasure of a warm window seat, the boom of heels crashing into cabinet panels, or a handy grille in which to feed pencils.

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