In which the authors take out the original unit designed specifically for K-12 and see how things have changed in the decades since it first went to school. Time and technology have solved some problems, yet an old-fashioned issue like ease of maintenance can still pose headaches (or backaches). All in all, this update about a well-known design option deserves a permanent spot in your three-ring reference binder.
For 70 or 80 years, the classroom unit ventilator had been a widely accepted solution for school heating and cooling. But operational and comfort problems arose, and the unit ventilator (UV) fell from favor. It does still have a place in modern school design though, providing that the problems can be solved and all codes can be addressed. This article discusses the perceived UV issues and one designer’s successful solutions.
A Short History LessonHerman Nelson invented the UV in 1917. It consisted of little more than a hole in the wall, a fan, and a steam radiator. It was designed to address a recognized problem of that era, namely that there was insufficient fresh air being introduced into schools and that it was better to bring the air in through an engineered system rather than to allow infiltration, open windows, and drafts. It was the first and maybe only HVAC equipment designed specifically and solely for schools.
In the years since 1917, cooling was added in many variations (chilled water, split DX, self-contained DX), and all possible heating sources (steam, hot water, resistance electric, and air and water source heat pump including geothermal) were available. The two big advantages of a UV school were (are) that they’re less expensive to build mechanically, and that they’re less expensive in the general construction since the building can be shorter as headroom for big ducts isn’t required.
By exact definition, a unit ventilator differs from a fancoil by virtue of the unit vent’s ability to introduce up to 100% outside air for economizer cooling. Most unit ventilator manufacturers offer multiple options and configurations, such that engineers have almost as much design flexibility with UVs as they do with air handlers. In addition to the traditional “under the window” cabinet-style unit vents (Figure 1), modern unit vents are vertical with ducted air distribution (Figure 2).
The PrerequisitesIrrespective of whether one HVAC solution is less costly than another, in order for any solution to be truly viable, it must deliver consistent IAQ, occupant comfort, and humidity control. Only after these primary issues have been adequately and thoroughly addressed should secondary items such as construction cost, operating economy, longevity, and maintainability be considered.
Another critical prerequisite is digital control. Many of the age-old UV problems can be solved by judicious application of modern DDC technology.
The Issues and The SolutionsIAQ (consistent fresh air). There are several aspects to this, involving equipment design, building system design, and construction issues. Theoretically, setting an outside air damper to provide the appropriate outside air should be easy and very repeatable. However, the old under-the window-unit-ventilators (UWUVs) have long damper shafts that are prone to twisting and binding, and fresh air becomes inconsistent. The modern vertical-unit-ventilator (VUV) has a much better aspect ratio (length vs. width), which means that damper issues are solved. There is also the airflow difference frequently seen on old UVs when the air filter would load up. This concern can be addressed by the use of constant torque motors that will automatically compensate for changes in suction-side static.
Two building system design issues deserve comment. The UWUVs typically have louvers that are close to the ground and they can be clogged by lawn clippings or snow drifts. Also, UVs typically have low static fans (unlike the high static fans of most air handlers) and they’re not capable of pushing excess/relief air out of the building. The VUVs typically have outside air louvers that are mounted much higher on the wall and are less prone to clogging. The relief air issues must be addressed with powered, not gravity, relief.
The construction concerns involve installation details.Most UWUVs are set against the outside wall and sealed with a foam gasket against a block wall. The straightness, squareness, and levelness of an 8- by 2.5- by 1.5-ft section of rough construction will have a huge impact on whether fresh air enters as intended, or room air leaks into the outside air (O/A) chamber because of an incomplete seal. This lack of proper sealing of the O/A chamber is a prime reason why many UVs do not provide adequate economizer cooling. The VUVs have ducted connections to the O/A louvers.
Poor air distribution and short circuiting of supply air.UWUVs have a single air outlet that is meant to make the air sweep across the ceiling to condition the whole room. This works very well in heating, as the outside wall and the roof are where the heat losses occur. It works adequately with some room configurations and poorly with others. And the concept can/will be terminated when the teacher piles books on top of the unit or hangs banners from the ceiling. A summer concern is the short-circuiting of conditioned air from the unit discharge back to the return because cold air drops. The short-circuiting gets worse if the fan speed is reduced for noise or capacity reasons. The VUVs have ducted air distribution to ceiling diffusers and can easily accommodate any room configuration with no short-circuiting.
Humidity control. Humidity control, or lack of it, is a concern any time the control scheme is “constant volume, valve control.” A school may be slightly more susceptible to this than other building types due to the higher outside air fraction required by the population density. A UV would be no different than an air handler or a fancoil if the control was constant volume, valve control. Modern UVs have either face/bypass (F/BP), reheat, or some sort of O/A preconditioning. The authors prefer F/BP as the least costly from both construction and operating standpoint. The F/BP approach, when properly applied, will keep space humidity below 60%, even in high humidity regions of the country.
Classroom noise. ANSI Standard S12.60 recommends that classroom equipment noise levels be kept below 35 dB in order to not interfere with student hearing. From a qualitative standpoint, 35dB is a very low number and a very quiet space. It is probably impossible to achieve with any system that has a single, large high-velocity outlet at ear level (UWUV). This requirement would probably also negate using anything with a compressor in the room, such as self-contained DX or heat pumps. Curtains, acoustical ceilings, and carpet will help attenuate the equipment noise. The solution is to duct the air distribution and design the VUV unit with a large, slow fan and well-insulated cabinet. Additionally, noise from both UWUVs and VUVs will be significantly lowered when fan speed modulation is used as a primary control scheme, meaning that the UVs will operate at medium or low speed (lower noise) most of the time.
Filtration efficiency.ASHRAE Standard 52.2 recommends a minimum filtration of MERV 6. Older UVs had 1-in. media filters with an approximate rating of MERV 2. The added static pressure drop associated with a higher filter rating may significantly affect airflow, and may be difficult to get with the low static fans in UWUVs. Again, the VUV has a fan that has more static capability and can meet the filter requirement.
Freeze-ups. This big complaint and trouble was a result of several problems, a prime one previously mentioned (leaky dampers), and all of them easily solvable by using F/BP. Years ago, some engineers felt there was a significant benefit to using a blow-through coil configuration rather than draw-through. Blow-through mixed the air before it came to the coil and minimized stratification (another prime freeze cause). However, a bigger freeze cause is valve control of the hot water coil. In a UWUV, the coil is 5 or 6 ft long and fed from one end. The room thermostat control is based on average temperature, and it was not uncommon to see temperature differences of 30°F or 40° from one end of the coil to the other.
Maintenance.A big plus for UVs vs. a central system such as VAV is that an outage will only affect one room, not the whole building. However, a big negative is that UWUVs are hard to work on when things do need maintenance. All work has to be done either kneeling or lying on the floor. The end compartments of UWUVs will get very congested with piping and controllers, making that equipment hard to get to (this is another area where F/BP offers advantages with fewer devices to pipe up). The VUV tends to have a lot more room for piping and controls, and all work can be done by one mechanic standing on the floor.
ConclusionThe UV still has a place in school design, but not the way it was done 20 years ago. The UV should be considered when doing renovations of existing UV schools. The UV should be strongly considered for buildings that have insufficient headroom for central air handler ductwork. And when properly applied, the UV can be less expensive to build, less expensive to operate and easier to maintain than other options, without compromising IAQ, occupant comfort, or humidity control. ES
Sidebar: Operation of a VUV
- Full-flow chilled water coil.
- Face and bypass dampers modulate by thermostatic control. The dampers are asymmetrical to compensate for the air pressure drop differential between face and bypass operation.
- The design is return air bypass. The chilled water coil is positioned so that the humid outdoor air preferentially flows through the chilled water coil. Mixing of the outdoor air and return air is minimized by the use of parallel blade outdoor air/return air dampers.
- The fan speed reduces as the room temperature reaches setpoint. If the fan motor is a PSC type, this is accomplished by stepping down through three fan speeds. If the motor is an ECM type, the speed reduction is through an analog signal. Design engineers should be aware of the very high aftermarket replacement cost of ECM motors and the potential burden imposed on the owner for future maintenance costs.
- As the supply fan speed reduces, the outdoor air damper is opened to maintain the ventilation rate. Alternatively the outdoor air/return air dampers can be decoupled from the controller and responsibility for outdoor air volume given to a CO2 sensor.
- 100% economizer function is included. This is an extremely important feature of the unit ventilator in terms of energy savings and is often not a function of other HVAC system designs.
Q: I am planning to replace some UWUV units with the VUV type. How is this done?
A: There are two methods:
- Remove the old UWUV and block off the wall opening. Use an insulated window panel to install a new intake louver for use with the VUV.
- Remove the UWUV and build bookshelves with an insulated plenum behind. Use the plenum for outdoor air intake from the existing wall louver into a rear plenum on the back of the new VUV.
A: By ensuring that a quick chilled water (CHW)/hot water (HW) automatic changeover function is incorporated in the design, the two-pipe system is an excellent choice. An operating benefit is also gained in that the HW loop can be kept at a much lower temperature than the four-pipe system because the coil in the VUV is considerably larger in a two-pipe unit. Depending on outdoor air temperature, the HW loop can be in the 90°F to 130° range, which results in significantly lower parasitic heat loss from piping compared to a conventional 180° loop.
Q: Why use face and bypass damper control instead of a modulating CHW valve?
A:A face and bypass damper system provides superior temperature and humidity control and allows full flow of CHW through the coil to ensure that the air passing through the coil reaches dewpoint at the fin surface. With modulating valve control, on a day that is cool but humid, the valve will be positioned by the PID control loop so that the room drybulb temperature is satisfied but there might be little or no condensate formed at the coil because of the low flow rate. And in winter, unless the water flow is completely stopped, it is virtually impossible to freeze a F/BP coil.
Q: I am designing for the climatic conditions found in Florida and the other states on the Gulf Coast. Is the unit ventilator suitable for this application?
A: Yes, with a variation. Economizer cooling is of no value in these regions. Specify a unit which has a dedicated outdoor air pretreatment coil included, in addition to the primary chilled water coil. This type of dual-path unit has been used in Florida since 1998 with great success. These units can handle up to 450 cfm of outdoor air.
Q: Is factory mounting of DDC an option? Can a condensate pump be included for rooms that are below grade?
A:Factory mounting of controls is much easier in the VUV than in the UWUV because the layout of the components in the VUV provides better accessibility. A condensate pump can be factory-installed in the VUV.