A Breath of Fresh Air (March 2000)
The reports repeatedly point to conditions that are now known to be dangerous but weren’t considered a problem years ago, such as the presence of asbestos or lead. In other instances, the reports point to human error, such as improper use and maintenance of equipment, which has led to problems in schools.
When it comes to equipment problems, the finger is frequently pointed at the ventilation system, which is often improperly used and/or maintained. Sometimes, however, the problem stems from a ventilation system that was improperly designed to begin with. Often the culprit in a poor design is an air intake that was improperly located; for example, being placed too close to a loading dock or bus stop. With a little care and consideration, engineers can avoid these problems and design the safest, healthiest ventilation systems possible for our schools.
Central Systems Gaining FavorWhen it comes to designing a healthy ventilation system for a school, many engineers favor a central system as opposed to a distributed system that utilizes unit ventilators located around the perimeter of the building. Chris Cummings, P.E., principal, R.D. Kimball Co. Inc. (Andover, MA) says there are several reasons why a central system is preferred.
“We’re finding a lot of unit ventilators that have been in place for 20 years or even longer have many components that have failed,” says Cummings. For example, an outdoor air damper may have failed in the open position, leading to a frozen coil and significant water damage in a classroom. An outdoor air damper could also fail in the closed position, which will cut off the ventilation air to the classroom. This condition can go unnoticed for a long period of time, leaving a classroom full of students without proper levels of fresh air.
What Cummings and R.D. Kimball Co. have been trying to do in the schools they work on is separate the ventilation component from the heating and cooling components. “We like to provide a central ventilation system, from which we can control where the outdoor air comes from and how the fresh air is filtered and tempered to meet the needs of the individual spaces,” Cummings said.
Another problem with unit ventilators, says Bill Turner, P.E., president, Turner Building Science (Harrison, ME) a wholly owned subsidiary of The H.L. Turner Group Inc. (Concord, NH) is that they’re at ground level and pull in all the pollutants at ground level, such as mold spores, dust, and pollen. “People like to park diesel buses around a building instead of parking it away from a building, so that creates problems, too,” says Turner.
Cummings likes to use central packaged air handlers that utilize 100% outdoor air. “We’ve been using packaged air-handling units with built-in air-to-air heat exchangers. This type of system has a few central locations where we bring in all the outdoor air, as opposed to the unit ventilator system with dozens of locations of fresh air intakes spread all around the building. The air is filtered, tempered, and delivered to the classrooms in the quantities that are needed.”
Exhaust from the classrooms is brought back to the central air-handling units where energy is recovered from the exhaust air and used to temper the incoming fresh ventilation air, helping to reduce energy costs. Cummings notes that this system is fairly expensive, but the payback is usually under five years. “With a unit ventilator system, it is not feasible to recover the energy from the exhaust air stream, and the opportunity to realize the energy savings is lost.”
Clint Hulsey, P.E., Hulsey Engineering (Lillian, TX) favors rooftop units, mainly because of the vandalism factor; they can’t be kicked or stolen. In addition, he believes there are often fewer polluting factors on the rooftop, so depending on the prevailing winds, the air is usually a little cleaner up there.
New Approach to Healthy VentilationTurner handles his central ventilation system design a bit differently. His firm has worked hard to come up with a displacement ventilation system, which supplies low velocity air near the floor and exhausts polluted air near the ceiling.
“This system does not mix clean fresh air with dirty air. It purposely stratifies the air in a building so all the bio-effluents from the people and the things they’re doing in the school leave in the exhaust. Occupants are bathed in fresh air that literally flows up around their bodies that’s been provided at the floor. That’s a radically different approach than most other designers,” notes Turner.
While it sounds as if this system might be more expensive, Turner notes that it only costs 3% to 5% more than a traditional ventilation system. “That means you need creative architecture and engineering, because you only get a limited budget. But based on some of the discussions we’ve had with companies in other industries, the amount of money we’re spending on ventilation systems in schools is in line with what other people are doing.”
Turner is very proud of the amount of research that went into designing the system. In order to make sure it worked, the company built its own office in Maine with a displacement system. They packed a conference room with 60 people per 1,000 sq ft with a 9-ft ceiling, which is twice the occupant density of a school classroom. And the system worked.
Additionally, the company built a mock-up elementary school classroom and proved again that it was possible to build a classroom and condition it, maintain it, and make it comfortable with this kind of system. After the first system was installed in a school, Turner says the company tested it extensively, trying to understand the limits of the design. “Since then we’ve done 12 more. With the approach that we’ve taken, we’re comfortable we can deliver a real quality product, but we’ve evolved that with time and experimentation. We just didn’t go read about it and do it,” Turner adds.
The Turner Group subsequently designed a 14,000-sq-ft home office building for themselves that utilizes these concepts successfully throughout the structure.
Thermal comfort and humidity are also large considerations in any system that Turner designs. If a school is unwilling to pay for air conditioning, he tries to convince them to spring for dehumidification. He says then at least the air is dehumidified before it’s delivered to the room, making it somewhat bearable.
The rest of the ventilation picture consists of proper filtration, according to Turner. He always provides at least minimum ASHRAE standards (dust spot of 30% to 40%) where filtration is concerned, and he usually tries to do better than that.
Location, Location, LocationNo matter which type of central ventilation system that is designed, the location of the air intake is critical. If the intake is placed where it will draw in fumes or other contaminants, it’s almost guaranteed that the building occupants will complain or even become sick.
Cummings notes that there are often few choices as to where an intake may be placed. “Especially in a city where there’s not a lot of open space, sometimes your options as to where you can locate your fresh air intake can be very limited. You have to try and find the best possible location available with the restrictions that exist. Often it’s not an easy choice.”
The goal is to find a clean source of fresh air that works within the structure and considers the impacts to the aesthetics of the building. Cummings says in addition to avoiding areas where there is a lot of vehicle exhaust, such as a bus drop off area, the fresh air intake needs to be located away from all ventilation system exhausts, plumbing vents, chimneys, dumpsters, kitchen exhausts, laboratory hood exhausts, emergency generator exhausts, and other potential sources of contaminants.
Sometimes locating an intake 20- to 30-ft away from a source of contaminated air is not enough. “I have seen cases where the exhaust from a chimney has traveled 100 ft across a roof, down a wall, and into a fresh air intake on the other side of a building,” says Cummings. “You have to consider how the building is constructed and where the prevailing winds will be coming from. Predicting the path of wind-entrained contaminants is very difficult.”
Turner adds that all the necessary guidelines for intake placement are located in Chapter 14 of ASHRAE Fundamentals.
Another important consideration is to design the air intake so that it doesn’t pull in water or snow. “If there is snow or rain, you want to make sure the intake has a place where the moisture can run out of the system without contaminating the duct liner,” says Turner. “In general, you need to insulate the outside air intake on the outside of the ductwork.”
Maintenance ConsiderationsWhen talking about proper ventilation systems, it’s absolutely necessary to also discuss maintenance. And, unfortunately, maintenance is sometimes in short supply in schools. As Hulsey notes, “Some of the schools have a good maintenance person who’s been there for a long time, and he’s pretty conscientious about his job. You can tell that he goes through and checks the units and washes the coils and changes the air filters.” Then there are the other schools that do the bare minimum, or less, to keep their schools maintained. Turner notes that an absolute “maintenance must” in any school is a walk-off, track-off mat system in the halls and at the exterior doors. “If you don’t have real good floor maintenance, one that uses high-efficiency vacuuming or auto scrubbers, you can design the best ventilation system you want, and the school is still going to have poor air quality and dirt everywhere. I think most custodial services around the country are on a learning curve as to what they need to do to provide good indoor air quality and proper cleaning of floors is on that curve.”
Hvac maintenance is absolutely critical as well. Turner says his company designs its systems so they’re readily accessible for maintenance; they aren’t hidden above a ceiling tile, and he even tries to avoid rooftops in cold climates. “We like somebody to be able to open a door and walk into a room and do the maintenance in a minimum amount of time, to the point where we even provide a maintenance checklist with the air handler when it’s commissioned, and we do require hvac commissioning for all our systems,” notes Turner.
“It’s been my experience over and over again that we spend somewhere in the range of $300- to $400/sq ft/year to have people come to work or kids go to school, and we have a hard time spending a couple of bucks a square foot to maintain the systems that provide the environmental comfort for those occupants. We need to rethink that,” he adds.
As schools become more aware of the potential problems that can arise from ventilation systems, let’s hope that they also give maintenance a closer look. As most engineers lament, the best designed system in the world will not function properly unless it’s properly maintained. ES
Sidebar Sometimes you make do with what you haveWhat’s an engineer to do if the school simply can’t come up with the money to fix its ventilation system? Just ask Clint Hulsey, P.E. He’s had to improvise on a number of occasions. He recalls an older building he worked on that didn’t have a ducted return air system. The school had a large central air handler, and they were trying to condition the whole building with it. However, with no ducted return air, there was virtually no fresh air or circulation.
Needless to say, the air quality wasn’t good, and the building smelled stale and musty. Not to mention that many of the rooms were never comfortable; if it was cold outside, it was cold in the rooms; if it was hot outside it was hot in the rooms. “When you’re going to have a central system you’ve got to come up with some zoning controls and a ducted return. That’s the only way to take care of it properly,” says Hulsey. The school had no money to fix the problem, however, so Hulsey was able to create six zones (due to the way the duct was arranged it was possible to add some zoning dampers). “We didn’t add a return air duct, because they couldn’t afford that. The chiller was also a little undersized, so we had to upsize it a bit. But that’s all we could do.”
Sidebar Sensor solution?It seems that mixed gas or volatile organic compound (VOC) sensors would be the solution to making sure a ventilation system is only pulling in good clean air. The logic is that if noxious fumes are pulled into the air intake, the sensor will automatically shut down the ventilation system, so the fumes are not spread throughout the building.
These sensors are problematic, however. According to Mike Schell, director of marketing and business development, Engelhard Sensors, Santa Barbara, CA, these sensors measure some VOCs plus a plethora of other compounds with no scaling or discrimination. “These sensors are broad band sensors that react differently to a wide range of gases. The biggest problem with the sensors is that they cannot really be calibrated to anything, and they have a strong tendency for short-term drift.”
Schell adds that in building controls, a sensor’s best use is to sense a catastrophic release of some compound that will cause a significant short-term change, allowing an alarm or some control strategy such as a ventilation purge to be used. “I would only see these type of sensors good for monitoring areas like a chemistry lab. They are not good sensors for monitoring or controlling general air quality.” Bill Turner, P.E., doesn’t think highly of VOC sensors either. “At the moment any VOC sensor that’s inexpensive is pretty much a joke. I don’t know what it measures, but I know it doesn’t measure it reliably. I have tried them in the past, and they were basically borderline useless. The units I tried had incredible interference problems, including moisture. Any sensor that detects moisture as an interference has got a problem.”
Sidebar Controlling outdoor sources of indoor air pollutionMany outdoor contaminants are the source of causing indoor air quality problems in schools. While it is often difficult to eliminate outdoor air pollution, it is not unreasonable to develop strategies for managing or controlling unwanted outdoor contaminants.
The New York State Education Department has come up with steps to control some outdoor pollution from contaminating the indoor air quality in schools. These include:
- Putting bird screens in place and inspecting them frequently;
- Ensuring outdoor air intakes are free of visible materials and standing water;
- Ensuring proper building pressurization to reduce infiltration;
- Using high-efficiency filtration in the air handlers;
- Vacuuming high-traffic areas frequently;
- Requiring vehicles to turn motors off at loading docks and near outdoor air intakes;
- Limiting use of gasoline-, propane-, and diesel- powered landscape equipment near outdoor air intakes when the building is occupied;
- Reducing outdoor air vent intake rates temporarily during times you know you cannot control outdoor air contaminants from coming in the building;
- Considering traffic patterns when placing outdoor air intakes on buildings during the design stage;
Relocating school bus drop-off and pick-up to areas some distance from outdoor air intakes; and
- Requiring school buses and other vehicles to turn off engines when near an outdoor air intake.