Outside Air Monitoring: Controlling the Thin Line

Dec. 2, 2011
Outside air monitoring provides a tool for precise balancing of air quality and use

Outside air monitoring. Like an apple a day, fresh air is a universal tenet of good health, and the more the better. But exactly how much of this good thing should be brought into a building? And how can one measure it?

One side of the challenge is ensuring enough healthy air for occupants, especially in spaces with dense or highly variable occupancies. IEQc1 – Outdoor Air Delivery Monitoring, a credit in USGBC’s LEED for New Construction, calls for carbon dioxide (CO2) and outdoor-airflow monitors that signal when fresh air is needed. These monitors must be programmed according to minimum setpoints defined by the definitive standard on outdoor air, ASHRAE 62.1-2007. Similar rules are included in widely used standards and in many state and local building codes, primarily for variable air volume (VAV) mechanical systems.

“The fact that codes and ASHRAE 62.1 feel that outside air monitoring [OAM] is required for VAV systems makes this an important issue for building owners,” says Steven T. Taylor, P.E., an ASHRAE Fellow and principal of Taylor Engineering in Alameda, CA.

The other side is that outside air delivery is an economic issue with broader implications. Buildings without outdoor air monitors and automated delivery controls may be ventilating interior spaces too much, leading to excessive energy use to condition surplus outside air. Buildings with demand-controlled ventilation (DCV) use sensors and monitors to deliver air on demand and as required.

“This brings up the issue of what is more important: better indoor air quality or reduced energy use,” says Thomas M. Lawrence, P.E., a public service associate in the University of Georgia’s Driftmier Engineering Center in Athens, GA. “A building should ensure proper outdoor airflow needed to maintain adequate IAQ. Too many people try to cut back on outdoor airflow to save energy without thought of the impact on IAQ.”

When CO2 levels are too high in a zone or throughout an entire building, building occupants may get headaches or feel more tired than usual. It’s a common problem in tightly packed work areas and interior zones with wide swings in how many people they accommodate. To deal with this, an OAM system’s sensors can signal outdoor vents and fans to activate. In some cases, the DCV subsystem jumps into action.

“CO2 monitoring is required by energy codes in densely occupied spaces for DCV as an energy conservation measure, not an IAQ measure,” Taylor explains. “LEED EQc1 also requires CO2 monitoring in densely occupied spaces – in this case, it is for IAQ, not energy savings.”

Clearly, OAM can be part of a strategy to reduce energy costs or improve the indoor environment – but can it do both?

Rationales for Retrofits
The first big question for America’s vast stock of existing buildings without OAM and DCV is, how do you know when you need it? The second question is, what are the best retrofit options and their costs?

To determine if a building needs OAM, first study the codes, says Taylor. Then consider whether the facility could be exposed to occupant complaints and lawsuits if it doesn’t comply with the rules. A third motivation is LEED for Existing Buildings, which has held its own standard for IAQ: the credit has listed minimum outside airflow rates according to ASHRAE 62.1 for CO2 levels 15% above those expected. But some facilities professionals complain that it isn’t clear exactly what those CO2 levels are supposed to be.

“Even with LEED, achieving a healthy indoor environment requires greater ventilation rates than those listed in ASHRAE 62.1, as these ventilation rates are intended merely to achieve ‘acceptable’ IAQ, where no more than 20% of the occupants are dissatisfied,” says David W. Bearg, P.E., a principal of IAQ firm Life Energy Associates, Concord, MA.

Another litmus test for choosing OAM is whether the building’s ventilation rates are excessive, which could be costing unnecessary energy. In these cases, a retrofit should immediately reduce operating costs, although Taylor cautions that some savings may be offset by OAM maintenance outlays.

On the other side of the coin is inadequate ventilation. If building owners have older HVAC systems and they are not sure if outdoor air volumes are appropriate, they should look into OAM, says Lawrence. “Another reason is if HVAC systems supply air to spaces with a wide variety of needs, such as conference rooms and office areas.”

Outdoor air monitoring can also help improve the health and performance of people within the building, says Bearg. “Consider looking into this aspect of ventilation performance assessment if occupant productivity is a priority,” he says. “After all, more money is spent on the people – for example, $300 per square foot, as compared with energy consumption at $3 per square foot.” Taylor agrees, noting that by addressing under-ventilating HVAC systems, facility managers can improve IAQ and boost organizational performance while reducing absenteeism.

Last, an OAM retrofit can help manage risk and uncertainty in building operations. If a building owner can’t say definitively how much ventilation is actually delivered throughout the building – especially in those areas with tightly packed or changing occupancies – monitoring can help reduce liability.

“Most building operators just assume, or hope, that HVAC ventilation is working as intended, but unless they actually measure CO2 values in occupied spaces, they have no way of knowing for sure,” explains Bearg.

Top Candidates for Retrofits
The most likely OAM retrofit applications are in buildings with VAV systems – a requirement in 62.1 as well as current local codes based on the International Mechanical Code (IMC) and the Uniform Mechanical Code (UMC), including California’s Ventilation Standard. If the HVAC system employs constant air volume (CAV) ventilation, the codes generally allow the use of a simple, fixed-minimum damper position to control outdoor air intake.

“LEED-NC EQ credit 1 requires outdoor air monitoring of non-densely occupied spaces for both CAV and VAV systems,” says Taylor. “So owners should always have some dynamic means of controlling outdoor airflow for VAV systems.” In constant-volume systems, OAM has been shown to help compensate for wind and stack effects, which may compromise proper ventilation rates. But most HVAC experts and building operators don’t feel these minor benefits are worth the cost and complexity of OAM.

Whether a building has CAV or VAV, if the building owner or occupants are pursuing a wellness program that includes ensuring a healthy indoor environment, OAM is still beneficial, says Bearg. The retrofit will serve the goals of improving energy and ventilation effectiveness, too.

Measure Twice, Retrofit Once
In pursuing an OAM retrofit, the selection of outdoor air devices and systems depends on the application and budget, says Taylor. A range of options are available to building owners.

The system choice and design generally boils down to the levels of ambient CO2 measured both outside and inside the building. Carbon dioxide levels vary by season and location, says Lawrence, but the trend is upward. It’s higher in towns and cities, conservatively estimated at about 400 parts per million (ppm) in most building locations. Better than guesswork, building owners can use actual site testing to gauge how urgent their OAM needs are. “This is particularly recommended for sites located in larger urban settings or near major roadways, where local CO2 concentrations can easily be several hundred ppm higher,” he warns.

It’s the difference between the outside and indoor measurements that are the most critical, especially if at periods of light occupancy the building interior level is similar to outdoor levels.

Two Approaches to Outdoor Air Monitoring

There are two basic ways to use outdoor air monitoring (OAM) in a building: direct measurement of incoming outdoor air and monitoring CO2 levels in spaces within the building. Each approach has benefits and drawbacks.

Direct measurement of outdoor air supply:

  • Tells how much outdoor air is coming in at the system level only.
  • Does not guarantee proper distribution to critical zones, such as conference rooms.
  • Uses setpoint triggers often based on the absolute minimum needed, leaving the possibility that the operator will not know some spaces actually need more outdoor air.
  • Requires regular checking and calibration.

CO2 monitoring in interior spaces:

  • Measures CO2 levels in rooms or zones as an indirect check on ventilation.
  • Is used – often incorrectly – as a “one-size-fits-all” approach to determine when the CO2 level is too high.
  • Requires knowledge of acceptable IAQ levels for various occupancy types.
  • Assumes the use of well-calibrated sensors for ambient air as well as interior spaces.

After studying the air quality conditions, there are two ways to implement OAM: direct measurement of incoming outdoor air volume at the air-handling intake or area monitoring of CO2 levels in a room or space. For the former, the building operator needs to know what the expected or required level of outdoor air is for the air-handling unit. The monitor will signal an alarm if the outdoor-air level falls below the minimum amount. For area monitoring, CO2 levels in the space act as a surrogate, indirectly determining if it’s adequately ventilated.

The two approaches have unique benefits and drawbacks. Direct measurement gives a conclusive reading at the air intake only and doesn’t say if every room and space is getting good ventilation, says Bearg. Just the opposite, area monitoring ignores outdoor-air volume and tries to tell by CO2 levels whether more ventilation is needed. As Lawrence points out, CO2 concentrations vary dramatically in buildings, from 960 ppm in a second-grade classroom to 1,975 ppm in a multiple-use assembly space.

“Decisions must be made regarding the balance between having adequate outdoor air ventilation to the occupied space while still considering the energy requirements needed to condition that ventilation air,” Lawrence wrote recently in a paper on comparing minimum outdoor air needs with the metabolic rates of building occupants in dozens of facility types – everything from fitness centers to supermarkets to office reception areas.

Success Factors for Retrofits
Collecting reliable, useful data on the building and its HVAC system is the key to a successful OAM retrofit, according to experienced owners. “First, you need to understand how your HVAC system attempts to deliver outdoor air for ventilation. After that, you should consider how variable the population is within the building,” Bearg explains. “Then a monitoring system can be designed to obtain the desired diagnostic feedback.”

Monitors can be zoned or centralized, depending on the best way to collect the desired information. A centralized, shared-sensor approach can be highly effective, says Bearg, who recommends that the system also monitor dewpoint temperature. If multiple sensors are used to compare indoor and outdoor CO2 levels, accuracy is critical – especially when the two levels are similar at times of low building occupancy. If the HVAC system already has air quality monitors, the new sensors need to be compatible.

No matter how the OAM system is designed, its vendor or installer should provide all needed testing and maintenance support to guarantee it functions properly, says Bearg. “This sophisticated HVAC control demands a quality assurance component to make sure sensors are accurately calibrated and the HVAC system is responding correctly.”

OAM is part of a whole-building engineering approach, and it’s becoming more important as other systems such as lighting and building envelopes become more efficient. Makers of OAM packages document a decent payback, estimated at as little as two years if productivity gains are included in the analysis. And let’s face it – there are even greater gains to be had with more alert and happier building occupants, whether they are nurses, high school students, office workers, or border patrol agents.

Better ventilation not only makes economic sense; it also affects perceptions of the property. “Accurate CO2 monitoring to provide assessment of the amount of outdoor air delivered to the occupied spaces is a key component of achieving a smart building,” Bearg concludes, “one where the optimization of building performance includes not only energy use but also healthy ventilation.”

C.C. Sullivan ([email protected]) is a marketing communications consultant and writer specializing in architecture, design, and construction technology.

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