Dedicated Outdoor Air Systems: Rx for Sick Buildings

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Dedicated Outdoor Air Systems: Rx for Sick Buildings

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It's been 27 years since bacteria spread by a hotel air-conditioning

system sickened 221 people and killed 34 at an American Legion

convention in Philadelphia, sounding a wake-up call to the American

public about the link between indoor air quality and human health.

Since then, there has not been another U.S. incident involving

multiple deaths from bad indoor air, yet concerns persist that

conventional heating, ventilation, and air-conditioning (HVAC) systems

are making people sick. Fortunately, the growing acceptance of a new

HVAC design known as a dedicated outdoor air system (DOAS) promises to

improve indoor air quality and comfort while lowering operating costs.

Most conventional HVAC systems circulate conditioned air through a

duct system to different parts of the building. Conventional HVAC

systems are designed to control both room temperature (the " sensible

load " ) and humidity (the " latent load " ) while providing sufficient

fresh air to dilute pollutants generated by building occupants and

equipment. Different amounts of outside air must be added to the

recirculating air and supplied to different parts of the building,

depending upon the number of occupants in each space.

In contrast, a DOAS allows the building designer to decouple the

latent and sensible loads, using separate systems to control

temperature, ventilation, and dehumidification. A DOAS provides the

exact amount of dehumidified ventilation air required in each part of

a building. And it can be used in conjunction with cooling systems

that discourage the growth of mold and microbes.

Diagnosis: Sick Buildings

" Sick building syndrome " (SBS) is a term that came into use in the

mid-1980s to describe situations in which building occupants

experience acute health effects and discomfort associated with time

spent in the building. Symptoms may include headache, dry cough, itchy

skin, dizziness, nausea, and eye, nose, or throat irritation.

Because these symptoms can be caused by factors outside the building

environment, some may question whether the link between an

individual's illness and the air quality in a specific building is

real or imagined. However, more than 100 published studies have now

established links between indoor air quality and human illness. And

because of the potential for lost productivity, increased insurance

costs, and lawsuits related to SBS, building owners and operators, as

well as tenants, must take occupant complaints about health and

comfort seriously.

A 1984 report titled Indoor Air Pollutants: Exposure and Health

Effects. Report on a WHO Meeting suggests that up to 30% of new and

remodeled buildings worldwide may be the subject of excessive

complaints related to indoor air quality. More recent reports,

including America's Schools Report Differing Conditions, a 1996

publication of the General Accounting Office, have found that at least

20% of all U.S. schools suffer from poor indoor air quality.

Studies have tied this problem to a variety of factors, including

malfunctioning or improperly designed and maintained HVAC systems, the

presence of volatile organic compounds, mold growth, dust, radon, and

asbestos. According to the U.S. Environmental Protection Agency, one

of the chief causes or contributing factors to poor indoor air quality

is inadequate ventilation.

Inadequate humidity control in particular has been linked to

discomfort (such as drowsiness and headache), mold growth, and the

incidence of respiratory illness. Asthma, the most common cause of

absenteeism in schools, has been tied to indoor air quality and mold,

both impacted by space humidity.

Until the mid-1970s, building ventilation standards called for

approximately 15 cubic feet per minute (cfm) outside air for each

building occupant. Following the 1973 Arab oil embargo, however,

national energy conservation measures called for a reduction of

outside air to 5 cfm per occupant. Experts thought this would be

sufficient to ensure adequate health and comfort, but they were

quickly proven wrong.

Complaints from building occupants increased, becoming commonplace in

the 1980s and 1990s and bringing " sick building syndrome " into the

public lexicon. In response, the American Society of Heating,

Refrigerating, and Air-Conditioning Engineers (ASHRAE) has since

revised its ventilation standard 62-2001 to provide a minimum of 15

cfm outside air per person, or 20 cfm per person in office spaces.

This standard has been adopted by all the major building codes, which

in turn have been incorporated into enforceable local building codes.

Yet problems with indoor air quality persist, particularly as it

relates to humidity. " The problem is that code recommendations are not

always followed, " says Fischer, a technology consultant with

SEMCO of Columbia, Missouri, one of the nation's largest manufacturers

of energy recovery systems. " People tend to want to do things the way

they have in the past, and many building designers and managers

apparently don't believe it's necessary to increase ventilation rates

to comply with ASHRAE standards. "

Furthermore, Fischer says, conventional packaged HVAC equipment--which

includes heating, ventilation, and air-conditioning all in one

unit--often is simply not capable of providing sufficient outdoor air

on a continuous basis without causing indoor humidity problems.

Updated Treatment

Although the DOAS idea is just now gaining acceptance in the building

community, the concept has been around for more than a decade. Stanley

Mumma, a professor of architectural engineering at The Pennsylvania

State University, began promoting DOAS in the early 1990s as a way to

improve energy efficiency while meeting tighter ventilation

requirements.

Mumma and diffuser

Breathing room. Stanley Mumma works in a demonstration space at The

Pennsylvania State University that is conditioned using a DOAS and

radiant cooling.

image credit: Pennsylvania State University

Most office and institutional buildings are conditioned by " variable

air volume, " or VAV, systems, which supply air at a constant

temperature but varying flow rate. Through his research, Mumma found

it was very difficult to ensure both proper temperature and proper

ventilation in all spaces with VAV systems. " For a given room, we

might only need two hundred cfm to meet the temperature setting, but

more to provide the needed ventilation for the occupants, " Mumma says.

" There's no good way to meet [both needs] with a VAV system,

especially with all the infiltration, exfiltration, and

short-circuiting of air flow in a building. With DOAS, if a room needs

two hundred cfm of ventilation air, we supply exactly that. "

Mumma also found that to even attempt to meet the new ASHRAE

ventilation standards, a VAV system generally required 20-70% more

outside air than a DOAS. That's because VAV systems must be set to

provide the proper amount of outside air for the space with the

highest ventilation requirements, overventilating the rest of the

building in the process. Cooling and dehumidifying all of that

additional air in the summer and humidifying it in the winter requires

more energy and thus greater cost. A DOAS, on the other hand, provides

only the amount of outside air needed for each space, thus reducing

the operating costs.

Finally, Mumma found that VAV systems--which attempt to control both

sensible and latent loads while supplying the proper amount of

ventilation--invariably lead to high relative humidity. The only

solution he could find was to use a DOAS.

Although outside air used for ventilation can also be used for

cooling, there are times and places (such as summertime in the South)

where a parallel cooling system must be used to make temperatures

comfortable. A DOAS can be used in tandem with virtually any type of

cooling system.

Mumma prefers using a radiant cooling system (one that circulates cool

water through ceiling panels) over those that employ forced air.

" Radiant cooling wins hands down over forced air systems in terms of

safety, comfort, and energy savings, " he says. " Forced air systems

that employ fan coil units with condensate pans are breeding grounds

for microbial growth. And any time you blow air around, you increase

occupant discomfort, as well as the chances of spreading germs and

other contaminants. "

Mumma has installed a demonstration DOAS at The Pennsylvania State

University and reports it has operated through the summer with no

problems. The results of this demonstration will be published in an

upcoming issue of the ASHRAE publication IAQ Applications.

Fischer agrees that radiant cooling offers the best option for large

commercial and institutional buildings in the future. However, in the

near term, he believes it is likely that most buildings will combine a

DOAS with conventional packaged cooling equipment. Radiant cooling

requires chilled water, which many facilities, especially schools,

avoid because of the perceived complexity of these systems' design and

maintenance, Fischer says. " [Facility managers] prefer packaged

cooling equipment that can be easily serviced by most HVAC contractors

or replaced if failure occurs. "

The Cost of the Cure

Under contract with the U.S. Department of Energy, Fischer and

Charlene Bayer, a principal research scientist at Georgia Tech

Research Institute, studied the impact of humidity control and

ventilation in 10 school facilities in Georgia. Five of the schools

were equipped with conventional HVAC systems and five with a DOAS.

Fischer and Bayer found that the 15 cfm per student ventilation rate

prescribed by ASHRAE was the minimum necessary to keep concentrations

of potentially dangerous airborne contaminants (such as formaldehyde

and total volatile organic compounds) below recognized guidelines set

by the Environmental Protection Agency, the National Institute for

Occupational Safety and Health, and other regulatory bodies.

None of the schools served by conventional HVAC systems were in

compliance with the 15 cfm ventilation standard, instead averaging

only 5.4 cfm per student. Fischer and Bayer say this was because the

conventional systems were simply unable to maintain comfort levels

with respect to humidity at higher ventilation rates. Conversely,

schools using a DOAS provided ventilation at the rate of 15 cfm per

student while maintaining the space humidity as desired. Average

absenteeism for schools using a DOAS was 9% lower than for the

conventional schools, a factor tied to improved comfort and health.

DOAS setups proved to be both energy-efficient and cost-effective,

reducing annual operating costs by $15,000-20,000 for a typical school

building. Fischer and Bayer's findings were reported in the May 2003

ASHRAE Journal.

Although it might seem more costly to design a building with separate

temperature control and ventilation systems, Mumma insists that a

properly designed DOAS is actually less expensive than a conventional

VAV system. That is because smaller heating and cooling units can be

used if they don't have to perform the double duty of removing the

latent load. Smaller units cost less to buy and are cheaper to

operate.

As an example, Mumma provides comparative cost estimates for heating,

cooling, and ventilating a theoretical 186,000 square foot office

building located in Philadelphia with a conventional VAV system versus

a DOAS using radiant cooling. Mumma estimates the installed cost for

the conventional system at approximately $1.4 million versus $1

million for the DOAS. He estimates annual operating costs at $77,350

for the conventional system and $59,730 for the DOAS.

Downing, associate director of the Advanced Technology

Development Center, which is part of Georgia Institute of Technology

in Atlanta, agrees that a DOAS can be economical. " If you start with a

blank sheet, you can design a DOAS for no more than a conventional

[system], " he says. " Retrofitting a DOAS on an existing building will

add to the cost, but you should be able to pay that off in energy

savings in less than four years. "

As building owners and operators strive to meet the new ASHRAE

standard, they are increasingly turning to DOAS technology. Hundreds

of schools have now been designed using DOAS setups, and they are

performing well, according to Fischer, with fewer complaints about

indoor air quality and better student performance. " This is the way

building design is going, " he says.