Helpful Hints
Key technologies used today that affect HVAC operation include chillers, thermal storage, variable frequency drives (VFD), and variable air volume (VAV) systems. Because heating and cooling can represent between 40 percent and 60 percent of building operating costs (including maintenance items), HVAC retrofit decisions should not be made without considering all factors -- including indoor air quality (IAQ).
For example, VAV systems use less energy than most other HVAC systems. However, if the system is not properly designed, it is neither energy-efficient nor healthy. All buildings require positive pressure in the range of 0.05 to 0.08 inches of H2O. The key to keeping a building positive is to maintain proper outside air throughout the turndown requirements of the system as dictated by the building loads, and by good control of the turndown cycle between supply and return fans.
The advantage of using VAVs is low energy use -- only the required heating and cooling is delivered. But when the cooling/heating requirement is satisfied, very little outside air may be delivered to the room. Therefore, IAQ problems can result.
To avoid that problem, have 40 percent to 50 percent minimum settings on VAV boxes so they can't close entirely and have a reheat coil so space is not overcooled. Also use VFD and direct digital controls to track return air and supply air fans.
But there are other, non-system-specific ways to improve HVAC operation -- and, so, IAQ. For example:
Consider a night purge cycle. HVAC systems often can be programmed to operate on a night "flush" cycle on 100 percent outside air to clear out any unwanted indoor pollutants on a daily basis. Note -- this strategy may not be advisable when the outside air is especially warm, humid or contaminated.
If you have insufficient air flow with a variable air volume (VAV) system, consider retrofitting your VAV boxes. They usually can be retrofitted to provide adequate air flow by installing minimum stops or fan-powered boxes.
If your HVAC system cannot deliver sufficient outside air or properly condition the outside air, consider retrofitting your system. Depending on the type and age of your HVAC system, there are many modifications you may be able to
make that will improve its performance. Retrofit options include adding separate outside air fans to each air handling unit; using energy recovery equipment; and using a make-up air unit. Make sure supply air diffusers serving the occupied spaces aren't obstructed. Obstructed supply air diffusers can ruin proper air balance. When this occurs, some areas receive too much supply air, while others receive too little. Improper air pressure relationships between the building and the outdoors, as well as between key areas within the building, also can develop, which can facilitate pollution transfer. If occupants complain about drafts, remedial options include
changing occupant locations, moving diffuser locations, or using different diffusers. Some options may require that the air system be rebalanced.
Use contaminant sensing as a means to ensure adequate ventilation. In building areas where occupancy or contaminant levels vary, consider the use of carbon dioxide, volatile organic compound, carbon monoxide or other contaminant-sensing inputs. Advances in today's HVAC system and component technology allow contaminant-sensing inputs to control the amount of indoor air introduced into a building area. But take precautions when using this technology: When contaminant-level thresholds are exceeded and the system calls for increased ventilation, make sure the outdoor air being brought in isn't itself contaminated.
IAQ - Source of Contamination
INDOOR AIR POLLUTION primarily is caused by an accumulation of contaminants that originate from inside the building, but contaminants also can be drawn in
from outdoors. If contaminant sources aren�t controlled, IAQ problems can result, despite proper HVAC design and maintenance.
There are myriad potential pollution sources, which singly or in combination may contribute to building-related illnesses. Sources outside the building
include:
- Contaminated outdoor air - pollen, dust, fungal
spores, industrial pollutants, general vehicle exhaust. - Emissions from nearby sources - vehicle exhaust,
loading docks, dumpster odors, re-entrained building
exhaust, unsanitary debris near outdoor air intakes. - Soil gas - radon, underground storage tank leakage,
contaminants from previous site uses, pesticides. - Moisture or standing water that promotes excess
microbial growth - problem areas include rooftops
after rainfall, crawlspace.
Sources from equipment include:
- HVAC system - dust or dirt in ductwork;
microbiological growth in drip pans, humidifiers,
ductwork and coils; improper use of biocides, sealants
or cleaning compounds; improper venting of combustion
products; refrigerant leakage. - Non-HVAC - emissions from office equipment, such
as volatile organic compounds (VOCs) and ozone;
emissions from supplies, such as solvents, toners and
ammonia; emissions from shops, laboratories and
cleaning processes; elevator motors and other
mechanical systems.
Sources from human activities include:
- Personal - smoking, cooking, body odor, cosmetic
odor. - Housekeeping - cleaning materials and procedures,
emissions from stored supplies or trash, use of
deodorizers and fragrances, airborne dust or dirt. - Maintenance - microorganisms in mist from
improperly maintained cooling towers; airborne dust or
dirt; VOCs from use of paint, caulk, adhesives and other
products; pesticides from pest-control activities; and
materials containing damaged asbestos.
Sources from building components and furnishings
include:
- Locations that produce/collect dust or fibers -
textured surfaces such as carpeting and curtains, open
shelving, old or deteriorated furnishings, materials
containing damaged asbestos. - Unsanitary conditions and water damage -
microbiological growth on or in soiled or
water-damaged furnishings, microbiological growth in
surface condensation areas, standing water from
clogged or poorly designed drains, dry taps that allow
sewer gas passage. - Chemicals released from building components or
furnishings - VOCs or inorganic compounds.
Other sources include:
- Accidental events - various liquid spills,
microbiological growth due to flooding or roof/piping
leaks, fire damage (soot, PCBs from electrical
equipment, odors). - Special-use areas and mixed-use buildings -
smoking lounges, laboratories, print shops and art
rooms, exercise rooms, beauty salons, food preparation
areas. - Redecorating, remodeling or repair activities -
emissions from new furnishings; dust and fibers from
demolition; odors, VOCs, and inorganic compounds
from paint, caulk and adhesives; microbiologicals
released from demolition or remodeling activities.
IAQ Monitoring
ASHRAE provides a target level of 1,000 parts per million (ppm) of CO2 as an indication of acceptable ventilation performance. OSHA, in its proposed IAQ rule, suggests an upper limit of 800 ppm to measure "acceptable" IAQ.
Regardless of the standard used, measuring IAQ by monitoring CO2 levels is an inexpensive way to assess IAQ, but it has limitations. Beware of how and when to use CO2 monitoring before you rely too heavily on it.
First, CO2 levels vary throughout the day as the occupancy load -- and CO2 generation -- increases and as the HVAC system responds to the corresponding thermal load. The typical CO2 concentration curve over a 24-hour period in a well-ventilated building fluctuates greatly.
If readings were taken throughout the day, they could be misleading without knowing the time of day or the shape of that building's 24-hour CO2 concentration curve. For that reason, sample and record CO2 continuously for at least 12 hours in
order to understand:
The HVAC system's performance, and its ability to respond to the ventilation load during the occupied cycle; and, the necessary system modifications.
Also, CO2 measurements indicate the HVAC system's effectiveness in diluting pollutants from only one source in the building: people. There are potentially many other sources of pollutants, including other gases, fumes, particulate fibers and
microbes.
Other pollutant sources include humidity and moisture, which can foster the growth of mold and mildew. Depending on the concentrations and the species, these pollutants can become one of the most toxic and expensive IAQ problems. Measuring airborne concentrations of mold and mildew is not effective or reliable; concentrations can vary widely over short periods of time and between areas.
Instead, measure relative humidity (RH) to determine if conditions are right for a mold or mildew problem. RH levels of 30 percent to 60 percent generally are acceptable; 40 percent to 60 percent are preferable. RH over 60 percent will provide conditions favorable for spore growth, and RH under 30 percent can cause skin and mucous membrane irritations.
Monitoring pollutant sources
Measuring temperature, RH and CO2 simultaneously is simple and can be done with one piece of equipment. The difficulty with sampling pollutants in indoor air is that most existing standards for evaluating airborne pollutants were developed for
industrial settings, where contaminants arise in higher concentrations. Standards of this type are rarely exceeded in commercial and institutional buildings.
Instead, most problems in non-industrial buildings arise from a combination of chemicals at very low levels for long periods
of time. Investigative strategies that focus on indoor air sampling and analysis, rather than identifying potential sources of
pollution, are much less effective.
Air sampling has its place in IAQ investigations, but not until its role is clearly defined and never at the outset of an investigation. Consider these two rules:
- Never sample unless you know what you are looking for
- Never sample until you know what your response will be to the results.
Responding to an IAQ problem
Many problems can be diagnosed and solved by staff who understand how to identify potential pollutant sources relating to the HVAC system. If the problem cannot be solved after initial attempts, you might need outside assistance.
The effort required to prevent most IAQ problems is usually much less than the time and cost required to resolve them after they develop. If minor problems evolve into an IAQ emergency, you might be able to identify a variety of building
deficiencies, but you often won't be able to say which one caused the emergency. This situation creates pressure to remedy every deficiency immediately instead of establishing a prioritized approach to IAQ improvements.
By contrast, many preventative measures identified in IAQ monitoring conducted before problems occur can be accomplished by in-house maintenance staff, and scheduled to meet your resources. If outside assistance is needed either to
diagnose an existing IAQ problem or prevent a future one from occurring, time spent in evaluating the engineer's qualifications, methods and experience will go a long way toward ensuring success.
