Mike Opitz, PE, LEED AP
In the early 21st century we U.S. citizens spend about 90% of our time indoors, meaning the quality of the indoor environment has an enormous impact on our health. In particular, the air we breathe needs to have acceptably low amounts of the contaminants common in buildings, such as carbon dioxide, excess moisture, odors, and chemical toxins.
The most common way of providing good Indoor Air Quality (IAQ) is by ventilation—the process of supplying and removing air to and from indoor spaces to prevent buildup of harmful contaminants.
Facility managers have been dealing with ventilation issues for decades, even centuries, because almost everyone notices and suffers from bad air quality when it’s present. Maintaining good indoor air quality keeps occupants happier and healthier, reduces trouble calls, and can increase productivity. This is why good ventilation practice fits within the United States Green Building Council’s (USGBC’s) “triple bottom line” of green building benefits: it’s good for the earth, good for the wallet, and good for human health and society as a whole.
To the casual observer most buildings may appear to be over-ventilated, while some seem to have no ventilation at all. Although both situations have been known to happen, typically these impressions arise from a common misconception: that central air ducts are required for ventilation, and that all air provided in them is “fresh air.” In fact, buildings employ a surprising variety of system types to do the ventilation job, and facilities managers need a basic understanding of when each is suitable.
Central air ducts are a common method of HVAC distribution, but they are subject to several different design and performance criteria because providing ventilation air isn’t their only function. This means that in most climates, for most of the year, the amount of airflow needed to provide adequate heating and cooling (i.e., the supply air flow) far exceeds the amount needed for ventilation (the outside air flow, or OA flow). In fact, the OA flow is commonly only 10-30% of the total supply air flow, with the remaining air being recirculated air returned from the conditioned spaces. The recirculated air is mixed with the ventilation air to provide the supply air. Only a few applications, such as hospital operating rooms, commonly use 100% outside air.
Other complications also arise. For example, some areas of buildings like kitchens and bathrooms don’t need pure ventilation air to be supplied directly because they use localized spot-exhaust fans. The exhaust fan pulls air out of the space, lowering the pressure, which draws in air from neighboring parts of the building. Sometimes this air is essentially “pristine” supply air; other times it has been partly “degraded” because it has already circulated through other spaces.
Moreover, building geometry also comes into play because interior zones may need dedicated mechanical ventilation year-round, whereas sometimes exterior zones can get by for at least part of the time based on air leakage through the building shell or operable windows. Some buildings, especially single-family residential homes, depend solely on air leakage for their fresh air. Using operable windows in commercial buildings is part of a recent revival of a strategy called natural ventilation.
If a facility manger is considering improving ventilation, the first step is finding out which of the above situations apply in the building, whether each is suitable for the application, and which could benefit from improvement.
Ventilation Standards and Sustainability
Any ventilation system must provide an adequate amount of outside air to prevent the symptoms of “sick building syndrome” that were so common during the 1980s. But how much outside air is enough? This has been a moving target historically, with values for typical spaces ranging from 4 cubic feet of air per minute (CFM) per occupant instituted in 1824 to as high as 30 CFM/person in 1893.
Today it’s widely agreed that the amount of ventilation air needed depends on how the space is used (e.g., office space needs less than laboratory space), and that the bottom line isn’t necessarily ventilation rate but indoor air quality itself. ASHRAE provides the most commonly referenced industry standards for ventilation and air quality in buildings in the U.S.:
- ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality
- ASHRAE Standard 62.2-2004, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.
Both standards share a common goal: to specify minimum ventilation rates and Indoor Air Quality needed to maintain occupant satisfaction and minimize health problems. Standard 62.1, the version applicable to commercial buildings, includes both a prescriptive-based compliance path (the ventilation rate procedure) and a performance-based path (the indoor air quality procedure).
The former is more common because of its simplicity, and is the only one most facility managers will need to deal with. The latter is offered for facilities that need maximum flexibility in their design and operation methods or already want to measure their Indoor Air Quality for other reasons.
Several specific aspects of ventilation impact occupant health, and therefore green building and the triple bottom line. Consequently, the USGBC’s LEED for Existing Buildings rating system has several ventilation-related elements:
- Environmental Quality prerequisite 1 (EQp1): outside air introduction and exhaust systems
- Environmental Quality prerequisite 2 (EQp2): environmental tobacco smoke control
- Environmental Quality credit 1 (EQc1): outdoor air delivery monitoring
- Environmental Quality credit 2 (EQc2): increased ventilation
- Environmental Quality credit 6.2 (EQc6.2): controllability of systems: temperature & ventilation.
Some of these items reference ASHRAE 62 directly and focus on ensuring the ventilation system is capable of meeting the standard (EQ p1), continuous monitoring and associated alarms when conditions are outside acceptable limits (EQ c1), and extra ventilation above and beyond ASHRAE 62’s requirements (EQ c2; this provides an extra margin of safety and also helps protect highly sensitive occupants). Others focus on control of specific, especially harmful contaminants (EQ p2) or localized control to suit the preferences of individual occupants (EQ c6.2).
As is the case with many green building strategies that depend heavily on mechanical system performance, for ventilation improvements facilities managers typically fall into one of two camps: 1) they already have a building that meets these requirements (or can with very little effort), or 2) they don’t, and meeting the requirements will require a significant renovation. If you’re in the latter camp, be sure to keep ventilation needs on the radar screen as renovation planning begins.
The Energy Penalty
Although many sustainable operations strategies are “no brainers” that essentially everyone should try to do, some involve definite trade-offs that require careful study and balancing. In most climates there is such a thing as over-ventilation because of the higher energy use it causes. Anytime the outside air conditions don’t match the desired indoor air conditions, which is most of the time, all incoming ventilation air and air leakage through the building shell must be conditioned to maintain thermal comfort. In other words, all that air has to be heated, cooled, humidified, or dehumidified.
In most U.S. climates the energy penalty to pre-condition outside air is substantial for all commercial buildings. In applications with very high ventilation because of high occupancy (auditoriums, convention centers) or a requirement of zero recirculated air (some health facilities), the ventilation air conditioning may be one of the largest energy uses in the building. High energy use raises not only operating costs, but also emissions from the burning of fuels, which amounts to a green penalty. The lesson here is that more ventilation is not necessarily better, and facility managers need to find a good balance that works in their situation.
One way to have your ventilation cake and eat it too is with an effective but under-used technology called heat recovery ventilators or energy recovery ventilators. These devices pre-condition the incoming ventilation air at much lower operating cost by “reusing” the energy and/or moisture in the outgoing exhaust air. This is done without contaminating the incoming ventilation air. Recovery efficiencies are typically 70-90%, meaning the ventilation pre-conditioning costs can be substantially reduced at the expense of some electricity to run the extra fans.
Ventilation a Win-Win Strategy
Providing optimized ventilation to building occupants is a win-win strategy for facilities managers: it reduces trouble calls by keeping occupants happier and healthier, it keeps the code officials satisfied, and demonstrates leadership in sustainable operations. If you suspect you have a sick building, or if major mechanical system renovations are in your near future, you owe it to all your stakeholders to seriously explore what enhanced ventilation can do for you.
Mike Opitz is Certification Manager, LEED for Existing Buildings, U.S. Green Building Council.
American Society of Heating, Refrigeration, and Air Conditioning Engineers (www.ashrae.org)
Publishes Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality.
U.S. Green Building Council (www.usgbc.org)
The USGBC’s LEED for Existing Buildings program addresses thermal comfort in several of its requirements and credits.
Simplified Design of HVAC Systems, William Bobenhausen, John Wiley & Sons, 1994. A superb introduction to thermal comfort issues and HVAC in general.
Heating and Cooling of Buildings: Design for Efficiency, 2nd ed., Kreider, Curtiss, and Rabl, McGraw Hill, 2002. A more technical review of how to meet HVAC requirements efficiently, including ventilation.
This article is reprinted with permission from FMLink and the U.S. Green Building Council. It was originally published on FMLink (www.fmlink.com), the online magazine for facilities managers, which has over 10,000 pages of news and feature articles, including a large sustainability section, in which this article was published; registration is free and will give one access to all of FMLink. The sustainability section is provided by the U.S. Green Building Council (www.usgbc.org), which is a non-profit organization committed to expanding sustainable building practices. USGBC is composed of more than 15,000 organizations from across the building industry that are working to advance structures that are environmentally responsible, profitable, and healthy places to live and work.