Published on : August 19, 2010
Achieving More Sustainable Operation of Hospitals through Ventilation
The sustainable operation of hospitals and other health care buildings requires as a priority assuring that the HVAC system is working as intended in the areas of ventilation and moisture management performance. Achieving intended operation in these two areas is necessary not only to provide a healthy indoor environment for the staff, patients and visitors, but also to operate the HVAC systems using the least amount of energy as possible.
Without accurately knowing how much ventilation is actually being provided to the most densely occupied spaces risks the unnecessary contamination by communicable viruses in the air; not knowing if effective moisture management is being achieved risks indoor mold growth and wasted energy.
As with any other building parameter that is managed effectively, performance in these two areas needs to be measured in an accurate and ongoing manner. Fortunately, centralized monitoring systems are now available to achieve this monitoring and thereby achieve “smarter buildings” where HVAC performance in both ventilation and moisture management can be assessed. As pointed out in the LBNL Report on CO2 Monitoring for Demand Controlled Ventilation in Commercial Buildings , the use of individual sensors for carbon dioxide (CO2) measurement can fail to achieve the design goals of saving energy while assuring that ventilation rates meet code requirements due to poor sensory accuracy. Centralized, shared-sensor monitoring systems provide a much higher degree of accuracy. Also, the latter approach to monitoring can simultaneously measure absolute humidity (dew point temperature) along with the CO2 measurement, as well as other IAQ-related gaseous parameters.
The need for a shared sensor approach to building management is important for two key reasons: More effective management in both achieving a healthy indoor environment and minimizing energy use. Improved building management through monitoring will not only determine if the intended amount of ventilation is actually being delivered to the building’s occupants, but the energy use needed for conditioning outdoor air is a critical component of the building’s operation.
This energy use is becoming a larger and larger component of total building energy use, because energy use for achieving thermal comfort is being reduced due to less leaky and better insulated building envelopes, and energy use for lighting is being reduced due to more energy efficient light fixtures and the greater use of daylighting.
Delivery of the intended amount of ventilation to the occupied areas at all times is crucial to the achievement of a healthy indoor environment of a sustainably operated hospital. Unfortunately, research has shown that the ventilation supplied to conference rooms and other spaces where people congregate relies on changes in thermal conditions to sense the presence of these people. The delay in response time for ventilation can result in ventilation deficiencies in the early portions of meetings.
Consequently, if one of the occupants is shedding a communicable able virus (such as the H1N1 flu virus, or its latest mutation) the disease agent is not rapidly diluted and removed from the space, and therefore poses an increased risk of infecting other individuals present. Monitoring CO2 concentrations in conference rooms identifies these periodic ventilation deficiencies occurring; this monitoring can also evaluate the effectiveness of any corrective measures employed to correct this problem.
Figure 1 presents a data plot of an example of elevated CO2 concentrations in a conference room in a hospital location. In this figure only one indoor and one outdoor location are presented. Plotting both indoor and outdoor values for CO2 simultaneously permits a determination of the maximum amount of ventilation that could be occurring at that time. This determination calculation involves dividing an appropriate per person CO2 generation rate with the difference between the indoor and outdoor CO2 values. At about 6:30 pm in Conference Room A, displayed in Figure 1, the indoor value of 1,270 ppm compared with the outdoor value of 450 ppm equals an indoor to outdoor difference of 820 ppm. Converting this CO2 difference to a ventilation rate involves using the expected activity level for office workers at 1.2 Met units, and the per person CO2 generation at this metabolic level of activity is 0.0106 cfm of pure CO2. In the actual equation however, this generation rate needs to be consistent with the parts per million values used for the CO2 concentrations, and so the numerator of this equation becomes 10,600.
Performing this calculation yields a result that the maximum ventilation rate at this time was no greater than 13 cfm of OA per person. Since this calculation procedure assumes that equilibrium conditions have been achieved, the actual ventilation rate might actually have been less, had the conditions persisted and the concentration increased.
Earlier in the day, around 12:18 a.m., another peak was observed for this conference room. This time, the indoor air value of 1,009 ppm and the outdoor air value of 454 ppm correspond to a ventilation rate no greater than 19 cfm of OA per person. Again, the ventilation rate in this location fails to achieve the ASHRAE Standard 62 minimum ventilation rate to achieve “acceptable indoor air quality”. While the title for this Standard includes IAQ, I do not consider this to be an IAQ-based standard, but merely a standard designed to achieve “perceived comfort”, as their definition of “acceptable IAQ” is one where “a substantial majority (80% or more) of the people exposed do not express dissatisfaction”.
Stated another way, this goal of this standard is to achieve a condition where no more than 20% of the people exposed are dissatisfied with the quality of the indoor environment. I therefore contend that the sustainable operation of healthcare facilities should require not only that the actual amount of ventilation being delivered to the occupied spaces be documented in an ongoing manner, but that the minimum ventilation rates recommended in ASHRAE Standard 62 be exceeded.
Figure 2 displays multiple locations in the same hospital and again, conference rooms are indicating that conditions of reduced ventilation are occurring with some frequency. For Conference Room B, with a measured CO2 reading of 1,957 ppm at 10:48 a.m., the ventilation rate would be no greater than 7 cfm of outdoor air per person at this time. The monitoring data displayed in Figure 2 also indicates that the majority of the other monitored locations are much more generously ventilated. With an outdoor CO2 value of about 450 ppm, and values recorded at higher than 1,000 ppm, the conference rooms are not meeting the ASHRAE minimum recommended ventilation rate of 20 cfm of outdoor air person.
Data from this building, along with other ventilation assessment studies performed clearly suggest that many occupants are at increased risk of exposure to communicable diseases due to low amounts of ventilation in conference rooms. Clearly additional investigations and research should be performed to better assess the magnitude of this problem.
Another important aspect of HVAC performance assessment, CO2 monitoring, can determine the percentage of outdoor (OA) in the supply air (SA). After measuring the CO2 concentration in these two air streams as well as the return air (RA), a simple mass balance calculation will yield the value of %OA in the SA. Figure 3 displays one example of this determination where the four (4) air handling units serving this hospital are not providing the same amount of OA in their SA. Despite the fact that it would be expected that all four AHUs would have the same amount of OA, AHU 4 frequently is conditioning 70% OA, while AHUs 1 and 3 are only providing around 40%. This data provides another example of the need for smarter buildings where the operators have easy access to how the HVAC systems are actually performing with respect to the amount of OA they are conditioning.
Ongoing assessment of effective moisture management is another important component of sustainable operation for hospitals. This is because localized elevations in indoor moisture can degrade IAQ by facilitating the growth of molds. In addition, over humidification can be a waste of energy. Evaluation of moisture management performance can be achieved by monitoring the dew point temperature at multiple occupied locations, the outdoors, and in the supply air from the AHUs.
With that monitoring in place, if any moisture intrusion is occurring at penetrations in the building envelope, this building defect will be identified when the indoor air humidity in this location experiences infiltration and will more closely resemble the outdoor conditions than the other indoor conditions.
In addition, the effectiveness of the air sealing of the building envelope can be assessed as well when there is a difference in absolute humidity between the indoors and the outdoors, as the humidity in those indoor locations where infiltration is occurring will begin to more closely resemble the outdoor conditions than in the rest of the building.
Another aspect of moisture management than can be achieved by monitoring of dew point temperatures is the identification of water leaks within the building. If, for instance, a chilled water valve starts leaking, the absolute humidity near this leak could become elevated and will be recorded as part of the monitoring effort. In effect, this monitoring becomes an early warning system to rapidly detect indoor elevated moisture levels. In this situation, it would not be enough to merely have the monitoring in place collecting data; it would need to be reviewed in a timely fashion as well.
The addition of moisture to the supply air during the time of the year when the outdoor conditions are dry is a frequent operational strategy in hospitals. This humidification can improve comfort for the occupants, but it can also waste energy. It is therefore another aspect of HVAC performance that can be assessed by the monitoring of supply air streams and occupied locations.
Conversely, the removal of moisture, or dehumidification, in the summer is also an HVAC function, and its performance can be evaluated my monitoring as well. In one monitoring installation, for instance, a comparison of the supply air dew points indicated that one of four AHUs was not dehumidifying as well as the other three units were.
In that hospital the monitoring system also measured carbon monoxide (CO) and hydrogen sulfide (H2S). The garage ventilation, relying just on CO concentrations, did not protect the occupants from the other air contaminants (oxides of nitrogen, aldehydes and other hydrocarbons) as indicated by the CO2 concentrations in the garage. The H2S review identified the presence of a dry trap allowing sewer gas to be detected in the garage as well.
Assessments of ventilation and moisture management performance are not only important for the achievement of a healthy indoor environment, they are also important for conserving energy. Fortunately the technology is now available for creating “smarter buildings” where this diagnostic feedback can be readily available to the building operators. For this potential to be realized, however, not only does a monitoring system need to be installed, but the information on ventilation and moisture management should be reviewed on a timely basis as well.
About the Author
David Bearg holds a B.S. from Northeastern University in Chemical Engineering and an M.S. from the Harvard School of Public Health in Environmental Health Sciences. He is also a Registered Professional Engineer in Massachusetts and a Certified Industrial Hygienist.
Mr. Bearg’s professional focus involves the achievement of healthy and productive indoor environments, typically achieved in conjunction with energy conservation. As part of his work he has authored the text, "Indoor Air Quality and HVAC Systems", as well as contributed to several other texts and has had papers published in technical journals and has made presentations at numerous conferences.
His avocation involves cultivating a self-sufficient lifestyle that includes extending the growing season with the use of a heat-storing greenhouse with automatic grey-water irrigation and the construction of moveable interior window insulation panels to reduce unnecessary heat loss in his home. More about these efforts can be viewed at www.sagefarm.net.