Optimizing the performance of a ventilation system maximizes its ability to provide the building's occupants with the intended amount of ventilation while using the least amount of energy necessary. Operators need to have reliable information on how much outdoor air for ventilation is being delivered by the ventilation system to achieve this high level of heating, ventilation, and air conditioning (HVAC) system performance. After all, if the system is not monitored, how can anyone do a good job managing it?

There are many building systems that require energy. Improvement in one area, while reducing the total energy load, increases the percentage of the load used for other functions. For instance, improvements to the lighting equipment or modifications to the building envelope will reduce overall energy use. At the same time, if the energy needed to condition the outdoor air for ventilation stays the same, that energy will represent a larger proportion of the total energy use for the building.

What is needed to optimize the overall performance of the ventilation system is an accurate way to assess how much of the conditioned outdoor air actually gets delivered to the people breathing it. Only then can the delivery components of the HVAC system be dynamically balanced. In addition, the minimum position of the outdoor air damper can also be set with certainty to achieve the ventilation goal with the least amount of energy consumed in the process.

The delivery of adequate quantities of outdoor air for ventilation is a basic requirement for adequate indoor air quality. In other words, if proper ventilation is not provided, the productivity of the employees involved may be reduced.

Increasing the effective ventilation rate may also increase productivity. A research study led by Professor Donald Milton of the Harvard School of Public Health looked at this relationship and focused on absences for sick leave as a measure of productivity. Milton found that when the amount of ventilation provided was generous (about 50 cubic feet per minute [cfm] per person), the rate of short-term sick leave was lower than for employees provided with moderate ventilation of 25 cfm/person. This study also quantified this difference and reported that the cost of sick leave attributable to ventilation at the current recommended rates was approximately $480 per employee per year. Estimating the cost of providing additional ventilation in Massachusetts at $80 per employee per year further resulted in net savings of $400 per employee per year that could be obtained with increased ventilation. Even if the ventilation goal is only the American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) recommended minimum, there should still be a mechanism is place for documenting that this goal is actually being achieved.

Metrics?

Table 2. The table presents the results of humidity monitoring.

While some operators may know how much outdoor air enters the HVAC system, there is no guarantee that this outdoor air is being distributed uniformly to where people are in the building. Problems with the distribution components of HVAC systems include closed fire dampers, variable air volume (VAV) boxes with low minimums, leaking supply air ducts, blocked return air plenums, and poor balance.

What then is the most accurate way to provide diagnostic information on the amount of ventilation being provided? Theoretically, knowing the percentage of outdoor air in the supply air and combining it with knowledge of the quantity of supply air delivered to an area would allow the amount of outdoor air delivered to be calculated. Unfortunately, determining the percentage of outdoor air in the supply air can be difficult, and this parameter can change through the day as the wind shifts, changing the pressure relationship at the outdoor air damper. Calculating the percentage of outdoor air in the supply air by the mixed-air temperature method is just not that accurate. In one reported 42-point traverse of a 79-square foot coil, the average mixed air temperature yielded a value of 26.2% for outdoor air in the supply air. With outdoor air of 30.3¿ Fahrenheit (F) and return air of 72.5¿F, the mean mixed air temperature of 61.4¿F had a standard deviation of 4.9¿F.

Recalculating the amount of outdoor air in the supply air indicated that the value was somewhere between 14.5 and 38.0%-not a very accurate conclusion. The potential variation in the amount of supply air provided to the occupants, an inherent property of the all-too-frequent variable air volume (VAV) systems in use, only increases the uncertainty.

The knowledge that people generate carbon dioxide (CO2) in fairly predictable amounts can also be used to assess the ventilation. Ventilation rates, as attested by the American Society of Testing and Materials STM, can be determined by the continuous measurement of indoor CO2 through the day. This approach can be very accurate because it assesses the ventilation rates, taking into consideration how all of the components of the HVAC system function together, and not just how much outdoor air enters the HVAC system. Also, since the calculation of ventilation rates from the maximum difference between indoor and outdoor CO2 concentrations yields a result in the form of cfm/person. This result is directly comparable with the minimum requirements listed in ASHRAE Standard 62.

CO2 monitoring for diagnostic purposes gives the operators of a building the ability to dynamically balance the distribution portion of the HVAC system to achieve an intended allocation of ventilation. In addition, they can position the minimum outdoor air damper to achieve the desired amount of ventilation so as to achieve this goal without wasting energy.

Measuring CO2 concentrations falls into the category of data logging, and the optimum approach will vary as a function of the building's size. That is, the optimum data logging approach will be different in a small building with only six measurement locations as opposed to a larger building with 48 sampling locations.

One approach to measuring CO2 involves running individual tubes from various locations to a shared sensor and central monitoring equipment. Because the same sensor monitors all locations, an observed difference between two locations will be real. Observed differences in a multiple sensor system may be due to a differing response or calibration of the two different sensors. Keeping just one high quality sensor working and in calibration is also effective in terms of both staff and costs.

Figure 1 shows actual monitoring results for two training rooms included among nine locations served by one air handling unit (AHU) in the building. One morning, the building system met the thermal requirements in rooms 108 and 112, and the local VAV boxes went to their minimums. Unfortunately this resulted in inadequate ventilation: less than 6 cfm of outdoor air per person. The rest of locations served by this AHU were well ventilated, at over 40 cfm of outdoor air per person, as indicated by the small increment between the return air value and the outdoor value.

The figure also indicates that the the indoor levels dropped quickly to the outdoor level, another measure of generous ventilation.

In addition, the shared-sensor approach to CO2 monitoring enables building operators to monitor of other parameters by adding additional sensors to the centrally located sensor cabinet where air from up to 48 locations in the buildings is automatically being delivered from one location at a time.

Dew point, for example, can be particularly useful to the building operators. The dew point temperature is, after all, a measure of the absolute humidity. Knowing the absolute humidity in their buildings can enable operators to do a better job of managing their buildings. This feedback will not only be used to assess the operation of any humidification systems; it will also be used to assess the ability of the HVAC system to dehumidify the indoor air as compared with the outdoors.

When there is a difference in absolute humidity between indoors and outdoors, any infiltration will also be reflected in the humidity monitoring data. Interior sources of moisture will also be detected, and this can save energy if leaking steam tables are detected.

A leaking chilled water valve will not only waste energy, but it can also be a precursor to the indoor growth of molds, which can severely degrade the quality of the building's environment. Figure 2 presents the results of humidity monitoring for this building on June 13, 1996; the monitoring shows that the outdoor humidity is higher at ground level as compared with rooftop level due to the moisture from the earth. A humidity level at the loading dock, intermediate between the outdoors and the other indoor locations, points out the source of infiltration. The elevated absolute humidity of the supply air from AHU-1, as compared with the interior locations, indicates that the HVAC system was not working as intended. A subsequent review of this equipment identified that a piece of sheet metal next to the cooling coil had ripped open allowing unintentional bypass of this dehumidification portion of the AHU. This figure also reveals that the local humidity control in Lab #5 that did not increase like most of the other interior locations when the HVAC system shut down for the night.