Using Data Loggers to Perform Environmental Monitoring

Morse Zehnter Associates
An energy consultant from Morse Zehnter Associates talks to Onset about using data loggers to identify and alleviate problems related to indoor air quality, temperature, and humidity control issues in a large South Florida school district.

How does Morse Zehnter apply portable data loggers in environmental monitoring projects?

We have been able to go into a large school district here in South Florida with a number of problems related to indoor air quality (IAQ), temperature, and humidity control issues. Upon our undertaking the study we received numerous IAQ related complaints. We initially used HOBO® data loggers to illustrate that a majority of the IAQ complaints were not based on age of equipment, which was what was suspected. The complaints largely stemmed from issues that had more to do with maintenance and capacity of equipment, including over-sizing and excess ventilation. We followed this with detailed renovation designs and followed up with ventilation designs for schools.

Specifically, we used five to ten HOBO® U12 temperature/humidity loggers in about 200 schools over the course of a few years and did the retrofit design in part based on the Onset data we collected. We then performed a pre- and post-evaluation of ventilation rates using occupant-generated carbon dioxide (CO2) as a tracer gas and estimated ventilation rate based on its decay after occupants left the space. In addition, we also measured the air flow of the zone, which included supply, return, ventilation or outside air, and exhaust, as comparison to the tracer gas analysis.

How do you monitor carbon dioxide levels in these buildings?

We couple the HOBO® U12 loggers with Telaire CO2 monitors to track carbon dioxide levels, typically for a monitoring period of one week. This was a part of a continuing services contract for code-compliant ventilation studies. Through careful interpretation of the data and applying the ASTM methods to the ASHRAE 62.1 Indoor Air Quality standard, in spaces that were determined applicable we were able to determine ventilation rates by the decay of occupant-generated CO2 levels. The data also allowed us to see what time during the day CO2 levels trailed off so we could advise when to shut off exhaust fans. Ultimately, by documenting this data, we were able to show building officials the ventilation rates that varied anywhere from 2 - 20 CFM of outside air per occupant and how that represented the relationship between operating costs and IAQ. The client was very receptive to the potential benefits of energy savings and improved IAQ.

How were the data loggers physically deployed in the school buildings?

We took a HOBO® U12 and interfaced the Telaire sensor to it. Then we concealed the equipment by placing them in unmarked brown metal thermostat boxes that were perforated to allow the ambient room air to pass through. The inconspicuous brown boxes were mainly used to secure and protect the instruments from curious occupants. This disguise is very handy when trying to data log CO2 readings as the exhaled breath directly onto the instrument can greatly affect the readings. The boxes were consistently placed within the breathing zone height of the room, but carefully placed away from the occupants to avoid direct exhalation, and away from doors, windows, and direct flow of supply air diffusers to avoid dilution of the room values. This technique follows both the ASTM and the ASHRAE standards methodology. We were also able to easily orient the logger in a way that allowed us to use the light logging capability as well. This was useful in helping us see when rooms were unoccupied or when corridor lights were left on, which can significantly add to operating costs as well.

What software tools did you use to interpret the data?

We initially utilized the HOBOware® Pro software, which is very useful for taking quick looks at the data. We can take snapshots to interpret data quickly. Once we found which selection of the logged data would be used for calculations and was as close as possible to a typical representative day, based upon interpretation of the week's data and interviews with teachers, we exported to Microsoft Excel for detailed analysis. In doing this, we were able to clearly see which HVAC zones were either over- or under-ventilating the respective classrooms. In the case of over-ventilation, many times this condition was exceeding the HVAC system's ability to dehumidify the excessive amount of ventilation air, thus leading to a lot of untreated moist areas, which ultimately can cause mold growth within the space.

What role do the data loggers play in mold investigations?

Speaking of HVAC equipment and ductwork, we've had projects where people complain they have mold coming out of their ductwork. Along with visual inspections, we can use HOBO data loggers in these projects to fully evaluate the conditions within HVAC equipment or ductwork. The HOBO data loggers have a range that approximates a very good direct digital control sensor that measures and controls operation of large built-up mechanical systems. We can take the HOBOs into classrooms and into mechanical systems, adhere them to the inside of a duct, and get a pretty good idea of how the units are providing air through the systems.

Overall, it sounds like school buildings pose a big challenge when it comes to balancing indoor air quality with energy savings.

Over- and under-ventilation is not just a South Florida phenomenon. It happens everywhere. In schools you're dealing with a balancing act of a rapidly-changing occupancy during the day, differing occupant densities within the same air handling zone, and changing of functional spaces. This can be a big challenge. Schools often get a bad rap for being mis-engineered – we hear it all the time – but it's not always the case. Using tools like HOBO data loggers and Telaire sensors, we can show real data to the people who are making the decisions. The data says everything.