Measuring CO2 in HVAC applications
Heating, ventilation, and air conditioning (HVAC) in buildings is typically not considered a challenging environment when it comes to CO2 sensors. While it’s true that ambient conditions are mostly benign, sensors still need to be reliable, easy to maintain, and offer long-term measurement stability. In this post, ESIS presents a discussion about why CO2 needs to be measured at all, and how demand-controlled ventilation (DCV) works. We then move on to look at how measurement helps maintain people’s wellbeing in a building, and how it impacts operating costs.
Why do we measure CO2 in HVAC applications?
While the most common reason for measuring CO2 is to save energy, the growing body of evidence demonstrating the direct link between indoor air quality (IAQ) and human wellbeing means that measurement is becoming important for maintaining healthy and productive working environments, too. Compliance with regulations is another driver, particularly if building owners are pursuing certifications that require the use of DCV and/or CO2 measurement.
How does DCV work?
The graph above illustrates the differences between regular time-controlled ventilation and DCV. The blue bars show the occupation level of a space such as an office over the course of a day, while the orange line shows how regular time-controlled ventilation works. In this case, ventilation is time controlled between 7 am and 6 pm to save energy by not ventilating at night. The grey line illustrates how DCV works with CO2 measurement. The CO2 concentration in a room is measured with sensors and the amount of fresh air is controlled to maintain the concentration at the desired level. The amount of fresh air or ventilation varies according to the building’s occupancy level.
One of the most important standards in relation to HVAC applications is the ASHRAE 189.1 green building standard, which places strict requirements on CO2 sensors in terms of accuracy and requires either that they should be capable of measuring outdoor CO2 concentration or that the concentration should be estimated based on local statistics. In practice, this means that many people opt for an outdoor sensor instead of an indoor one.
One of the longest standing regulations is the Californian Energy Commission’s CEC-400-2008-001 standard (Efficiency standards for residential and non-residential buildings). The LEED v.4 green building standard awards credits for CO2 measurement, with two credits available for CO2 monitoring in occupied spaces. There are also requirements relating to the accuracy, calibration interval, and maintenance of sensors – something that we are seeing more and more of, and which makes perfect sense. Why demand measurement if you are not demanding maintenance too?
Indoor Air Quality (IAQ) is an emerging topic. For example, the WELL building standard from the International WELL Building Institute is largely based on the LEED and ASHRAE standards. Currently, the focus is shifting away from technical requirements more towards occupant wellbeing. In 2010 the World Health Organization published an indoor air quality guide that includes guidelines for selected pollutants and discusses their potential health risks.
The impact of HVAC measurement on building operating costs
A breakdown of operating costs in a typical office building might look something like this:
Energy costs account for about 1% of the total operating costs of a building, with rental or capital costs totalling 9%, and employee wages 90%. This means that employee wellbeing and productivity is probably the most important consideration in relation to HVAC. Not optimizing your HVAC systems with the help of CO2 measurement might hit you in the much bigger 90% slice by reducing productivity.
The link between CO2 levels and employee productivity
Research has shown that CO2 concentration has a big impact on human cognitive performance. One study conducted by a group from Harvard looked at how specific skills were affected by varying CO2 concentrations. The study found that skills such as crisis response, information usage, and strategic thinking were significantly impaired even at a concentration level of 1000 ppm, which is considered to be a healthy level to work in.
Elevated CO2 levels could result in impaired employee learning, decreased productivity, poor decision-making, mistakes, and even hazardous situations. In general, the research seems to agree that good indoor air quality can contribute to a significant increase in employee productivity.
It’s easy to choose and install a CO2 sensor for demand controlled ventilation with the Vaisala GMW80 and GMW90 ranges which are designed for the purpose. Transmitters are available with a variety of outputs, and can have an LCD display to show the measurements. Models are also available with built in humidity and temperature sensors.
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Article courtesy of VAISALA.