CO2 Sensors, Goldilocks, and Creating a Building Management System that Works Just Right

Goldilocks was on to something. Her trick for finding the happy medium between extremes is a principle that applies to a lot of different areas – including building ventilation. Getting things just right in this area can make buildings more efficient and people healthier. And it all starts with a CO2 sensor, a field device at the foundation of a building management system.

As people breathe, they exhale CO2, a colorless and odorless gas. So, the level of CO2 in the air is an indication of how many people are present. It also is a marker for indoor air quality. The higher the CO2, the unhealthier the air inside a building may be.

In some cases, the result of bad indoor air can be headaches, coughs, dizziness, nausea and illness – symptoms of sick building syndrome. The causes, according to the Environmental Protection Agency, are inadequate ventilation, as well as indoor, outdoor and biological contaminants. In addition to eliminating contaminants, the solution is increased ventilation, with the recommendation being a minimum of 15 cubic feet per minute (cfm) of outdoor air per person.

Great! With standards in place, one would think that the problem is solved, right?

Actually, it isn’t. Swapping conditioned indoor air for unconditioned outdoor air may make everyone breathe easier but it isn’t energy efficient. Every time air comes in from the outside it has to be heated or cooled and its humidity adjusted. To give you an idea of the impact, the standards adopted in the 1970s for energy conservation called for ventilation of 5 cfm per occupant. The standards have since been revised back up to 15 cfm.

But, a lot of buildings were built while the standard was low, so they may have indoor air issues. As for all the others, remember that the ventilation figure was originally dropped to up energy efficiency, something that everybody wants to see be as high as possible.

So, too much ventilation means wasting energy. Too little means people might get sick. All buildings have a varying number of people in them, with that number changing throughout the day and day to day. Weekends, for instance, may present a light occupant load for some buildings while for others the same may be true for mornings.

What do you do when a building has variable occupancy? Set the ventilation for the highest possible load? Or do you make the system somewhat smarter and use a timer? Do you do your best and hope?

You should let Goldilocks be your guide – along with a CO2 sensor.

Monitoring the concentration of CO2 provides a measure of the air quality. If the CO2 level gets too high, ventilation can be adjusted to bring in more outside air. As long as the CO2 level remains acceptably low, less outside air can be brought in per minute. So, the demands of health and energy efficiency can be better balanced than would be the case if a fan were running full blast all the time or if a timer were used.

The U.S. Green Building Council has some guidelines on this and their recommendation is to use a CO2 sensor for those cases where ventilation is adjustable. Indeed, the proper use of a CO2 sensor can help you earn a point in a green building, or LEED, certification.

Now, as for what CO2 sensors look like, they may be boxes that sit next to, and are sometimes mistaken for, thermostats. The sensors sample the air and report back to a controller, just like other field devices: thermostats, valves, sensors, actuators, other sensors and input and output devices. They’re all part of the firm foundation any modern ventilation system needs.

With a CO2 sensor, you can make sure the air inside a building is – and stays – just right. That’s something Goldilocks would approve of.

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