BSEE-JUL21-P10-11 Vaisala_Layout 1 23/06/2021 15:04 Page 11
AIR CONDITIONING, COOLING & VENTILATION
NonCovid advantages of CO2 and humidity monitoring
The ASHRAE Green Standard 189.1(USA) and the European standard FprEN 16798-3 recommend using demand controlled ventilation (DCV) to reduce energy usage while promoting healthy indoor air. From an HVAC design perspective, CO2 is an ideal proxy for indoor air quality where the building is predominantly occupied by people. Humidity would be either better or at least a useful additional parameter, especially in buildings that are used to store artwork, books, wine, historic artefacts etc., or in buildings that are themselves in need of conservation. Typically, outdoor air contains 250 to 400 ppm CO2. In contrast, exhaled breath contains around 50,000ppm CO2 which represents a 100 fold increase over inhaled gas, so without adequate ventilation, when people are indoors, CO2 levels will gradually rise.
Both the comfort and performance of people inside buildings can be affected by CO2 levels. Occupied spaces with good air exchange may contain 350-1,000 ppm, but anything above this can induce drowsiness, with levels above 2,000 ppm causing headaches, sleepiness, poor concentration, loss of attention, increased heart rate and slight nausea. Exposure to very high levels
(from oil/gas burners or gas leaks) can even result in fatalities from asphyxiation.
Recommended minimum ventilation rates are provided for a wide variety of indoor spaces in ANSI/ASHRAE Standard 62.1-2019 Ventilation for Acceptable Indoor Air Quality.
Several studies have evaluated the effects of CO2 concentration on cognitive function. For example, Allen et al (2016) found that cognitive function scores were 15% lower for the moderate CO2 day (~945 ppm) and 50% lower on a day with CO2 concentrations of ~1,400 ppm in comparison with two ‘Green+’ days (~ 540 ppm). On average, a 400-ppm increase in CO2 was associated with a 21% decrease in a typical participant’s cognitive scores. DCV based on CO2 measurements can therefore deliver improvements in well-being and productivity that far outweigh the costs of the DCV system itself.
Choosing the right CO2 transmitter
It is important to resist the temptation to purchase the cheapest sensors that meet the required specification. This is because, whilst accuracy and range are important; the ongoing performance of the BMS will rely on the stability of the sensors. Suppliers of HVAC systems will
naturally prefer sensors that you can ‘fit and forget’. Consequently, it is necessary to select sensors that do not require frequent recalibration to prevent drift. However, the selection process is further complicated by sensors that claim to compensate for drift by implementing a software solution which assumes that the lowest measured readings are the same as the average outdoor concentration of CO2. The danger with this type of algorithm is that small errors are compounded as time passes; leading to very significant errors in the longer term. As an attempt to avoid true calibration, these software algorithm sensors are not applicable in spaces that are continuously occupied, and can also be fooled by building automation systems that aggressively ramp down fresh-air intake during off-peak hours. In some cases even the concrete in the walls may absorb CO2 and thereby ‘trick’ the algorithm and create further inaccuracy.
There is potential for a slight conflict of interest between a BMS supplier/installer and a building owner/facility manager. For the former, the system must work perfectly immediately, and for at least the period of the warranty, but for the latter the requirement is more long-term.
The cost of a good sensor fades into insignificance in comparison
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with the benefits that it provides. Energy savings from accurate, need- based controls can be considerable, but even more importantly, the health and well-being of people inside of the building are protected and indoor conditions improve workplace performance.
The ideal solution is therefore to opt for Vaisala CARBOCAP® CO2 sensors. This is because they employ dual-wavelength NDIR technology capable of thriving in a variety of environments and able to conduct true self-calibration with an internal reference. The cost of this technology is insignificant in comparison with the energy costs of a BMS that is not efficient or with the cost of maintenance when low-cost sensors drift or fail.
It is not uncommon for Vaisala’s sensors to operate trouble-free for up to 15 years. This stability and reliability has been recognised around the world… and beyond. Vaisala sensors continue to operate on NASA’s Curiosity Rover, which was launched in 2011, and on-board the Perseverance Rover which landed on Mars in February 2021.
In summary, here on Earth, disease prevention measures can be re- enforced by smart ventilation with reliable CO2 measurements. Furthermore, good indoor air quality can have a significant positive impact on the health and well-being of people inside buildings.
Read the latest at:
www.bsee.co.uk
BUILDING SERVICES & ENVIRONMENTAL ENGINEER JULY 2021 11
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