Ventilation Energy Savings While Maintaining Premium Air Quality

Intelligent air quality
Adults consume two to three liters of liquids and one to two kilograms of food per day. While hygiene and safety of edibles receive great attention, air quality gets very little even though on average we inhale 15 kg of air per day — 80% of which indoors.

From the classroom to the cubicle, the benefits of maintaining good indoor air quality extend beyond protecting the occupants’ health.

Students in schools with healthy air are more proficient at retaining information and teachers have fewer sick days.

For employers, studies show that improving indoor air quality directly correlates with higher productivity and a more satisfied workforce.

Moreover, the advent of “green buildings” and emission-dependent energy taxes has created awareness for both indoor air quality and ventilation energy costs.

Consequently, in modern or reconstructed buildings, the alternatives of either having minimal ventilation with poor air quality on the one hand or permanent ventilation with high ventilation energy costs on the other are impractical.

A balance between the two extremes exists in “demand
controlled ventilation” or DCV.

This paper is focused on air quality sensors for DCV. It describes typical indoor air contaminants, their sources, and their impact on human health.

Moreover, it confronts current indoor air quality standards with modern ventilation demands and compares today’s commercially available air quality sensor technologies accordingly.

Finally, suggestions for improvement of typical ventilation scenarios by using AP:s intelligent air quality solutions are provided.

Anatomy of Indoor Air
Clean air is comprised of 21% oxygen, 78% nitrogen and 1% argon.

However, indoor environments are different where other noble gases, carbon monoxide (CO), carbon dioxide (CO₂) and volatile organic compounds (VOCs), also known as mixed gas, exist with different prominence.

When it comes to the impact on health the latter two are the most important ones: CO₂ for its HVAC (heat, ventilation, and air conditioning) industry awareness, and VOCs for their criticality.

. . . beyond CO2

The Role and Impact of VOCs in Indoor Air
There are estimated to be 5,000 to 10,000 different VOCs, which are two to five times more likely to be found indoors than outdoors. Indoor VOCs are hydrocarbons that originate from two primary sources:

• bio-effluents, that include odors from human respiration, transpiration and metabolism

• vapors generated from building materials and furnishings.

VOCs cause eye irritations, headache, drowsiness or dizziness, and contribute to a condition known as “sick building syndrome” or SBS, whereby adequate ventilation must be provided.

Aside from industrial conditions and comfort aspects such as temperature control, VOCs are the most critical reason to ventilate. Some typical indoor contaminants and their sources are shown in Table 1.

Clearly, humans represent the greatest source of VOCs, directly and indirectly; far beyond building materials,
furniture and office equipment, and thereby dominate the demand for ventilation.

Table 1 - Typical Indoor Air Contaminants (VOCs and others)

The Role and Impact of CO₂ in Indoor Air
Although CO2 is listed twice in Table 1 and plays a major role in modern ventilation control, it has no permanent effect on humans, especially in small doses. Exposures on submarines and the International Space Station confirm that even heavy CO₂ concentrations of 1% (10,000ppm) show no irreversible impact on occupant well-being.

Due to the lack of suitable VOC sensing devices, CO₂ has served historically as an adequate air quality indicator.

Moreover, since the amount of CO₂ is proportional to the amount of VOCs produced by human respiration and transpiration (Metabolic Rule) CO₂ levels reflect the total amount of VOCs (TVOCs) as illustrated in Diagram 1.

Diagram 1 - CO₂ and VOCs from Business Meeting Sessions

Therefore, the ease of targeting CO₂ as a single analyte versus thousands of VOCs, combined with the availability of suitable CO₂ measuring technology has made CO₂ the surrogate of occupant-generated pollution in indoor environments.

As such, CO₂ levels are today’s standard indoor air quality reference for DCV with tangible air quality definitions as initially introduced by Max von Pettenkofer and adopted by most HVAC industry standards. See Table 2.

The Volatility of Volatile Organic Compounds
Diagram 1 illustrates more than just the correlation between
VOCs and CO₂. Importantly, the diagram also demonstrates
that VOCs are much more volatile, or sudden in their occurrence.

An increase of human bio-effluents or the intermittent
use of odorous materials such as cleaning supplies, perfumes or cigarette smoke is not uncommon. Diagram 1 shows spikes

of these events; thus, relying exclusively on CO₂ as a ventilation reference will lead to unsatisfactory results.

Ventilation should react on demand toward all contamination sources, not only CO₂. This points out the weakness of CO₂ - based DCV. Detecting a broader range of contaminants optimizes ventilation energy savings and minimizes the impact on human occupants.
 

Indoor Air Quality References From Past to Present
Historically, CO₂ has been the detection gas of choice since it is a reasonable reference and its detection is fairly easy. Mixed gas or VOC detectors suffered early criticism due to long-term stability problems and the inability to calibrate output units.

Further, without suitable threshold values, HVAC planners using VOC detectors could not easily set ventilation rates and VOC sensor drift made the entire ventilation system functionally unpredictable.

Although the motivation to measure the root cause for contaminated air was appropriate, the implementation was not successful.

AP:s Approach — Close to Human Perception
Taking into account the lack of VOC standards, AP:s iAQ, intelligent Air Quality, sensor takes advantage of its Reversed Metabolic Rule technology, RMR. RMR technology calibrates measured VOC concentrations to CO₂ - equivalent ppm-values, thereby achieving full compatibility to CO₂ - standards.

Moreover, the iAQ sensor captures all VOC odor emissions that are completely invisible to CO2 sensors as Diagram 2 demonstrates.

Importantly, AP:s control algorithms correct for sensor drift and aging and thereby provide consistency. The iAQ sensor overcomes deficiencies of CO₂ measurement by detecting the true root-cause of ventilation demand, VOCs.

Further, the iAQ sensor remedies deficiencies of typical VOC sensing technologies by signal-adherence to established CO₂ standards and stringent drift compensation for extended sensor life. The iAQ sensor emulates the human perception of air quality and even detects odorless, potentially hazardous substances such as carbon monoxide.

Diagram 2 - Typical scenarios where CO₂ sensors fail as DCV reference

Which Reference to Follow
Today, various types of DCV sensors are available including occupation detection, CO2 detection, humidity measurement and VOC sensing.

Table 3 compares the performance of the latter three air quality sensor technologies over various applications, clearly depicting the advantage of AP:s intelligent Air Quality technology.
 

Table 3 - Performers of various Air Quality Sensors over typical ventilation scenarios

When and How to Ventilate
The answer is: On demand. Most VOC events are unpredictable as they are dominated by human metabolism
and behavior, which accounts for more than 85% of all ventilation cases.

The remainder comes from building material emissions common in new buildings and after refurbishments or from furnishings and coatings.

To dilute these emissions sufficiently, low-rate, permanent ventilation at 5-10% of maximum is adequate.

Table 1 lists relevant substances and recommended ventilation scenarios. VOC emissions rarely occur in isolation; therefore, a combination of both ventilation types is the ideal solution.

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