Design of Ventilation Systems in Office Buildings

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Table of Contents

  1. Introduction
  2. Ventilation Types in Office Buildings
    1. Natural Ventilation
    2. Air Conditioner
    3. Mixed Mode
  3. Thermal Comfort
  4. Occupant Satisfaction
    1. Natural Ventilation, Air Conditioner and Mixed Mode
    2. Occupant Behaviour on Window Control
  5. Conclusions
  6. References
  7. Appendix
  1. Introduction
  1. Ventilation Types in Office Buildings

One of the main adaptable opportunities in an office building is about ventilation systems. The ventilation systems can be through cooling or to get some fresh air due to occupant’s behaviours. However, this possibility has decreased during past years with a modern approach to office buildings, architectural society decided to use air conditioner systems instead of allowing natural ventilation through windows. Still both ventilation and cooling methods are under effect of these systems. On the other hand, there are buildings with mixed mode operation that allows occupants to control their window systems and at the same time air conditioner. In general the ventilation types in office buildings can be collected with these three groups which are Natural Ventilated, Air Conditioned and Mixed Mode Usage.

  1. Natural Ventilation

With focusing on a dictionary meaning, it would be easier to explain ventilation as the exchange of outside air in terms of to achieve better or acceptable indoor air quality. Also, natural ventilation has a meaning of combining two functionalities that are outdoor air exchange for cooling and ventilation also considering the effect on occupants, occupied space and building mass. This cooling and ventilation effects are in through operable windows in buildings.

Advantages

As a behavioural conclusion for humankind, people are likely to change the environment that they are inside or be able to change. That is why mainly during summers people are mostly using their windows to benefit from fresh air and gather a small amount of breeze through window that would affect their thermal acceptance due to the comfort ratio. To have a natural ventilation also means reducing the energy usage, gas emissions and operating costs over mechanical ventilation or other ventilation systems which have a significant economic effect. Apparently while determining the ventilation systems climatic conditions are the primary input. In a climate that will be difficult to achieve comfort temperatures during the summer just with natural ventilation, people prefer mechanical ventilation, which is going to increase the cost of building expenses in a critic way.

In naturally ventilated buildings, thermal conditions are showing differences than the air-conditioned building also with considering the prediction value due to changeable environmental factors. However to allow occupants to have their control over windows is the only way to let them interact with thermal conditions in the occupied space.

Also ASHRAE Standard 55-2004 has concluded the research done in 160 buildings on different continents that occupants who have control over natural ventilation due their thermal acceptance with considering their desires and characteristics or window control, are tolerating wider range of temperatures compared to the air conditioned buildings. This research has showed the effect of adaptive opportunities on occupant behaviour and thermal acceptance in a clear way. In naturally ventilated buildings adaptive opportunities are not just limited to the opening of windows or not. As a personal control, clothing level can be decided with considering the indoor environment. While pointing to the indoor environment, indoor air quality is one of the most critic titles. Research have mentioned that indoor air quality has much higher in naturally ventilated building than the air conditioned. About this research, one critic study has found the ratio of sick building syndrome in the air conditioned building is significantly greater than naturally ventilated building.

Relevant facts

Like all of the problems that every building element have, natural ventilation has its limitations too. These points are mostly because of the climatic conditions where natural ventilation cannot meet the expectations. While considering about the natural ventilation the main topics supposed to be focused on are outside temperatures, humidity, and air velocity, air quality which depends on the environment that can show variety due to density of area and also noise level.

  1. Mechanical Ventilation

In mechanical ventilation air, exchange is occurring with a help of mechanically powered equipment, which are consuming electricity and cost. These kind of equipment have also mechanically cooling function to arrange the temperature of space. Mechanical ventilation and cooling can work together or individually. Some of the buildings are just using mechanical ventilation instead of cooling function. These systems are often controlling by a central operational mechanism.

Advantages

As stated before mechanical ventilations and cooling are working without considering the outside temperature, climatic conditions. This can provide flexibility for the design of the buildings in the approach for most of the architects. Architectural flexibility can also be counted as operational flexibility due to each occupied space by considering the occupants. These systems can control air movement, infiltration and allow for moisture management. Mechanically, like natural ventilation, the aim is to increase indoor air quality or balance it due to thermal comfort.

Relevant facts

Although mechanical ventilation systems are working to control the indoor thermal conditions, usually they are criticized for being limited controlled. The main reason for this is the lack of considering the relation between architecture and ventilation. Planning involves spaces, occupancy and also occupant behaviours. When the control ratio is not well balanced for occupants and space, this can bring discomfort and low efficient thermal control. Also in most of the cases, people working in open space working areas do not have adjustments over the ventilation systems. On the other hand, with changing the open space into separated ones, maybe the personal control will increase, but system cost and complexity will increase too.

Like natural ventilation, noise can be a problem for mechanical ventilation too, but this time not because of the environment conditions, it is due to system elements itself.

  1. Mixed Mode

Mixed mode is a combination of natural ventilation and mechanical cooling. Like totally natural ventilated building, in mixed mode occupants can control their windows to get fresh air or change the internal temperature.

One of the limitations of natural ventilation was due to extreme climate conditions. That is why in mixed mode, whenever natural ventilation is not enough to achieve the comfort band, mechanical systems are starting to operate. If these two methods are going to be used in an efficient way, it can decrease cooling loads and relatively the costs.

Advantages

While the mechanical systems are working, as stated in the previous chapter they can perform in every climate conditions. To allow building to be able to use natural ventilation for acceptable conditions can satisfy the occupants and building management. Window opening can be manually or automatically set due to particular program and schedule. This program can show a variety of different climates.

Relevant facts

It will be clear to mention the first costs are going to be higher than other systems. To have both operable windows and mechanical ventilation/cooling will bring weight to the costs. However, these systems are likely to reduce the operational costs, not the first cost.

Also, because if the system is occupant dependent, there can be some problems due to control. Occupants need to understand when or how mechanical systems are working. If mechanical cooling and natural ventilation are operating at the same time, this will increase the loads and costs too. That is why one of the argument is to have educated occupants in a particular building or to prepare user guides.

  1. Thermal Comfort

ASHRAE Standard 55

Main purpose of the Standard 55 is to understand the relation between thermal comfort and indoor environment. The scale of the Standard is not building dependent. Therefore, it can be used for domestic and non-domestic buildings. Because of the primary factor is occupancy, it can be also used for transportation vehicles.

Due to Standard 55, the acceptable comfort band supposed to be %80 for the occupants who are under the same environmental conditions and space as shown in the Figure X.X. Dissatisfaction ratio can explain differences in researches or definitions of local discomfort.

Thermal Comfort

Because of people’s adaptations, behaviours, cultural impacts or occupancy patterns are different from each other; it would be difficult to assess a single comfort definition. That is why a thermal comfort can depend to occupants who are responding to their thermal environment. In general this sense defines the heat exchange of the human body with its environment. Moreover, in this equation there are some factors that can affect this feeling. As mentioned before, these elements can be collected in two groups that are thermal environment and occupant dependent. For the environment, air temperature, radiant temperature, humidity and the air speed can be counted, for occupancy, clothing level and metabolic rate that can show the difference between each activity, season or space.

PMV and PPD

PMV, in longer definition, Predicted Mean Vote is the result of occupant questionnaires based on ASHRAE thermal sensation scale to presume the quality of the thermal environment. This sensation scale is from +3 to -3 which defines hot to cold. After getting the results of PMV, PPD (Predicted Percentage Dissatisfied) can be calculated too. The values being used while calculation the PPD are +-2 and +-3, which tells the discomfort of the occupants as hot, slightly hot or cold, slightly cold.

  1. Occupant Satisfaction

To make a comparison between ventilation types in office buildings in terms of occupant behaviour and satisfaction, this paper depends on Kano’s satisfaction model. This model is suitable to understand the relation of occupants and indoor environment quality for commercial buildings. In this model, indoor environmental quality factors have divided into three groups. Basic, Bonus and Proportional Factors. Basic factors can be explained by meeting the minimum requirements. It is not covering the whole occupant satisfaction, but it can be useful to understand the basic dissatisfaction. Therefore, instead of the positive results, Basic Factors are likely to negative performance. On the other hand, Bonus factors depend on the positive outcomes which are complete satisfaction. For Proportional Factor, it is the bridge between Basic and Bonus factors, satisfaction and dissatisfaction. Proportional factor is mostly considering the performance and related with satisfaction. This relation is linear. When the performance is high, satisfaction level is also high. If the performance is low, satisfaction is low too. As graphically, these factors can be seen in Figure X.X

Referring the research that has been made for estimating the satisfaction values by Kim Jungsoo, in the survey that is used, occupants are expected to give votes from +3 to -3. The ones, who voted for +3 or +2, has been counted as satisfied and occupants who chose -3 or -2 for dissatisfied. The ones for +1, 0 and -1, they are counted as a reference group. Thus, this three sub-groups (satisfied, dissatisfied and reference) are making the 7-point scale satisfaction table. Because of the psychological effect is not the same between making a decision for +2 to +1 and +1 to 0, researchers wanted to simplify this equation by sub-groups as shown in the Figure X.X

  1. Natural Ventilation, Air Conditioner and Mixed Mode

The database, which has been used on previous research paper by Kim, are from the Centre for Built Environment of the University of California, Berkeley. In this paper that questionnaire has used as a reference to understand occupant satisfaction levels due to different ventilation types. In that CBE survey, occupants are expected to make a choice between +3(very satisfied) and -3(very dissatisfied). The study is covering office buildings mainly in USA, but there are also buildings from different countries too. Table X.X shows the questions that have been asked in the survey. After receiving the results, dataset has divided for their ventilation systems, Natural Ventilation (NV), the Air conditioned (AC) and Mixed Mode (MM) as shown in the Table X.X.

Figure X.X displays the mean satisfaction rating for different ventilation types. For overall satisfaction, it is clear to state that occupants in natural ventilation is higher than air-conditioned buildings. In the same figure, for the natural ventilation there are more advantages like amount of light, noise level, sound privacy, amount of space, visual privacy, ease of interaction. These elements are also affecting the satisfaction values in a significant way.

On the other hand, Figure X.X shows that in overall satisfaction in AC buildings, thermal comfort is not as important as the other ventilation types. There is a definite relation between unsatisfied thermal performance and overall dissatisfaction that can be concluded as AC is getting realized while the comfort level is decreasing. That also show the sensitivity between them.

In naturally ventilated buildings occupants, have control on their windows and also for their personal factors which are mostly their clothing level as mentioned in this paper before. They would like to adjust themselves rather than having a dissatisfaction. This can be explained by the significant importance of adaptive opportunities. The study has also showed that people in naturally ventilated buildings can tolerate a wider range of temperatures than the MM and AC buildings. Also, their expectation level is adjusting with considering these factors.

  1. Occupant Behaviour on Window Control

One of the detailed researches has done about the relation between ventilation systems and occupant satisfaction which is concluded by ASHRAE RP-884 also confirms that in naturally ventilated buildings people are accepting wider range of temperatures than the occupants have HVAC systems in their buildings. On the other hand, ASHRAE RP-1161 has expressly looked at the operable windows, local thermal conditions and its effect on occupant behaviour, which is completely in the case of Natural Ventilation.

For this research, the selected building is Berkeley Civic Centre in San Francisco, which is an office building with nearly 230 employees that can be seen in Figure X.X. The department has designed as open plan except some private rooms. Building has natural ventilation with an opportunity for cross ventilation totally too.

The survey that they had included “basic demographics, personal workspace characteristics, various personal environmental control opportunities, window operation and satisfaction with various environmental attributes”. Brager, Gail 2004.

First of all, the questions about thermal sensation and air movement preference due to warm or cool season are showing a critic difference between each other as shown in Table XX. As a result, it can be understood for occupants that they would like to feel cooler in the warmer season with %41 more than warmer in cool season with %14 ratio. That means people react warm weather more than cold weather. Also, air movement table is approving about situation, and people want more air changes during warm air. This can be explained by the range of adaptive opportunities like clothing in a warm season. During cold weather, while they can increase the clothing level without any significant limitations, in warm weather this opportunity is limited.

In Figure X.X, it shows the chart between thermal sensation and air movement preference. It is clear to state that when occupants are feeling warmer, they are expecting for more air movement. However for the cold or neutral temperatures the overall is likely in the no change values.

The survey continues with grouping the occupants with their access ratios to windows. The ones, who have direct control over windows, are called PC_HI, and other occupants, who do not have control or have limited control, are called PC_LO. Table X.X represents that in the warmer season the difference between PC_HI and PC_LO is not high due to thermal sensation. However for the cool season the difference is significant especially for the Top and Va values.

The outcomes from the neutral temperatures as shown in Table X.X, and it is clearly pointing the difference between PC_HI and PC_LO due to seasons. In the warm season occupants who have more control over windows are comfortable at higher temperatures than other occupants. Figure X.X also approves this thesis. In a warm season occupants who have low control are expecting cooler temperatures, on the other hand, warmer in the cool season. This is the primary finding of the effect of satisfaction levels.

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