HVAC System: Zones and Designing a Zonal System - Part 2

In the previous article we learned what a zonal system is. In this article we will learn how to apply the zonal system to any HVAC system. We will first go through the factors that should be taken into consideration while zoning a HVAC system. Also we will have a brief look at the problems that are generally faced during and after the installation of HVAC system.

A zonal design can be affected by any one or a combine effect of the following factors.

Solar gain

Change in the position and amount of heat radiated from the sun affect the requisite cooling and heating of a zone. We will try to see how it affects the design later in the article.

Occupancy by people and equipments

The population density of a particular zone and also the number of equipments and the space occupied in the room affects the temperature and design of the zone. For example, if the number of people in a room is more than its normal capacity, then the room will require more of cooling. Also if the room is filled with more number of heat releasing and high heat load equipments such as computers or servers, then too the amount of cooling required will be more. This kind of rooms will require more of ventilation and air filtering too.

Apart from this, there are also few narrow spaces or compartments that should be considered as separate zones. Narrow passages surrounding exhaust ducts of washroom or kitchen, and spaces surrounding the exhaust pipe or passage of chimneys are few examples of such spaces. All these spaces should be considered as separate zones and special ventilation and thermal load sensors should be provided to them.

Spaces used for limited time

Consider a normal office that operates five days a week, from Monday to Friday from 9 AM to 6 PM. Now this same company has a costumer care department that works round the clock through out the week. Thus both these departments have different scheduled timings and should come under different zones and provided independent thermostats that can be controlled manually.

Similarly, think about a seminar room in the same office. A seminar room is not used that often and thus should be provided with a manual sensor which can be operated whenever the room is in use. This way a large amount of energy can be saved.

Location of the zone

If we consider a multistory building, the topmost floor of the building will be subjected to more variation in temperature and humidity. The roof of the top floor will absorb more heat or humidity according to the weather and thus will experience continuous change in thermal load.

Similarly, consider an internal room that is surrounded by walls from all the four sides. In hot climate this room will remain cooler and will not require cooling as compared to the surrounding rooms which will require continuous cooling. In the same way in winters, the surrounding rooms will lose more heat through their walls compared to the internal room and thus will require more heating.

Pressure and humidity

In some special buildings the pressure and humidity of the spaces are also controlled. If humidity is not kept in check it can seep in through the walls of the building and spoil the contents of the building. For example, in genuine and high quality museums and art galleries, the pressures of the spaces are controlled with the aim to protect the invaluable and delicate paintings.

Also in the indoor swimming pool spaces, humidity sensors are provided to keep in check the humidity level.

For this purpose special humidity sensors are provided which measure relative humidity. Dehumidifiers that keep in check the humidity level are installed in such special places.

Pressure of a space or zone is kept under control mainly to prevent air borne contaminants from entering the building. We use the benefit of differential pressure for this.

In hospitals that treat TB (tuberculosis) diseases, it is imperative that pressure of the spaces and zones are kept in check. The spaces are particularly kept under negative pressure as compared to surrounding areas to prevent the TB germs from proliferating to surrounding areas.

Similarly, kitchen, toilets, smoking rooms, chemical laboratories etc are also kept under negative pressure to contain the smell which can be removed by supplying extra forced air.

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In the next article we will take the topic further and learn about the designing consideration for zoning a HVAC system.

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Fundamentals of HVAC system by Robert McDowall

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