HVAC System : What is a Zone? Part -1

We all know that the main aim of a HVAC (heating, ventilation and air conditioning) system is to provide and maintain a supply of good quality of air inside the building through the process of filtration, ventilation and temperature monitoring. In short, the requirement of an HVAC system is to provide desired thermal comfort through out the building. But it is often seen that not all the “spaces” inside the building are evenly controlled. This means that there are several spots in the spaces where there is continuous thermal variation,i.e. some are hot and some are cold.It is imperative that these differences in thermal loads are controlled not only for thermal comfort of the buliding but also for savings on energy bills.

These spots in the spaces with fluctuating thermal loads are known as zones.

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A zone is particularly that area of the building that is continuously subjected to variation in thermal load. For example, the areas near a window which is under continuous change in temperature due to variation of heat from the sun. Also it can be some interior area such as an underground store or a big attic where sunlight and heat cannot be easily reached and the natural temperature of the zone is already low.

In the same way, imagine an airport with an HVAC system. An airport is a massive open space with extreme variable occupancy and high difference in thermal loads. Areas such as lobbies, security checks, and passenger check-ins are constantly subjected to thermal load variations and have their own specific requirement of thermal load.

Also, a building can have one or more than one zone. As the number of zones increases the number of controlling devices increases. All the buildings nowadays have multiple zones.

Let’s try to understand zoning in buildings having HVAC system. For that we first need to know what solar gain is. The heating and cooling of the building depends on the heat from the sun. The change in the thermal loads due to the changes in the position of the sun is known as the solar gain.

To understand how solar gain leads to zoning lets take an example.

Suppose that we have a house with an HVAC system. The rooms or zones in the houses have been given the names as shown in the figure. We will try to understand the zoning on the basis of the heating pattern and the movement of the sun.

The sun rises in the east. In the morning hours, the easterly walls will be directly heated by the sun. Thus the NE and SE zones will be at a higher temperature than the rest of the building and thus will need more cooling to keep the whole temperature of the building at one constant temperature.

Now as the early hours progresses to midday, the thermal load on NE and SE will reduce and internal part of the building will also be experiencing equal thermal load as NE, SE, NW and SW .Thus the whole building will be requiring almost same amount of cooling capacity.

Now as the sun moves towards the west in the evening, the NW and the SW zones will be under direct solar gain and thus will need more cooling than the rest of the zones.

In this way, depending on the solar gain, the temperatures of the zones are controlled and maintained.

Generally,sensors are used to control the temperature of these zones. A zone can also have its own independent HVAC system(But this is not generally seen in practice). Each zone has its own individual thermostat. The controlling of the zone depends on the level of heating and cooling required. The dampers on the basis of thermostat, automatically provide the amount of heating or cooling required.We will learn in details about designing and controlling the zones in the next article.

Fundamentals of HVAC system : Robert McDowall.

images:

http://www.colemanac.com/homeowners/index.asp