Zoning Design
There are several types of zones. These zones are differentiated based on what is to be controlled, and the variability of what is to be controlled. The most common control parameters are: thermal (temperature), humidity, ventilation, operating periods, freeze protection, pressure and importance.
The most common reason for needing zones is variation in thermal loads. Consider the simple building floor plan shown in Figure 5.2. Let us assume it has the following characteristics:
Well-insulated
A multi-story building, identical plan on every floor Provided with significant areas of window for all exterior spaces Low loads due to people and equipment in all spaces Located in the northern hemisphere
In this example, we will first consider the perimeter zone requirements on intermediate floors due to changes in thermal loads. These changes can occur because of the movement of the sun around the building during the course of a sunny day. These changes in thermal loads take place because the spaces receive solar heat from the sun, called solar gain.
The designer’s objective is to use zones to keep all spaces at the set-point temperature. The set-point temperature is the temperature that the thermostat is set to maintain.
Early in the morning, the sun rises in the east. It shines on the easterly walls and through the east windows into spaces NE and SE. Relative to the rest of the building, these spaces, NE and SE, need more cooling to stay at the set-point temperature.
As the morning progresses towards midday, the sun moves around to the south so that the SE, S and SW spaces receive solar gain. However, the solar heat load for the NE space has dropped, since the sun has moved around the building.
As the afternoon progresses, the sun moves around to the west to provide solar gain to spaces SW and NW.
Zoning Design Considerations
While most of the spaces have been experiencing a period of solar gain, the two N spaces have had no direct solar gain. Thus, the load in the two N spaces is only dependent on the outside temperature and internal loads, like lights. These two factors are approximately the same for each space. Therefore these two N spaces could share a common thermostat to control their temperature and it would not matter whether the thermostat was located in one space or the other. These two spaces would then be a single zone, sharing a single thermostat for the temperature control of the two spaces.
The two S spaces have similar thermal conditions with high solar gain through the middle of the day. Both of the two S spaces could also share a thermostat, since they have similar solar and other loads.
The remaining spaces: NE, SE, SW, and NW, all have different solar gains at different times. In order to maintain the set-point temperature, they would each need their own thermostat.
Thus, if we wanted to deal with the solar gain variability in each of these eight spaces, we would need six zones. Note that this discussion is considering zoning on the basis of only solar loads.
In real life there may not be enough funds allocated for six zones. Thus, the designer might combine the two N spaces with the NE space; on the basis that a little overheating in NE space in the early morning would be acceptable. Then the choice is between N and NE spaces for the thermostat location. Since it is generally better to keep the majority happy, the designer would choose to put the thermostat in an N space. However, if the designer knew that the NE space was going to be allocated to an important person, the choice could be to put the thermostat in the NE space!
In a similar way the two S spaces and the SE space could be combined, since they all experience the midday solar gain. Lastly the SW and NW spaces could be combined, since they both experience the high solar gain of the late afternoon.
In this way, the six zones could be reduced to three. The effect would be to have considerable loss of temperature control performance, but there would also be a coincident reduction in the installation cost.
The balance between performance and cost is a constant challenge for the designer. Too few zones could lead to unacceptable performance and potential liability, while excessive zoning increases costs and maintenance requirements.
Interior and Roof Zones
The discussion so far has ignored both the internal zone and the effect of the roof. The internal zones on intermediate floors are surrounded by conditioned spaces. As a result, they never need heating, are not affected by solar gain and need cooling when occupied all year. In a cool climate this can often create a situation where all exterior zones require heating but the interior zones still require cooling. The different behavior of interior zones can be dealt with by putting them on a separate system.
The top floor perimeter zones are also different from the intermediate floor zones since they have the added summer roof solar gain and the winter heat loss. On the top floor, interior zones are also affected by solar gain and winter heat loss. As a result the top floor design needs special consideration with additional cooling and heating abilities.
Choosing zones is always a cost/benefit trade-off issue. In an ideal world, every occupant would have control of their own part of the space. In practice the cost is generally not warranted. As a result the designer has to go through a selection process, like we did in this example, to decide which spaces in a building can be combined. In our example, we only considered solar gain, but in a real building the designer must consider all relevant factors. Common factors are outlined below:
Thermal Variation
Solar gain. As shown in the example, solar gain through windows can create a significant difference in cooling load, or the need for heating, at varying times of the day according to window orientation.
Wall or roof heat gains or heat losses. The spaces under the roof in a multi-floor building will experience more heat gain in summer, or heat loss in winter, than spaces on the lower floors.
Occupancy. The use of spaces and the importance of maintaining good temperature control will influence how critical zoning is.
Equipment and associated heat loads. Equipment that gives off significant heat may require a separate zone in order to maintain a reasonable temperature for the occupants. For example, a row of private offices may have worked well as a single zone, but the addition of a personal computer and a server in one of those offices would make it very warm compared to the other offices. This office could require separate zone design.
Freeze protection in cold climates. In a cold climate, the perimeter walls and roof lose heat to the outside. Therefore, it is often advantageous to designate the perimeter spaces as separate heating zones from those in the core of the building.
Ventilation with Outside Air
Occupancy by people. In a typical office building, the population density is relatively low. However, conference rooms have a fairly high potential population density and therefore, a very variable, and not continuous, ventilation load. Therefore, conference rooms are often treated as different zones for ventilation and for time of operation, compared to the offices in a building.
Exhausts from washrooms. As noted in Chapter 4, washrooms may be treated as a separate zone and provided only with exhaust. The exhausted air may be up of air from the surrounding spaces.
Exhausts from equipment and fume hoods. Often, equipment is required to operate continuously, although the majority of the building is only occupied during working hours, Monday to Friday. In these cases, it may be advantageous to treat the spaces with continuous exhaust as a separate zone or even service them from a separate system.
Time of Operation
Timed. In many buildings, the time of operation of spaces differs. For example, an office building might have several floors occupied by tenants who are happy with full service only during working hours from Monday to Friday. One floor could be occupied by a weather forecasting organization that required full operation 24 hours-a-day seven days a week. In this case it might be advantageous to have the building zoned to only provide service when and where needed.
On demand – manual control or manual start for timed run. In many buildings there are spaces that are only used on occasion. They may be designed as separate zones, which are switched on when needed. The activation can be by means of an occupancy sensor, or by a manual start switch in the space, which runs the zone for a predetermined time. For example a low-use lecture theatre in a university building might be provided with a push button start that would energize the controls to run the space air conditioning for two hours before switching off.
Humidity
High humidity in hot humid climates for mold protection. In hot, humid climates, the moisture can infiltrate into the building through leaks in the walls, doors and windows. This can cause the building contents to mold unless dehumidification is activated.
Humidity sensors can be installed in individual representative zones that will measure relative humidity. If these sensors detect excess humidity in these zones, they can trigger the system to provide system wide dehumidification. The control system can be designed to provide dehumidification without ventilation during unoccupied hours.
Museum and art gallery requirements for good humidity control. High quality museums and art galleries have to maintain accurate control of the humidity in the storage and exhibit areas. This humidity control is generally not required in other spaces like offices, restaurants, merchandising and lobby areas. Therefore museum and art gallery often have at least two systems, to provide the collections with the required humidity control.
Pressure
Air flows from places at a higher pressure to places at a lower pressure.
A difference in pressure can be used to control the movement of airborne contaminants in the building. For example, in a hospital, the tuberculosis (TB) patient rooms can be kept at a negative pressure compared to surrounding areas, to ensure that no TB germs, known as bacilli, migrate into surrounding areas.
In a similar way, kitchens, smoking rooms, and toilets are kept negative to contain the smells by exhausting more air than is supplied to the spaces.
Conversely, a photographic processing laboratory is kept at a positive pressure to minimize the entry of dust.
Zoning Problems
One recurring problem with zoning is changes in building use after the design has been completed. If there are likely to be significant changes in layout or use, then the designer should choose a system and select zones that will make zone modification as economical and easy as practical.
Having reviewed the reasons for choosing to zone a building, let’s consider the control of the zone.