Controlling the Zone
The most common zone control device is the thermostat. It should be placed where it is most representative of the occupants’ thermal experience. A thermostat is usually mounted on the wall. It is designed to keep a constant temperature where it is, but it has no intelligence; it does not know what is going on around it. The following are some of the issues to be aware of when choosing the thermostat location.
e Mounting the thermostat in a location where the sun can shine on it will cause it to overcool the zone when the sun shines on it. The sun provides considerable radiant heat to the thermostat. The thermostat interprets the radiant heat as though the whole location had grown too warm, and it will signal the air conditioning system that it requires a lower air temperature. As a result, the occupants will be cold, and cooling expenses will escalate.
e In many hotels, the thermostat is mounted by the door to the meeting room.
If the door is left open, a cold or warm draft from the corridor can significantly, and randomly, influence the thermostat.
e In some conference or assembly rooms, the thermostat is mounted above lighting dimmer switches. These switches produce heat that rises up into the thermostat. This makes the thermostat think that the room is warmer than it actually is. If the dimmers are left alone and their output is constant, the thermostat can be set at a set point that allows for the heating from the dimmers. Unfortunately the dimmers heat output changes if the dimmer setting is adjusted, so adjusting the lighting level will alter the thermostat performance.
e Mounting a thermostat on an outside wall can also cause problems. If the wall becomes warm due to the sun shining on it, the thermostat will lower the air temperature to compensate. This offsets the increased radiant temperature of the wall on the occupants, but usually the effect is far too much and the room becomes cool for the occupants. In a similar way, in the winter the wall becomes cool and a cool draft will move down the wall over the thermostat, causing it to raise the air temperature to compensate.
e There are times when heat from equipment can offset the thermostat. A computer mounted on a desk under a thermostat can easily generate enough heat to cause the thermostat to lower the air temperature. If the computer is only turned on periodically, (perhaps to drive a printer,) this offset will occur at apparently random times, creating a difficult problem for the maintenance staff to resolve.
e If the thermostat is mounted where it is directly affected by the heating or the cooling of the space, it will likely not maintain comfortable conditions.
For example, let us imagine that the air-conditioning system air-supply blows directly onto the thermostat. In the heating mode, the thermostat will warm up quickly when the hot air stream blows over it. Therefore, it will quickly determine that the room is warm enough and turn off the heat. The result will be rapid cycling of the thermostat and the room will be kept cooler than the set-point temperature. Conversely, when in the cooling mode, the thermostat will be quickly cooled and will cycle rapidly, keeping the room warmer than the set-point temperature.
If the system has been adjusted to work satisfactorily during the heating season, then when the system changes over to cooling, the thermostat will keep the zone warmer than it did when in the heating mode. Complaints will result and the thermostat will get adjusted to satisfactory operation in the cooling mode. When the season changes, the shift will reverse and readjustment will be required once more. This is the sort of regular seasonal problem that occurs in many air-conditioning systems.
e Wall-mounted thermostats generally have a cable connecting them to the rest of the control system. The hole, tubing or conduit can allow air from an adjoining space or the ceiling to blow into the thermostat, giving it a false signal.
e Lastly, mounting a thermostat near an opening window can also cause random air temperature variations as outside air blows, or does not blow, over the thermostat.
Humidity
While this discussion has been all about thermostats and poor temperature control, the issues are very similar for humidity, which is controlled by humidistats. The result of failing to consider placement of the humidistat will be poor humidity control. Remember, as we discussed in Chapter 2, section 2.2.1, if the temperature rises, then relative humidity drops and conversely, if the temperature falls then the humidity rises.
The Next Step
Having considered the issues around zones, we are now going to consider typical systems that provide zone control. In Chapter 6 we will be considering single zone systems and in Chapter 7, systems with many zones.