How building controls work

The central plant makes the heating and cooling for the building. It sends the energy through the building as hot water or chilled water. Each room does not need its own furnace and air conditioner. A small number of large machines in a mechanical room (or on the roof) do the work for all the spaces.

• Chillers make chilled water (usually approximately 44°F) for cooling. A chiller is water-cooled (with a cooling tower) or air-cooled.
• Boilers make hot water or steam for heating.
• Cooling towers release the heat from a water-cooled chiller to the outdoor air, mostly by evaporation of water.
• Pumps move the water through two loops: one chilled and one hot. Large plants divide each loop into a primary loop (through the equipment) and a secondary loop (through the building).

The BAS stages this equipment. It calculates how many chillers or boilers must operate. It resets the water temperatures for the demand and the weather. It sequences the pumps and the towers to keep the building comfortable with the minimum energy. If a plant cannot supply sufficient cooling on a hot afternoon, the cause is frequently the staging, the reset, or the flow, not a defective machine.

An AHU is a large unit, usually on the roof or in a mechanical room. It conditions the air and moves it through ducts to the spaces. A typical unit mixes return air from the building with fresh outdoor air. The air goes through filters and heating or cooling coils. A supply fan then moves the air to the zones.

The BAS controls these parts in an AHU:

• Dampers for the outdoor air, the return air, and the exhaust air control the quantity of fresh air. The economizer logic uses cool outdoor air for free cooling when the weather permits.
• Coils cool the air (chilled water or refrigerant) and heat the air (hot water, electricity, or gas). The central plant supplies the coils.
• Fans supply and return the air. Many fans have variable-frequency drives that change the fan speed with the demand.
• Sensors measure the supply, mixed, and return air temperatures, the duct static pressure, the outdoor air conditions, and sometimes the CO₂ level.

The AHU keeps a supply-air-temperature target and a duct-static-pressure target. The zone equipment downstream then always receives conditioned air at a sufficient pressure.

Variable Air Volume (VAV) is the most common method to supply many zones from one air handler. The AHU supplies cool air continuously. Each zone has a VAV box (a terminal unit). The box controls the quantity of air that goes into the space.

• The box has a damper. The damper opens and closes to keep the zone at its temperature setpoint. For more cooling, it opens. For less cooling, it closes to a minimum airflow.
• Many boxes also have reheat (a hot-water coil or an electric coil). When a zone is at the minimum airflow but is too cold, the box heats the air. This is why one room can receive cold air and warm air from the same system.
• Pressure-independent boxes measure their own airflow and control to a CFM target. They stay stable when the pressure in the main duct changes.

If one zone is not comfortable, the cause is usually at its box: the damper, the actuator, the airflow sensor, or the reheat valve. If many zones change together, examine the AHU or the plant.

Graphics are drawings of the floor plan and the equipment, with live values. A good AHU graphic shows the fan status, the damper positions, the temperatures, and the setpoints. An operator can read the system state quickly. The operator can click a point to command it or override it. Dashboards collect several graphics or important values on one screen. The operator then sees the condition of the full site in one place.

The BAS records point values over time: temperatures, setpoints, outputs, and energy use. These trends (or histories) are the most useful troubleshooting tool in the building. Put the zone temperature, the setpoint, and the damper output on one chart. The chart usually shows if the problem is the control tuning, a mechanical fault, or a defective sensor.

Trends also show the performance across days and weeks, not only at the time you are at the equipment.

The BAS continuously monitors for conditions that are out of limits: a space that is too hot, a fan that is commanded on but has no status, a dirty filter, or lost communication with a controller. When a condition goes out of limits, the BAS activates an alarm. It can send a notification to the staff by email or text.

A good alarm configuration shows real problems and stays quiet at other times. If a system sends many nuisance alarms at the same time, the alarm configuration usually needs adjustment. Alarms are frequently the first indication of a failure, before an occupant reports a problem.

Analytics are a newer layer above the BAS. This layer is also called fault detection and diagnostics (FDD). It applies rules to the trend data to find problems automatically: heating and cooling at the same time, valves that leak through, an economizer that stays closed, or a schedule in override.

The operator does not have to examine the graphics one screen at a time. The analytics show a list of faults in order of priority. They frequently estimate the energy cost or the comfort cost of each fault. The team can then repair the expensive faults first.

Access control decides who can open each door, and when. A reader at the door reads a credential (a card, a fob, or a telephone). A controller compares the permissions of that person with the schedule. If the person has permission, the controller releases the lock.

The main parts are the readers, the door controllers or panels, the electric locks or strikes, a request-to-exit sensor (so that an exit does not cause an alarm), and a door-position switch (so that the system knows if the door is open or closed). The system records each entry and each denied attempt. It applies the schedules (for example, a door is unlocked from 7 am to 6 pm on weekdays). It activates alarms for a door that is forced open or that stays open too long. It frequently uses the same network and front end as the BAS.

Surveillance records what occurs on and around a property. Modern systems use IP cameras. The cameras send video to a Video Management System (VMS) or a network video recorder (NVR). The system stores the video. Operators can watch live video or play back recorded video.

Cameras are different in lens and field of view, resolution, low-light performance, and built-in analytics (motion, line-crossing, license-plate reading, and people-counting). Video frequently connects to access control and alarms. A forced-door event can automatically show the nearest camera. The usual concerns are the storage and retention time, the network bandwidth, and the camera positions and coverage.