Chapter 17

1. Chapter 17 Building Automation System Inputs

2. Information – Input If the point is used to supply information in which the processor will use to make decisions it is an input. • Control – Output If the point sends out a signal that controls a device the point is an output. • Digital – 2 possible positions or values • Analog – more than 2 possible positions or values

3. Building Automation System Inputs • Device that senses and sends building condition information to a controller. • Input information is used to make control decisions. • The inputs allow controllers to make decisions and change the state of output devices. • Inputs may be analog or digital.

4. Analog inputs Analog inputs • Device that senses a variable such as temperature, pressure, or humidity and causes a proportional electric signal change at the building automation system controller. • Enable HVAC technicians to obtain readout of variable inputs from a PC or other operator interface devices. • Are normally more expensive than digital inputs.

5. Temperature Sensors Temperature Sensors • A device which measures the temperature in a duct, or room and sends a signal to a controller. • Most common analog input. • Available in a wide variety of packages and mounting configuration. • Wall mounted sensors are the most common temperature sensor. • Duct-mounted sensors are used to measure temperature inside ductwork. • Well-mounted temperature sensors are used to sense water temperature in piping systems. • Outside air sensors are also currently used as well. • Positive-temperature-coefficient sensors, the resistance increases as the temperature increases. • Negative-temperature-coefficient sensors, the resistance decreases as the temperature increases. • Sensor characteristics vary based on the material they are constructed from. • May be thermistor or resistance temperature detectors.

6. Thermistor Thermistor • A resistor made of semiconductor material in which electrical resistance varies based on changes in temperature. • They are inexpensive but are known for inaccuracy when long wire runs are used. • Offsets can be made in software to adjust for the inaccuracy of thermistors. • Must be properly located

7. Resistance Temperature Detector (RTD) Resistance Temperature Detector (RTD) • Resistor made of a material for which the electrical resistance is a known function of the temperature. • They are made of long pieces of nickel, silicon, or platinum. • They are accurate but expensive. • Can be adjusted through software or may be adjusted by burning off part of the precious metal. • Output is often 4mA to 20mA or 0 VDC to 10 VDC. • Must be properly located

8. Humidity Sensor Humidity Sensor • A device which senses the amount of moisture in the air and sends a signal to a controller. • The most common humidity sensor measures relative humidity (% RH) there are others that measure dew point or absolute humidity. • Use hygroscopic element for measurement of humidity. • More difficult than temperature sensing. • Have to be maintained more so than temperature sensors since they are susceptible to dirt on the element. • Should be checked annually for proper operation. • May be polymer or capacitive sensors. • Capacitive sensors use a thin film of hygroscopic element to alter the capacitance of a circuit. Are extremely sensitive to a change in surrounding humidity. • Bulk polymer sensor is a polymer sensor that is saturated with a salty compound. • Output is often 4mA to 20mA or 0 VDC to 10 VDC. • Must be properly located

9. Pressure sensors Pressure sensors • A device that measures the pressure in a duct, pipe, or room and sends a signal to a controller. • Used to provide an indication of the actual pressure in a pipe or duct. • Output is often 4mA to 20mA or 0 VDC to 10 VDC. • Must be properly located

10. Light Sensors Light Sensors • Lighting loads may be 20% or more of the building load in a commercial building. • Can be used to save power consumption with indoor and outdoor lights. • Conserve power by controlling lights by a foot-candle reading. • Output is often 4mA to 20mA or 0 VDC to 10 VDC. • Must be properly located.

11. Specialized Analog Inputs Specialized Analog Inputs • Can contain any desirable variable • Common input devices • Carbon dioxide – used to sense inside air quality • Carbon monoxide – used to sense the amount of toxic exhaust fumes inside parking garages • Refrigerant sensor – used to sense the amount of refrigerant in the air due to leaks • Gas sensor – used to sense the amount of gas in the air

12. Digital Inputs Digital Inputs • A device that produces an ON/OFF signal • Don’t give as much information as analog inputs • Less expensive than analog inputs • Often used as status points in system • Often use dry contacts to designate status • Digital inputs can include: • Thermostats • Humidistat • Differential pressure switches • Flow switches • Light level switches • Accumulators • Current-sensing relays • Timed override initiators • Specialized digital inputs

13. Thermostat Thermostat • A temperature-actuated switch • Limit thermostats are thermostats that maintain a temperature above or below an adjustable set point. • Normally used to indicate an improper temperature level and are not used as a primary temperature controller. • Normally inserted into a duct or strapped to a pipe. • Have set point that determines the set point at which they switch. • The building automation only reads the position of the two position switch.

14. Humidistat Humidistat • Device that senses humidity level in the air • Often used to indicate humidity level in a duct or building space • Have set point and either a normally open or normally closed set of contacts • Normally less expensive that analog humidistat

15. Differential Pressure Switches Differential Pressure Switches • Input device that switches open or closed because a difference between 2 pressures • Often use bellows or diaphragm elements • Set point must be adjusted and the NO or NC switches must be properly connected • Often used with fan and pump status • One side is usually installed on the inlet and the other on the outlet side • Shouldn’t be located too close to fan or pump where high pulsation or vibration occur

16. Flow Switches Flow Switches • Switch that contains a paddle when contacted by air or water flow. • Can be used to indicate the flow status of a pump or fan • Not as good as differential pressure switches because of fan or pump pulsation which leads to false indications

17. Light Level Switches Light Level Switches • Indicates if a light level is above or below a set point • Less expensive than analog light switches sensors • Light level switch set point adjustment may consist of a shutter over the sensing element that is adjusted to trip at a higher or lower light level

18. Accumulators Accumulators • A device that records the number of occurrences of a signal • Commonly used to measure the electrical demand for a building or load

19. Current Sensing Relays Current Sensing Relays • Device that surrounds a wire and detects the electromagnetic field due to electricity passing through the wire • Used to obtain the operational status of pumps, fans, compressors, and lighting loads • Installed so that the current carrying wires of a motor pass through the center of a relay. • When pump starts the relay senses and sends a signal that pump is running as long as current is present • Not as good as differential pressure switches or flow switches because it only ensures that the motor is running not that there is actual flow

20. Timed Override Indicators Timed Override Indicators • A device that, when closed, sends a signal to a controller which indicates that a timed override period is to begin • Timed overrides are used in such circumstances as a unit which is operated by a building automation system and has a button to allow for additional run time for a predetermined time period. • Normally momentary contacts