Final Exam

Question 1

Two different control approaches

Answer 1

Pneumatic

Electronic

Question 2

Pneumatic

Answer 2

Used compressed air as the operating medium

Almost never used today in modern construction

Stand alone, can’t be networked

Question 3

Electric and electronic

Answer 3

Typically powered by a 24 VAC power source (usually a transformer mounted on the terminal unit)

Are being replaced by DDC

stand alone, can’t be networked

Question 4

DDC (direct digital control)

Answer 4

Most popular and flexible option for most modern buildings

Digital refers to the use of digit computers in these systems

They use analog sensors along with digital computer programs to control HVAC systems

Question 5

Temp measurements for DDC applications are made by 3 principle methods

Answer 5

Thermocouples

Resistance temperature detectors (RTDs)

Thermistors

Question 6

Thermocouples

Answer 6

Consists of two dissimilar metals chosen to produce a measurable voltage at the temp of interest

Question 7

RTDs

Answer 7

Use small, responsive sensing sections constructed from metals whose resistance-temp characteristic is well established and reproducible

Question 8

First order resistance equation

R = Ro (1 + kT)

Answer 8

R - resistance in ohms
Ro - resistance at reference temp (0 degrees C) in ohms
k - temp coef of resistance
T - RTD temp

Question 9

Two common materials for RTDs

Answer 9

Platinum

Balco (40% nickel, 60% iron alloy)

Question 10

Thermistors

Answer 10

Semiconductors with the property that resistance is a strong but nonlinear function of temperature

Question 11

VAV CONTROL SYSTEM DIAGRAM

Answer 11

See diagram in notes

14 actions

Question 12

1

Answer 12

When the fan is energized, the control system is activated. Prior to activation, during unoccupied periods, the perimeter zone baseboard heating is under the control of room thermostats.

Question 13

2

Answer 13

Return and supply fan interlocks are used to prevent pressure imbalances in the supply air ductwork.

Question 14

3

Answer 14

The mixed-air sensor controls the outdoor air dampers to provide the proper coil air inlet temp. The dampers will be at their minimum position at about 40 degrees Fahrenheit typically.

Question 15

4

Answer 15

The dampers minimum position controls the minimum outdoor airflow

Question 16

5

Answer 16

As the upper limit for economizer operation is reached, the OA dampers are returned to their minimum position.

Question 17

6

Answer 17

The return air temp is used to control the morning warm up cycle after night setback.

Question 18

7

Answer 18

The OA damper is not permitted to open during morning warm up by action of the relay shown

Question 19

8

Answer 19

Likewise, the cooling coil valve is de energized (NC) during morning warm up

Question 20

9

Answer 20

All VAV box dampers are moved to fully open during morning warm up action of the relay override. This minimizes warm up time because the boxes will almost always be in the closed position. Perimeter zone coils and baseboard units are under the control of the local thermostat.

Question 21

10

Answer 21

During operating periods, the PI static pressure controller controls both supply and return fan speeds (or inlet vane positions) to maintain approximately 1.0 inWG of static pressure at the pressure sensor location.

Question 22

11

Answer 22

The low temp sensor initiates freeze protection measures

Question 23

12

Answer 23

At each zone, room thermostats control VAV boxes (and fans, of present); as the zone temperature rises, the boxes open more.

Question 24

13

Answer 24

At each perimeter zone, room thermostats close VAV dampers to their minimum settings and activate zone heat as the zone temp falls.

Question 25

14

Answer 25

The controller, using temp information for all zones, modulates OA dampers and the cooling control valve to provide air sufficiently cooled to meet the load of the warmest zone.

Question 26

BACnet

Answer 26

Building automation and control network

Question 27

BACnet definition

Answer 27

A standard communication protocol developed by the American society of heating, refrigeration and air conditioning engineers (ASHRAE), which has been adopted by the American national standards institute (ANSI) and also by the international standards organization (ISO)

Question 28

CAV CONTROL SYSTEM DIAGRAM

Answer 28

9 Actions

Question 29

1

Answer 29

When the fan is energized, the control system is activated

Question 30

2

Answer 30

The minimum air quality is set (usually only once during commissioning, as described earlier)

Question 31

3

Answer 31

The OA temperature sensor supplied a signal to the damper controller

Question 32

4

Answer 32

The RA temperature sensor supplied a signal to the damper controller

Question 33

5

Answer 33

The damper controller positions the 4 dampers to use outside or return air, depending on which is cooler

Question 34

6

Answer 34

The mixed-air low temperature controller controls the OA dampers to keep excessively low temperature air from entering the coils. If a preheat system we’re included, this sensor would control it.

Question 35

7

Answer 35

Optionally, the space sensor could reset the coil discharge air PI controller

Question 36

8

Answer 36

The discharge air controller controls the

• heating air damper
• outdoor air damper
• exhaust air damper
• return air damper
• cooling coil valve after economizer cycle upper limit is reached

Question 37

9

Answer 37

The low temperature sensor initiates freeze protection measures, as described previously