HVAC System Trend Analysis
Content
A S H RA E
JOURNAL
The following article was published in ASHRAE Journal, February 1997. © Copyright 1997 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.
Figure 1: Graph shows the unit was running during unoccupied periods on Wednesday, Saturday and the second Monday (Monday 2).
HVAC System Trend Analysis
By Stephen B. Austin, P .E.
Member ASHRAE
H
VAC System Trend Analysis is essential to ensuring proper performance of an air handling system. You can’t tell exactly what is going on by watching a unit’s instantaneous operation. You need to see the simultaneous interaction of all the components in the system. And you need to see this interaction over an extended period of time under a variety of conditions. • Have you verified that your HVAC system is operating as efficiently as it was designed? • Is the system providing the savings it was justified on? • Could some of your hardware be defective? • Have devices been misapplied? • Could the current sequence of operation be reducing equipment life? We thought the plant air handling units were operating reasonably well. In fact, we watched the system’s graphical interface and everything looked fine. We
44 ASHRAE Journal
spent a lot of time and effort in calibrating devices and performing preventive maintenance on equipment. However, when we began to systematically collect and analyze trend data, we were surprised by what we found. While most engineers will agree with the concept, the author’s experience is that few people actually perform comprehensive, systematic HVAC System Trend Analysis. This article is intended to emphasize the considerable benefits of expending the necessary effort. The following discussion is a summary of the surprises we uncovered and other useful information including: • How to setup a trend; • How to analyze a trend; • What types of problems are typically encountered; and • What can be done to correct some of these problems. Background The plant air handling system consists of over 100 air handling units of various capacities and arrangements. They provide conditioned air for a large pharmaceutical and chemical manufac-
turing facility. A Direct Digital Control / Energy Management System (DDC/ EMS) provides individual unit control and system time-event programs. Typical DDC/EMS control functions are shown in Table 1. What’s Wrong With This Picture? Figure 1 is an example of an actual HVAC System Trend Analysis. This
See Austin Continued on page 46
About the Author
Stephen B. Austin, P.E., is a principle project engineer with Glaxo Wellcome, Inc. in Research Triangle Park, N.C. He has more than 15 years of experience in energy, automation and utilities. He has a master’s degree in mechanical engineering from North Carolina State University, and is a registered Professional Engineer in North and South Carolina. He is a member of ASHRAE and the International Society of Pharmaceutical Engineers (ISPE).
February, 1997
Advertisement in the print edition formerly in this space.
February, 1997
ASHRAE Journal
45
Austin Continued from page 44
Table 1: Typical DDC / Ems Control Functions
Discharge Air Temperature Reset - The discharge air temperature is reset from 55°F to 65°F (12.8°C to 18°C) as the space temperature decreases from 74°F to 70°F (23°C to 21°C). Return Air Humidity Control - The chilled water coil is modulated as required to keep the return air humidity below 62%. Reheat is used to prevent over-cooling the space. Economizer Cycle - An economizer routine (enthalpy control) uses either outside air or return air, whichever has a lower heat content. To keep the space from being over cooled, economizer control will mix return air with outside air if necessary. A minimum ventilation rate of 10% outside air is used during occupancy. Night Cycle - Night Cycle shuts off an air handling unit during unoccupied periods if the space) and 82°F (28°C). The sample unit has an unoccupied period of 5 p.m. to 5:30 a.m. Night Purge - Night purge pre-cools the space with outside air during Night Cycle to reduce the amount of mechanical cooling required to cool the space during startup. The Night Purge criteria are: 1. Less than six hours before occupancy 2. Outdoor air temperature is above summer-winter switchover point (50°F or 10°C) 3. Outdoor air dewpoint is less than 50°F (10°C) 4. Space temperature is at least 65°F (18°C) 5. Space temperature is at least 10°F (6°C) above outdoor air temperature Optimum Stop - Optimum Stops the air handling unit as much as an hour before the end of an occupied period if the program predicts the space will maintain conditions through occupancy. Optimum Start - Optimum Start starts the unit prior to occupancy, but no earlier than necessary, to have the space conditions satisfied at occupancy.
sample air handling unit is a single zone unit with supply and return air fans; a set of interlocked dampers for outside return, and mixed air; and chilled water and reheat coils. The unit uses the DDC/ EMS control strategies shown in Table 1. Why Is This Unit On At Night?
Figure 1 shows that on Wednesday, Saturday and the second Monday mornings the unit was running during unoccupied periods. Night Cycle cooling mode is supposed to keep the air handler off during these periods unless the space temperature reaches 82°F (31°C). The space temperature had only reached 77°F (25°C) but the unit was on and maintaining 68°F (20°C). What is going on? In this case, Night Purge started the unit to pre-cool the space before occupancy. Night Purge will use outside air to pre-cool the space if all the Night Purge criteria are satisfied. (See Table 1). Since all five Night Purge criteria were satisfied, the fan started and the outdoor air dampers went to 100% open. If any one of these conditions had failed, the unit would have stopped. That explains why the air handler was running during an unoccupied period, but...
Why Is This Unit Short Cycling?
Figure 1 shows short cycling of the fan and economizer before the occupied periods on Wednesday, Saturday, and the second Monday mornings. The unit stopped and started on average every 16 minutes during these periods. Two things caused this to happen. First, the unit would stop during Night Purge when the space temperature dropped just below the Night Purge setpoint of 68°F (20°C). When the temperature increased slightly, the unit would restart. This is shown on Wednesday and the second Monday. The unit cycled to maintain 68°F space temperature. Second, after Night Purge satisfied its five criteria, the unit started. If any of these conditions had failed, the unit would stop. That is what happened on Saturday morning. Before the space temperature reached 68°F, the outdoor air increased to within 10°F (6°C) of the
46 ASHRAE Journal
space temperature, Night Purge criteria #5. The unit stopped. Within a few minutes, the space temperature increased so the conditions for Night Purge were satisfied again. These cycles repeated over and over. Not only did this cost energy compared to a more stable operation, but more importantly, it put a great deal of unnecessary stress on belts and motors. The problem was a result of no deadband on the Night Purge criteria or temperature setpoint. We solved the excessive cycling by programming a minimum off-time of 23 minutes and a minimum on-time of 7 minutes for all our units. This is consistent with the motor manufacturer’s recommendation for the minimum time between starts. Now the units won’t short cycle regardless of temperature or conditions. We noticed from other trends the same short cycling occurred when a unit was in Night Cycle. The minimum on- and offtimes for the fans corrected these shortcycling situations.
Why Is The Economizer Cycling?
Figure 2 shows the economizer operation in action on Tuesday. The space temperature was above its cooling setpoint, 74°F (23°C). Cooling was required. The economizer routine evaluated whether return air or outside air had lower heat content. From startup until 11:20 a.m., outside air had a higher heat content so the economizer held the outdoor air dampers in their minimum position, 10%. The chilled water valve modulated to maintain space conditions. At 11:20 a.m. the outdoor air enthalpy dropped below the return air enthalpy so the economizer went full open. Chilled water was still required, but not as much if return air had been used. Shortly afterward, the outdoor air conditions changed slightly and return air became more economical to use. During the next six hours the economizer routine was bouncing back and forth between return air and outside air.
See Austin Continued on page 48
February, 1997
Advertisement in the print edition formerly in this space.
February, 1997
ASHRAE Journal
47
Austin Continued from page 46
This situation can vary the air balance between different air handling units and cause temporary fluctuations in room pressures. The variance is caused by the differences in the pressure losses of the return and exhaust ducts. The economizer short cycling occurred because there was no deadband on the enthalpy calculation. We remedied this by adding a make-and-break point for switchover. The program will not switch the primary air stream until the secondary air stream is one Btu per pound lower. This prevents the economizer from short cycling. As an additional check, we verified the unit’s proper economizer and chilled water valve operation. The chilled water valve should be closed unless the economizer is at minimum or maximum position. The trend data confirmed that the routine was operating as it was intended. Where Did That Blip Come From?
Figure 2: Graph shows the economizer short-cycling on Tuesday because there was no deadband on the enthalpy calculation.
Figure 3 shows on Wednesday the unit stopped at 4 p.m., one hour before the 5 p.m. scheduled unoccupied period. The fan stopped and the economizer went to the closed position. The unit shut down because the Optimum Stop routine determined, based on history, that the space would not exceed its occupied cooling setpoint for the rest of the occupied period. However, the unit restarted at 4:05 p.m. and stayed on until 5 p.m., Figure 3: The unit stopped Wednesday at 4 p.m., one hour before the scheduled the beginning of the unoccupied period. unoccupied period, and then restarted due to an internal clock reset. Optimum Stop stopped the unit as it was supposed to, but the unit restarted within a few minutes. Why? As it turns out, the master clock was resetting the individual controllers’ clocks periodically. Due to the amount of communication bus traffic, the master clock and the controller clocks would get slightly out of synch. After a controller’s internal clock was reset, it looked at all global commands and followed a command priority. Because the unit was technically in Occupied Mode (the highest priority), it overrode the Optimum Stop command. The unit was commanded back on when the clock reset. This might not seem like a huge waste of energy but it did keep the units on Figure 4: On Friday at 12:30 a.m. the outside air temperature increased abruptlonger than necessary. The problem was ly from 31°F (-.55°C) to 55°F (12.7°C). At 6 a.m. it dropped back to 33°F (.55°C).
48 ASHRAE Journal
February, 1997
Advertisement in the print edition formerly in this space.
Figure 5: How to perform a typical HVAC trend analysis.
corrected by adjusting the priority of the Optimum Stop command so the program would not override it with an Occupied Mode command. Can Weather Change That Fast?
Figure 4 shows on Friday morning the outside air temperature increased abruptly at 12:30 a.m. from 31°F to 55°F (-.55°C to 12.8°C). It stayed constant for several hours and then at 6 a.m. it dropped back to 33°F (.55°C). Something similar occurs the following day at the same time. What caused the outdoor temperature to change so quickly? The problem was that the outdoor air sensor for this unit was located in the outdoor air intake of another air handling unit. That unit was off for Night Cycle from 12:30 a.m. to 6 a.m. During this time the unit was not bringing in outside air across the sensor. Warm room air was backing up into the outdoor air intake and heating the sensor. When the unit started, outside air was brought across the sensor and the true outdoor air temperature was measured. Due to sensor placement, anytime the unit was off the reported reading was much higher than the true outdoor air temperature. The colder the outside air, the more dramatic the difference. To make matters worse, this sensor provided an outdoor air signal for several units. The problem caused all the units to stay out of economizer operation when they should have been using the economizer. Energy was wasted. We corrected the situation by installing three temperature and humidity sensors to serve the entire site. (If you use one
February, 1997
Advertisement in the print edition formerly in this space.
49
sensor, you do not know when it is bad. If you use two sensors, you know when one is bad, but you do not know which one). We located the sensors in shielded boxes to give reliable temperature and humidity readings. We compare the readings, average the two closest, and transmit the average to all controllers. If the third reading is more than 10% different from the average, we generate a calibration alarm for that device. That Space Doesn’t Require Much Cooling Does It?
ficient to keep the space temperature from reaching its maximum setpoint. Reheat prevented the space from over cooling. This prompted us to replace the valve with an industrial type that sealed properly under the system water pressure. Applications
Figure 1 shows that on Wednesday through Saturday the unit maintains space conditions without chilled water or outside air for cooling. (There was some economizer control on Friday afternoon). While at first this seems like a good thing (no energy consumption), on second thought you realize something must be wrong. What we found is that the chilled water valve was not closing completely and was leaking. This amount of cooling was suf-
Figure 5 shows the steps in performing a typical HVAC System Trend Analysis. Consider using Trend Analysis as a tool for preventive maintenance. As a group of air handlers are undergoing preventive maintenance, run a trend on one of the units. This allows a method to maintain ongoing, systematic trending. Trend Analysis should be used to commission new installations. This will prevent future surprises. Include Trend Analysis in the specifications for all new project work.
Conclusion Trend Analysis is a powerful tool for HVAC troubleshooting. It can help: • Solve operating problems;
• Increase energy efficiency; • Locate defective hardware; • Identify out-of-calibration devices; • Improve occupant comfort; and • Extend equipment life. It would be impossible to anticipate all variables when developing and implementing a sequence of operation for an air handling system. You must be able to see the unit in action, and the interaction between the different devices. You must also be able to see the unit’s operation over a significant period of time and over a wide range of conditions. HVAC Trend Analysis can do all these things.
Please circle the appropriate number on the Reader Service Card at the back of the publication. Extremely Helpful ........................ 458 Helpful ....................................... 459 Somewhat Helpful ....................... 460 Not Helpful................................. 461
Advertisement in the print edition formerly in this space.
Advertisement in the print edition formerly in this space.
50
ASHRAE Journal
February, 1997
JOURNAL
The following article was published in ASHRAE Journal, February 1997. © Copyright 1997 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.
Figure 1: Graph shows the unit was running during unoccupied periods on Wednesday, Saturday and the second Monday (Monday 2).
HVAC System Trend Analysis
By Stephen B. Austin, P .E.
Member ASHRAE
H
VAC System Trend Analysis is essential to ensuring proper performance of an air handling system. You can’t tell exactly what is going on by watching a unit’s instantaneous operation. You need to see the simultaneous interaction of all the components in the system. And you need to see this interaction over an extended period of time under a variety of conditions. • Have you verified that your HVAC system is operating as efficiently as it was designed? • Is the system providing the savings it was justified on? • Could some of your hardware be defective? • Have devices been misapplied? • Could the current sequence of operation be reducing equipment life? We thought the plant air handling units were operating reasonably well. In fact, we watched the system’s graphical interface and everything looked fine. We
44 ASHRAE Journal
spent a lot of time and effort in calibrating devices and performing preventive maintenance on equipment. However, when we began to systematically collect and analyze trend data, we were surprised by what we found. While most engineers will agree with the concept, the author’s experience is that few people actually perform comprehensive, systematic HVAC System Trend Analysis. This article is intended to emphasize the considerable benefits of expending the necessary effort. The following discussion is a summary of the surprises we uncovered and other useful information including: • How to setup a trend; • How to analyze a trend; • What types of problems are typically encountered; and • What can be done to correct some of these problems. Background The plant air handling system consists of over 100 air handling units of various capacities and arrangements. They provide conditioned air for a large pharmaceutical and chemical manufac-
turing facility. A Direct Digital Control / Energy Management System (DDC/ EMS) provides individual unit control and system time-event programs. Typical DDC/EMS control functions are shown in Table 1. What’s Wrong With This Picture? Figure 1 is an example of an actual HVAC System Trend Analysis. This
See Austin Continued on page 46
About the Author
Stephen B. Austin, P.E., is a principle project engineer with Glaxo Wellcome, Inc. in Research Triangle Park, N.C. He has more than 15 years of experience in energy, automation and utilities. He has a master’s degree in mechanical engineering from North Carolina State University, and is a registered Professional Engineer in North and South Carolina. He is a member of ASHRAE and the International Society of Pharmaceutical Engineers (ISPE).
February, 1997
Advertisement in the print edition formerly in this space.
February, 1997
ASHRAE Journal
45
Austin Continued from page 44
Table 1: Typical DDC / Ems Control Functions
Discharge Air Temperature Reset - The discharge air temperature is reset from 55°F to 65°F (12.8°C to 18°C) as the space temperature decreases from 74°F to 70°F (23°C to 21°C). Return Air Humidity Control - The chilled water coil is modulated as required to keep the return air humidity below 62%. Reheat is used to prevent over-cooling the space. Economizer Cycle - An economizer routine (enthalpy control) uses either outside air or return air, whichever has a lower heat content. To keep the space from being over cooled, economizer control will mix return air with outside air if necessary. A minimum ventilation rate of 10% outside air is used during occupancy. Night Cycle - Night Cycle shuts off an air handling unit during unoccupied periods if the space) and 82°F (28°C). The sample unit has an unoccupied period of 5 p.m. to 5:30 a.m. Night Purge - Night purge pre-cools the space with outside air during Night Cycle to reduce the amount of mechanical cooling required to cool the space during startup. The Night Purge criteria are: 1. Less than six hours before occupancy 2. Outdoor air temperature is above summer-winter switchover point (50°F or 10°C) 3. Outdoor air dewpoint is less than 50°F (10°C) 4. Space temperature is at least 65°F (18°C) 5. Space temperature is at least 10°F (6°C) above outdoor air temperature Optimum Stop - Optimum Stops the air handling unit as much as an hour before the end of an occupied period if the program predicts the space will maintain conditions through occupancy. Optimum Start - Optimum Start starts the unit prior to occupancy, but no earlier than necessary, to have the space conditions satisfied at occupancy.
sample air handling unit is a single zone unit with supply and return air fans; a set of interlocked dampers for outside return, and mixed air; and chilled water and reheat coils. The unit uses the DDC/ EMS control strategies shown in Table 1. Why Is This Unit On At Night?
Figure 1 shows that on Wednesday, Saturday and the second Monday mornings the unit was running during unoccupied periods. Night Cycle cooling mode is supposed to keep the air handler off during these periods unless the space temperature reaches 82°F (31°C). The space temperature had only reached 77°F (25°C) but the unit was on and maintaining 68°F (20°C). What is going on? In this case, Night Purge started the unit to pre-cool the space before occupancy. Night Purge will use outside air to pre-cool the space if all the Night Purge criteria are satisfied. (See Table 1). Since all five Night Purge criteria were satisfied, the fan started and the outdoor air dampers went to 100% open. If any one of these conditions had failed, the unit would have stopped. That explains why the air handler was running during an unoccupied period, but...
Why Is This Unit Short Cycling?
Figure 1 shows short cycling of the fan and economizer before the occupied periods on Wednesday, Saturday, and the second Monday mornings. The unit stopped and started on average every 16 minutes during these periods. Two things caused this to happen. First, the unit would stop during Night Purge when the space temperature dropped just below the Night Purge setpoint of 68°F (20°C). When the temperature increased slightly, the unit would restart. This is shown on Wednesday and the second Monday. The unit cycled to maintain 68°F space temperature. Second, after Night Purge satisfied its five criteria, the unit started. If any of these conditions had failed, the unit would stop. That is what happened on Saturday morning. Before the space temperature reached 68°F, the outdoor air increased to within 10°F (6°C) of the
46 ASHRAE Journal
space temperature, Night Purge criteria #5. The unit stopped. Within a few minutes, the space temperature increased so the conditions for Night Purge were satisfied again. These cycles repeated over and over. Not only did this cost energy compared to a more stable operation, but more importantly, it put a great deal of unnecessary stress on belts and motors. The problem was a result of no deadband on the Night Purge criteria or temperature setpoint. We solved the excessive cycling by programming a minimum off-time of 23 minutes and a minimum on-time of 7 minutes for all our units. This is consistent with the motor manufacturer’s recommendation for the minimum time between starts. Now the units won’t short cycle regardless of temperature or conditions. We noticed from other trends the same short cycling occurred when a unit was in Night Cycle. The minimum on- and offtimes for the fans corrected these shortcycling situations.
Why Is The Economizer Cycling?
Figure 2 shows the economizer operation in action on Tuesday. The space temperature was above its cooling setpoint, 74°F (23°C). Cooling was required. The economizer routine evaluated whether return air or outside air had lower heat content. From startup until 11:20 a.m., outside air had a higher heat content so the economizer held the outdoor air dampers in their minimum position, 10%. The chilled water valve modulated to maintain space conditions. At 11:20 a.m. the outdoor air enthalpy dropped below the return air enthalpy so the economizer went full open. Chilled water was still required, but not as much if return air had been used. Shortly afterward, the outdoor air conditions changed slightly and return air became more economical to use. During the next six hours the economizer routine was bouncing back and forth between return air and outside air.
See Austin Continued on page 48
February, 1997
Advertisement in the print edition formerly in this space.
February, 1997
ASHRAE Journal
47
Austin Continued from page 46
This situation can vary the air balance between different air handling units and cause temporary fluctuations in room pressures. The variance is caused by the differences in the pressure losses of the return and exhaust ducts. The economizer short cycling occurred because there was no deadband on the enthalpy calculation. We remedied this by adding a make-and-break point for switchover. The program will not switch the primary air stream until the secondary air stream is one Btu per pound lower. This prevents the economizer from short cycling. As an additional check, we verified the unit’s proper economizer and chilled water valve operation. The chilled water valve should be closed unless the economizer is at minimum or maximum position. The trend data confirmed that the routine was operating as it was intended. Where Did That Blip Come From?
Figure 2: Graph shows the economizer short-cycling on Tuesday because there was no deadband on the enthalpy calculation.
Figure 3 shows on Wednesday the unit stopped at 4 p.m., one hour before the 5 p.m. scheduled unoccupied period. The fan stopped and the economizer went to the closed position. The unit shut down because the Optimum Stop routine determined, based on history, that the space would not exceed its occupied cooling setpoint for the rest of the occupied period. However, the unit restarted at 4:05 p.m. and stayed on until 5 p.m., Figure 3: The unit stopped Wednesday at 4 p.m., one hour before the scheduled the beginning of the unoccupied period. unoccupied period, and then restarted due to an internal clock reset. Optimum Stop stopped the unit as it was supposed to, but the unit restarted within a few minutes. Why? As it turns out, the master clock was resetting the individual controllers’ clocks periodically. Due to the amount of communication bus traffic, the master clock and the controller clocks would get slightly out of synch. After a controller’s internal clock was reset, it looked at all global commands and followed a command priority. Because the unit was technically in Occupied Mode (the highest priority), it overrode the Optimum Stop command. The unit was commanded back on when the clock reset. This might not seem like a huge waste of energy but it did keep the units on Figure 4: On Friday at 12:30 a.m. the outside air temperature increased abruptlonger than necessary. The problem was ly from 31°F (-.55°C) to 55°F (12.7°C). At 6 a.m. it dropped back to 33°F (.55°C).
48 ASHRAE Journal
February, 1997
Advertisement in the print edition formerly in this space.
Figure 5: How to perform a typical HVAC trend analysis.
corrected by adjusting the priority of the Optimum Stop command so the program would not override it with an Occupied Mode command. Can Weather Change That Fast?
Figure 4 shows on Friday morning the outside air temperature increased abruptly at 12:30 a.m. from 31°F to 55°F (-.55°C to 12.8°C). It stayed constant for several hours and then at 6 a.m. it dropped back to 33°F (.55°C). Something similar occurs the following day at the same time. What caused the outdoor temperature to change so quickly? The problem was that the outdoor air sensor for this unit was located in the outdoor air intake of another air handling unit. That unit was off for Night Cycle from 12:30 a.m. to 6 a.m. During this time the unit was not bringing in outside air across the sensor. Warm room air was backing up into the outdoor air intake and heating the sensor. When the unit started, outside air was brought across the sensor and the true outdoor air temperature was measured. Due to sensor placement, anytime the unit was off the reported reading was much higher than the true outdoor air temperature. The colder the outside air, the more dramatic the difference. To make matters worse, this sensor provided an outdoor air signal for several units. The problem caused all the units to stay out of economizer operation when they should have been using the economizer. Energy was wasted. We corrected the situation by installing three temperature and humidity sensors to serve the entire site. (If you use one
February, 1997
Advertisement in the print edition formerly in this space.
49
sensor, you do not know when it is bad. If you use two sensors, you know when one is bad, but you do not know which one). We located the sensors in shielded boxes to give reliable temperature and humidity readings. We compare the readings, average the two closest, and transmit the average to all controllers. If the third reading is more than 10% different from the average, we generate a calibration alarm for that device. That Space Doesn’t Require Much Cooling Does It?
ficient to keep the space temperature from reaching its maximum setpoint. Reheat prevented the space from over cooling. This prompted us to replace the valve with an industrial type that sealed properly under the system water pressure. Applications
Figure 1 shows that on Wednesday through Saturday the unit maintains space conditions without chilled water or outside air for cooling. (There was some economizer control on Friday afternoon). While at first this seems like a good thing (no energy consumption), on second thought you realize something must be wrong. What we found is that the chilled water valve was not closing completely and was leaking. This amount of cooling was suf-
Figure 5 shows the steps in performing a typical HVAC System Trend Analysis. Consider using Trend Analysis as a tool for preventive maintenance. As a group of air handlers are undergoing preventive maintenance, run a trend on one of the units. This allows a method to maintain ongoing, systematic trending. Trend Analysis should be used to commission new installations. This will prevent future surprises. Include Trend Analysis in the specifications for all new project work.
Conclusion Trend Analysis is a powerful tool for HVAC troubleshooting. It can help: • Solve operating problems;
• Increase energy efficiency; • Locate defective hardware; • Identify out-of-calibration devices; • Improve occupant comfort; and • Extend equipment life. It would be impossible to anticipate all variables when developing and implementing a sequence of operation for an air handling system. You must be able to see the unit in action, and the interaction between the different devices. You must also be able to see the unit’s operation over a significant period of time and over a wide range of conditions. HVAC Trend Analysis can do all these things.
Please circle the appropriate number on the Reader Service Card at the back of the publication. Extremely Helpful ........................ 458 Helpful ....................................... 459 Somewhat Helpful ....................... 460 Not Helpful................................. 461
Advertisement in the print edition formerly in this space.
Advertisement in the print edition formerly in this space.
50
ASHRAE Journal
February, 1997
