Chilled-water supply stays at constant design temperature

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Introduction

Some chilled-water systems have a reset logic based on outside-air temperature (OSAT). This conserves energy when the cooling load is less than the design load. The reset logic allows the chilled-water supply temperature (CHWST) to rise above the design temperature and increases the chiller efficiency by reducing the amount of work it has to do. The air-handling-unit (AHU) fans will compensate for the corresponding elevated supply-air temperature (SAT) as needed, but using less energy.

How This Wastes Energy

Keeping the CHWST lower than necessary causes the chiller to operate in a less efficient region of its performance curve. The rule of thumb is that raising the CHWST one degree Fahrenheit reduces chiller energy consumption roughly 2%. Lower CHWST can also cause higher heating bills because of reheating at terminal units.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Checked  Inspection   Step Type Description
  1 Explanation  A small process load (e.g., server room) is connected to the system and requires a constant chilled-water temperature.
  2 Explanation Control over chiller setpoints (either local or via the direct digital control system) is absent.
  3 Problem Temperature-reset schedule is not programmed, or is overridden.
  4 Problem Temperature sensor is miscalibrated or improperly located.
  5 Problem HVAC zone may require low supply-air temperature to compensate for a lack of airflow.
  6 Problem Lack of insulation or long piping runs cause excessive heat gains in the piping distribution system.
  7 Problem Distribution system is undersized.
  8 Problem Chilled-water system is improperly balanced.

How to Find the Problem(s) by Inspection

Inspection Step 1

Review the design documents for your chilled-water system to see if a process load is served by the system. Possible loads include a fan coil serving a small data room, an elevator machinery room, or a lighting-control room. Any one of these would force the chilled-water system to maintain a steady CHWST since the process load is constant.

Inspection Step 2

Familiarize yourself with your chiller's capabilities. Can it reset the CHWST through the direct-digital-control (DDC) system? If it has an OSAT sensor installed, can it do it using control logic integral to the chiller controls, Can it reset CHWST based on the AHU that requires the most cooling? Older chillers often have no temperature-reset capability. Your system will operate at a set chilled water temperature if reset logic is not included in the stand-alone software running the unit, or if there is no control from the DDC system.

If you have no automatic-reset capability, you may want to consider adjusting the temperature manually for seasonal conditions. This will require that a detailed record be kept showing occupant complaints as compared to OSAT and CHWST. Always operate the CHWST as high as possible without causing tenant complaints. Be aware, though, that allowing the AHU cooling-coil temperature to rise too high may allow humidity to become unacceptably high.

Inspection Step 3

Inspect the override logs of the DDC system to verify that the reset-schedule setpoints have not been altered. Verify the original setpoints as commissioned or in the operating plan. If the setpoints have been overridden, find out why before correcting the problem. Some reasons for override may be:

  • A process load connected to the system requires constant chilled-water temperatures.
  • An AHU that can't meet its loads with a higher chilled-water temperature due to a water- or air-balance problem.

Inspection Step 4

Check sensor location and calibration:

  • See if the CHWST probe is correctly installed and calibrated. If the probe is not located properly in the water stream, it will provide elevated temperatures to the DDC system, forcing the chiller to drop the supply-water temperature.
  • Inspect the OSAT sensor location and installation. Is it located where it provides an accurate reading? Check to see if it shares a DDC system point from another controller. If communication was lost between controllers, the reset logic may be using the last OSAT value transmitted.

Inspection Step 5

Check the trouble log of the HVAC system. A zone may lack airflow and require supply air at a temperature below its normal setpoint to meet its load. If this is the case, resolve the airflow problem before addressing the reset logic.

Inspection Step 6

Review your chilled-water piping. You may have excessive heat gain in the piping due to very long runs or inadequate insulation. Wet insulation, due to a break in the vapor barrier or a leak can increase temperature rise dramatically.

Inspection Step 7

Review the design drawings for flow and piping size. The piping may be undersized such that the chilled water must be cooled below its design setpoint due to a lack of adequate flow.

Inspection Step 8

Review the latest test-and-balance report.

  • Verify that the system was properly balanced at the time the report was prepared. Compare those readings to the current conditions. Inspect your system for possible bypassing through 3-way valves. Check balancing valves for proper position. Passing too much water through a coil can actually reduce the heat transfer rate by not allowing the water enough time in the coil. (More flow is not always a good thing.)
  • Sometimes water flow at the far end of the system is inadequate due to poor balancing. The pump may be at the design rate but most of the flow is passing through the units closest to the pumps due to the higher pressure differential. The most common response to this is to turn on another pump, which only makes the problem worse.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Chilled-water supply temperature (CHWST)
  • Chilled-water return temperature (CHWRT)
  • Cooling-coil valve positions-feedback if possible, and not output from the DDC system
  • Secondary pump speed-feedback from the variable speed drive (VSD) if possible, and not output from the DDC system

Graph the CHWST with respect to the other four points. If the OSAT line is flat, then the OSAT sensor is the problem. If a cooling-coil valve is always 100% open, then that unit may be undercooling (or over-flowing). If the secondary pump speed is constant at 100%, the system demand may exceed the chiller capacity. (The return-water temperature can also be above design in this case.)

Normal Operation

The graph below illustrates normal, efficient operation of the system. The system starts at 09:00 (based on an OSAT of 60 degrees) with the CHWST at 50 degrees, and the cooling valve 20% open. As the OSAT rises, the CHWST starts to drop to its design point of 45 degrees. The CHWRT starts to rise at the same time, showing that the chilled water is bringing back more heat to the chiller. The pump speed increases as the load increases. The chiller is operating at maximum differential temperature at 14:00. The AHU-cooling-coil valve continues to open as the zone load increases until it is open 100%. If your graph looks like this then the problem should be solved.

graphic figure

Normal system operation

Abnormal Operation - Absence of Reset Sechedule

The graph below illustrates abnormal operation of the system. The system starts at 09:00 (when the OSAT reaches 60 degrees) with the CHWST at 45 degrees, and the cooling valve 20% open. As the OSAT rises, the CHWST stays constant at its design point of 45 degrees. The CHWRT starts to rise, showing that the chilled water is bringing back more heat to the chiller. The pump speed increases as the load increases. The chiller is operating at maximum differential temperature at 14:00. The AHU-cooling-coil valve continues to open as the zone load increases, until it is 100% open. If your graph looks like this, the problem is a lack of reset.

graphic figure

Abnormal operation: reset problem

Abnormal Operation - Water Flow or Balance

The graph below illustrates abnormal operation of the system. The system starts at 09:00 (when the OSAT reaches 60 degrees) with the CHWST at 45 degrees. As the OSAT rises, the CHWST stays constant at its design point of 45 degrees, trying to satisfy the AHU demand. The CHWRT starts to rise, showing that the chilled water is bringing back more heat to the chiller from the entire system. The chiller is operating at maximum differential temperature at 14:00. If your graph looks like this, then the problem is either an airflow or water-flow issue. If you have trended the chilled-water pump speed and it is constant at 100%, you may have a water-flow or water-balance problem. In either case, go back to Inspection Step 5 and re-investigate. Continue trending to verify your new findings.

graphic figure

Abnormal operation: chilled-water valve and VFD at 100%

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
  • Test-and-balance technician

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