| LOW SUCTION PRESSURE FAULTS CAUSED BY LOW FLOW |
Low suction pressure fault caused by evaporator flow near or below the minimum allowed. |
If flow is near the minimum or below the minimum allowed in the IOM, it may be causing the low suction pressure fault. Low flow should be suspected if only a single compressor on the chiller is running "fully loaded" on a two system chiller and a resulting temperature differential greater than 8° F across the evaporator is observed; on a three compressor system, greater than 5.3° F of temperature differential with a single compressor running is suspect; on a four compressor system, greater than 4°F with only a single compressor running is suspect. |
Check approximate flow across the evaporator using a flow meter or the pressure drop charts in the IOM. Running a single system on the chiller and checking the temperature differential across the evaporator (return minus leaving water temp) is also a quick means for checking for low flow, if the flow cannot be measured using a flow meter or there is doubt about the accuracy of the flow pressure drop measurement. |
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| LOW SUCTION PRESSURE FAULTS CAUSED BY LOW SUBCOOLING |
Low suction pressure fault caused by low refrigerant charge or non-condensables in the system. |
Low refrigerant charge is often responsible for low suction pressure faults. If there is no liquid or there are bubbles consistently observed in the liquid line sight glass, the system is under charged or there are non-condensables in the system. If high subcooling is noted along with bubbles in the sight glass, there are non-condensables in the system. |
Run the suspect system to full speed and check for bubbles or a loss of liquid in the liquid line sight glass. A full column of liquid should normally be observed except during transients, such as fan cycling or a rapid speed increase. Check the "liquid line subcooling"; it should normally be 7 to 12° F on a 95° F day. If needed, turn fans OFF to approximately duplicate these conditions in colder ambients. To verify correct charge, check subcooling along with verifying there are no bubbles in the sight glass. Do not weigh the charge. Always measure liquid pressure as close to the feed valve as possible on the liquid line and measure liquid line temperature as close as possible to the feed valve when checking "liquid subcooling". |
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| LOW SUCTION PRESSURE FAULTS CAUSED BY AN INCORRECT TRANSDUCER OUTPUT |
Low suction pressure fault caused by an incorrect suction pressure transducer output. |
The suction pressure transducer output may be incorrect; the micro panel display reading will not agree with a gauge installed on the suction line (the two readings should be reasonably close). |
Install a manifold gauge on the suction line and compare the micro panel display reading to the gauge. Make sure the manifold gauges are reading correctly before condemning the transducer, since gauge accuracy is very important when working with R-134A at low pressures. If there is still doubt the micro panel is reading the pressure correctly, the transducer from the suspect system can be plugged into the input of another system that is providing a correct reading. Gauge reading comparison on a properly operating system versus the panel display is also a useful means of checking gauge accuracy. |
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| LOW SUCTION PRESSURE FAULTS CAUSED BY A DEFECTIVE FEED OR DRAIN VALVE |
Low suction pressure fault caused by a defective feed valve or drain valve. |
A defective feed valve that causes a low suction pressure fault will often be accompanied by a legitimate 0% (empty) flash tank level or very low actual flash tank level percentage resulting from lack of refrigerant flow, and the feed valve will appear to be opening appreciably more than other system(s) on the chiller. A defective drain valve will often appear to be open near the 100% point if it is defective and starving the evaporator. A low suction fault with a drain valve position near 100% and a micro panel display level indicating more than about 5% in the flash tank is often caused by a defective level sensor outputting a higher than actual level, rather than a bad drain valve. |
Operate the valve in the SERVICE mode (see IOM). Assure the valve is operating by viewing the valve sight glass while running it from 0% to 100% and 100% to 0%; the valve should visibly open and close smoothly. When the test is complete, close the valve to the 0% position and recycle power to the system. On some old style valves without integral sight glasses, the only tests possible are audibly listening for valve motor operation or touching the valve to feel possible internal movement — this only assures the motor is operating and does not allow diagnosis of a valve that is sticking or uncoupled/broken from the drive motor. Always check any plugs in the wiring between the valve and valve controller as well as between the chiller microprocessor board and the valve controller, since a bad connection or corrosion on the pins will cause problems with valve operation and possibly cause the valve to lose steps. Check valve wiring after a controlled shutdown, with power off. Be aware that in almost all cases, drain and feed valves that are replaced are not defective. Contact Product Technical Support before changing a feed or drain valve. |
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| LOW SUCTION PRESSURE FAULTS CAUSED BY AN INCORRECT LEVEL SENSOR OUTPUT |
Low suction pressure fault caused by a level sensor reading higher than the actual flash tank level. |
Level sensors that cause actual low suction pressure faults typically have a positive offset error of more than 10% based on the micro panel display percentage; normally a problem sensor will read high in comparison to the actual tank level. If the sensor reading is more than a value of 10% above the actual level percentage, there is a risk of the flash tank running out of refrigerant when discharge pressures are low or the compressor is accelerating. When the flash tank runs out of refrigerant due to level sensor error, the drain valve is unable to control superheat and suction pressure drops. In some cases, the system only goes into suction limiting and will not accelerate in speed; in other cases, it will fault on low suction pressure. |
Review the history buffer: when the flash tank is empty and the evaporator is starved, the drain valve position is often near 100% (typically over 90%) at the time of the fault, and the flash tank level typically shows 5% or higher although the tank may be empty. Operate the drain and feed valves in the SERVICE mode to rule out a problem valve, wiring, or controller. With a solid column of liquid in the liquid line sight glass, run the system with enough load to open the economizer valve and obtain a level in or near the middle of the flash tank sight glass (bottom of glass = 32%, middle = 35%, top = 38%); the FEED VALVE LEVEL SETPOINT (SERVICE 2878 ENTER) can be temporarily adjusted upward (key in 45.0) to speed this up — DO NOT change any other programmed point under the password protected displays. Compare the display to the glass: the display must indicate between 25% and 45%, and less than 10% above the actual sight glass level percentage; if not within this range and low suction pressure faults occur, the level sensor should be replaced. To check for an intermittent sensor, "lightly" tap the electronics enclosure of the level sensor with the handle of a screwdriver and watch the display for a significant change; also check plugs between the level sensor and the chiller control (microprocessor) board, since a corroded or loose pin connection will cause intermittent or erratic level sensor displays. Unless the level sensor is reading a displayed level numerically more than 10% above the actual level in the sight glass, it should not be changed without contacting factory technical support. |
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