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Chiller Problems - Microprocessor Low Flow error

This article explains how to fix your chiller when the compressor won't run and there is a microprocessor low flow error.

Low flow error message on microprocessor controller. Compressor will not run.

Micro-Processor "Low Flo" faults can be caused by:

1. Evaporator flow issues related to your pump.

2. Evaporator flow issues related to a restricted filter or strainer.

3. Excessive air in chillers pumping system that is causing false readings on your flow sensor.

4. If your system is running glycol, your concentration of glycol has changed since the systems was originally commissioned.

********* Important Update ********
On 12-31-15, PSG Controls (makers of chiller controls from 2000 to 2012 for both Legacy and Drake brand chillers) went out of business. As a result, replacement parts for this process chiller control system are expected to run out in 2016.

As of 1-1-2013, Legacy Chiller Systems has developed a retrofit package for the PSG process chiller control system. For more information CLICK HERE

Before calling factory technical support, it is recommended that user check/try the following:

1. On all PACT and EST model chillers, make sure the tank recirculation pump is running.

2. Make sure your pump is turning in the correct direction. You can check for pump direction by CAREFULLY sliding a nylon wire tie through the air vent at the end of the pump housing. When your wire tie makes contact with the plastic cooling fan the end will be forced up or down depending on the direction of spin. If the wire tie end goes UP the pump is spinning CW and if DOWN CCW. Located at the end of the pump, there is an arrow embossed into the SS head of the pump. This arrow will indicate the design rotation. Your pump MUST be spinning the same direction. On THREE PHASE systems only, if your pump is spinning the wrong direction, STOP, turn the main power OFF to the chiller and reverse any two legs of power on the main buss. Turn power back on and recheck rotation. On Single-phase chillers, if your pump is running the wrong direction, please call the factory for further instruction.

3. Clean ALL filters and strainers: Restrictions to flow through the chillers cooling system WILL cause Low Flow errors. On PACT and EST systems, with integrated water storage tanks, a common problem is a plugged strainer located just above the tank recirculation pump discharge. Before proceeding with any other steps, turn the power to the chiller off and clean the strainer. On all chiller models, the factory recommends a filter or strainer on the FROM PROCESS loop line to prevent debris from entering the chiller. If you have an ES,PAC system such filters and strainers must be cleaned, as they WILL cause low flow errors.

4. Bleed air from the high side of differential flow switch. Inside the chiller located on the back of the electrical bulkhead, there is a blue box with two chrome towers one marked "HIGH" and one marked "LOW". With the pump running, using a wrench, carefully loosen the brass compression nut at the high connection. DO NOT remove this nut. The goal is to loosen the nut enough to bleed the air out of the high side tubing. Once you see only fluid coming out of the tube (no bubbles) re-secure the nut being careful to not over tighten.

5. If your system is running glycol, check your freeze point. To do this, you will need an optical-Refractometer. These can be purchased at
:> Once your freeze point is confirmed, it should be checked against the original design glycol percentage for your system. That information can be found in your Legacy factory order confirmation. In order to prevent glycol concentration related flow errors, your actual percentage should be within 2% of your design.

After performing steps 1-5 above and your low flow error has not cleared then proceed to step #5 below.

5. Since you have already located the differential flow switch in step #4 above, remove the four SS screws holding the cover on. Inside the box you will see an arm with a small RED hex nut at the end. The red hex nut is the calibration adjustment for this control. With the pump running turn the hex nut CW until the switch closes and the error clears.

VERY IMPORTANT you must cycle the pump ON and OFF manually (using the pump motor starters RED manual stop button)to make sure the switch opens when the pump stops. Each time pump is turned off, you should get the low flow error. If you do not, further adjustment to the flow switch is needed. If the chillers cooling system runs without confirmed water flow through the evaporator you will cause non-warranty damage to your chiller.

Recommendations in this article should be preformed by QUALIFED service personnel who have extensive experience with high voltage/control electrical and plumbing systems. Individuals who lack such experience can cause damage to this equipment that would not be covered under warranty. More importantly, injury and even death can occur.

All chiller models

Posted 11/20/2005 :: 10293 views