PREVAC - Frequently Asked Questions


What does a “High Temperature” LED indicator mean?

There are two different sensors that can indicate “high temperature”. One is a thermistor, and the other is a temperature switch, both located on top of the stainless steal vessel.


  • IF PREVAC IS STILL ON, the thermistor controls the water temperature, limiting it to 110 F. If the temp goes above the 110 F set point, it will drop out 1 phase of the 3 phases and reduce the wattage by half in order to maintain the 110 F set-point. This is when the high temp LED light will illuminate. Everything will now be running on half power to maintain the proper water temperature. This is a normal part of the PREVAC operation.

  • Some causes for this situation are: (1) You are close to evaporating all of your refrigerant, (2) Cold condenser water or (3) Very cold equipment room, causing refrigerant migration.

  • IF THE PREVAC IS OFF, the open/close temperature switch is open, due to water temperature at or exceeding 135 F. 

  • Some causes of this: (1) The chiller pump is operating, (2) The isolation valve is closed, or (3) PREVAC pump is not operating.

The last thing to note is that they both reference ground. If they lose their reference to ground, the high temp indicator will light up. To check that your ground connection is secure, from inside the controller, using a volton meter, first disconnect power. Check the thermistor (yellow wire) to ground. It should read, at room temperature, approximately 48 ohms.  The temperature switch (orange wire) to ground should be straight continuity.

“Pressure Not Rising” LED Indicator is illuminated. What does that mean?

It’s a timing circuit that shuts down the PREVAC when something is preventing PREVAC from bringing the pressure to the set-point. 

This is caused by water flow through the chiller (condenser OR evaporator). Address the reason for the water flow.

Why does the readout shows a dash and a 1 (-1)?

The pressure transducer is not connected.

How do I check water flow through the PREVAC vessel?

At full power, check the inlet temperature and the outlet temperature. A temperature rise of 5 F -10 F through the vessel means you have good water flow. The lower the temperature rise, the better the water flow. 

If you have a temperature rise of around 15 degrees, you have a problem with water flow. This means one of two things: 

  • The valves aren’t open.


  • The pump is cavitated or restricted. Check this by measuring your motor’s amps. They should be at .9 amps. Lower amperage, such as .6 or .5 means that the pump is cavitated. If it’s at 1.5 amps, there’s some sort of restriction.

How do I make sure all of the heaters are properly operating?

Make sure power is off to the PREVAC. Right from the terminal, with an ohm meter, check from H1 to H2, H1 to H3, and from H2 to H3. For the 480v system, all the readings should be the same, around 23 ohms. To check an individual heater, the reading should be 69 ohms.

For the 240v system, from line to line 9.8 ohms. The individual, 28.9 ohms.

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