If a newer Trane roof top unit is not operating either on initial startup, or following maintenance where the main electrical supply lines were disconnected from the main contactor, the problem may be an incorrect phase condition. The Phase monitor may be preventing RTU operation. Give us a call today at 402-238-2425 for all your Omaha HVAC needs.

Omaha Trane Comfort Specialist describes Phase Monitor Preventing RTU Operation Due to Out of Phase Condition in Three Phase UnitsOmaha HVAC provider describes Phase Monitor Preventing RTU Operation Due to Out of Phase Condition in Three Phase UnitsAccurate Heating & Cooling has BBB A+ Rating

The photo below is of a typical Trane RTU.

Omaha commercial building HVAC service and replacement









As an heating & cooling contractor servicing the HVAC needs of the Omaha metro area, Accurate Heating & Cooling works on roof top units which are common units that provide heating and cooling for commercial applications. Newer Trane roof top units have phase monitors that sense the voltage between the three conductors of the three phase power supply. The phase monitor is located inside the access panel with the other electrical control wiring and devices. The phase monitor has two LED lights (one red light and one green light) that provide indication. Phase monitors protect motors and compressors against problems caused by phase loss, phase imbalance, and phase reversal.

Phase Monitor LED Indicators

The green LED illuminates when the three phase supply conductors are properly connected to the main contactor (a three phase contactor). This is the normal arrangement for a unit.

The red LED illuminates when the three phase supply conductors are improperly connected to the main contactor (when there is a phase error). When the red LED is lit, the phase monitor prevents the unit from operating. There are two conditions that will result in the red LED being illuminated.

  • If any of the three phase conductors (L1, L2, and L3) are connected to the main contactor in an improper sequence (when L1 is not connected to T1, L2 is not connected to T2, or L3 is not connected to T3 on the contactor), then the red LED will be lit and the unit will be prevented from operating.
  • If line to line voltage is not between 180 volts and 633 volts.

The only time we have found the red LED illuminated on the Trane phase monitor is when initially connecting the line leads (L1, L2, and L3) to the contactor in the wrong phase sequence, or if the line leads are disconnected from the contactor for some maintenance reason and they are then subsequently reconnected incorrectly.

Phase Monitor Preventing RTU Operation












Safety First

WARNING: These repair activities involve working on high voltage equipment that poses serious electrical shock risk. Follow all safe electrical work practices and ensure that only qualified, trained service technicians perform these repair activities. Hazardous Voltage! Failure to disconnect power before servicing could result in death or serious injury. Disconnect all electric power, including remote disconnects, before servicing. Follow proper lockout/tagout procedures to ensure the power cannot be inadvertently energized.

When a Trane roof top unit will not operate (phase monitor is preventing RTU operation) and the red LED is illuminated on the phase monitor, then switch any two of the electrical line leads supplying the main contactor. Then check the LED status on the phase monitor. If the red LED goes out and the green LED illuminates, then you know that you have now established the correct phase sequence. A standard digital multimeter (DMM) does not have a feature to read voltage phase, so just swap any two of the leads and check for the green LED being lit on the phase monitor.

Simplified Schematic Drawing of Phase Monitor Circuit Preventing RTU Operation

Schematic drawing of how a RTU phase monitor is connected in roof top HVAC unit







Phase Monitor Preventing RTU Operation ensure electrical leads connected correctly












Phase Monitor Preventing RTU Operation switching any two electrical leads












Photo of Phase monitor preventing RTU operation

Phase Monitor Preventing RTU Operation green light












Helpful to Color Code Line Wiring

It is a good practice to color code the individual line leads connecting to the main contactor before removing them from the contactor for any maintenance. Electrical tape in various colors is available and it is handy to wrap a strip of colored electrical tape around the electrical lead an inch or so from the end of the conductor. Have a different color for each lead. Then when the leads are reconnected to the contactor later, they can be reconnected in the same arrangement. Taking photos before and after is also a helpful check to ensure the phases are reconnected following maintenance in the correct order.

Additional Information

Check out our Home Page here to learn more about us.

And for information on the maintenance items homeowners can perform to keep their HVAC system operating efficiently, refer to this blog.

For additional technical information on Trane RTU phase monitors, refer to the Installation and Operation manual. A website that makes some of these manuals available can be found here. There are sections of the manual that provide guidance on phase monitor wiring that is preventing RTU operation.

For more information on the technical aspects of phase monitor relays, a good source is Eaton which is one of the suppliers of these monitors.

When performing maintenance on roof top units, it is good practice to ensure that the low voltage transformer high side is wired for the correct line voltage. It is common for the low voltage transformer that is shipped with the roof top unit to have options for either 230v or 208v primary voltage. Based on results of a voltage test using your voltage meter, ensure that the correct primary side terminals are connected.

While it is very rare, low voltage transformers can fail, and when they do the failure is often the result of some other component failing.

To interpret the model number descriptions on Trane roof top units, refer to the Trane Roof Top Unit (RTU) Model Number Description blog entry on this site (still under additional development). There is a similar blog that will help to decode Trane residential furnace model numbers. For a description of Armstrong furnace model numbers, refer to this blog. And check out this blog on how to troubleshoot a RTU Gas Regulator Intermittent Failure.

Gas valves are found in both residential furnaces and commercial building roof top units. Gas valve failure can be difficult to diagnose. Refer to the blog on troubleshooting a failed gas valve for some helpful hints. Once in a while we run into a RTU that makes use of electric heating elements instead of the normal gas heat. For information on such an older York Electric Heat RTU, refer to this blog.

Other Things That Can Prevent a RTU From Operating

Here are a few situations that can prevent the roof top unit from operating normally that are easy to overlook during troubleshooting.

  • Check the air filters to make sure they are not dirty
  • Check the evaporator to ensure there is no dirt accumulation
  • Check the condenser to ensure there is no dirt accumulation
  • Ensure the flame sensor (in furnace mode) is clean and sending a proper microamp signal
  • Check for any error codes
  • Check that all electrical connections are tight (threaded fasteners should be checked with a screwdriver or nut driver for tightness and spade terminal connector fasteners can be tightened with a pair of needle nose pliers if loose)
  • Check for proper gas pressure on the inlet and outlet of the gas valve (in furnace mode)
  • Once in a while we will find moisture in the gas regulator supplying a roof top unit. At night the moisture freezes and prevents natural gas from being supplied to the unit but during the day, when the sun hits the regulator and melts the ice, the unit runs fine.

For a description of what the MERV rating on air filters means, refer to this blog.

For a good discussion on what air filter would be best, refer to this blog.

And for a description on how to control a furnace second stage heat using a time delay relay, refer to this blog.

Check out this blog for information on how to correctly wire a geothermal water valve in a once through system.

Your Trusted Omaha HVAC Provider

As a trusted Omaha furnace repair and a/c repair contractor, call Accurate Heating & Cooling for all your Omaha furnace service, Omaha air conditioner service, Omaha commercial roof top unit service, Omaha mini split service, Omaha geothermal unit service, and Omaha home comfort needs. We also professionally install hvac equipment. We are licensed, bonded, insured, and experienced. Call us today at 402-238-2425.

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