Rheem Unstable Thermostat Power or Internal Fault

Thermostat powers on but frequently reboots or restarts by itself.

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Loose R or C connection, Undersized or failing transformer, Short cycling due to low-voltage short, Internal thermostat power regulator issue

How to Fix / Troubleshooting

Steps:

  • Inspect wiring: Remove the thermostat and tighten all low-voltage terminal screws, especially R and C. Ensure no bare wires are touching.
  • Measure voltage stability: With a multimeter, monitor voltage between R and C at the thermostat while the system starts and stops. Voltage should remain near 24VAC without large dips.
  • Reduce accessory load: Temporarily disconnect non-essential 24V accessories (humidifier, UV lights) from the R/C circuit to see if reboots stop.
  • Check transformer rating: Confirm the VA rating of the indoor unit's transformer is adequate for the Rheem smart thermostat plus all accessories (often 40VA or higher is recommended).
  • If voltage is stable but reboots continue: The thermostat's internal power supply may be failing; replace the thermostat.
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Repair Difficulty

Medium 3/5

Required Part

Thermostat Main Control Board
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