Lux
Voltage Sag or Electrical Noise on 24VAC Supply
Thermostat reboots or resets when heating or cooling starts
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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
Weak 24VAC transformer, Shared power with high-load accessories (humidifier, UV light), Loose R or C connection at Lux wall plate
How to Fix / Troubleshooting
Safety: Turn off HVAC power before inspecting wiring.
- Inspect R and C connections: Ensure the R and C wires are tightly secured at both the Lux wall plate and the furnace control board.
- Check accessory wiring: If a humidifier, dehumidifier, or UV light is powered from the same transformer, verify proper wiring and that loads are within transformer rating.
- Measure voltage under load: With a multimeter, measure 24VAC between R and C while the system is idle and while it is running. A large drop indicates transformer or wiring issues.
- Upgrade transformer: Have an HVAC technician replace the transformer with a higher VA rating if it is undersized.
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Repair Difficulty
Hard
4/5
Required Part
HVAC 24VAC Control Transformer
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