Rheem
Low Voltage Drop or Short During Furnace Operation
Thermostat goes blank or resets when furnace 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 24V transformer, Short in thermostat wiring, High inrush load from accessories, Loose R or C connections
How to Fix / Troubleshooting
Safety first: Turn off power before working on low-voltage wiring.
- Check R and C connections: Tighten all low-voltage screw terminals on the control board and thermostat.
- Inspect thermostat cable: Look for damage where the cable passes through metal or is stapled.
- Measure voltage: Monitor 24V between R and C while the furnace starts. If it drops significantly, the transformer may be undersized or failing.
- Isolate accessories: Temporarily disconnect humidifiers, Wi-Fi thermostats, or other 24V loads to see if the problem resolves.
If voltage remains unstable, replacing the transformer with a properly sized Rheem-compatible unit may be necessary.
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Repair Difficulty
Medium
3/5
Required Part
24V Control Transformer
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