E15
Rheem
External Control / Thermostat Input Fault
Boiler shows E15 and may not respond to thermostat or zone calls
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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
Open or shorted thermostat wiring, Incorrect thermostat type (e.g., line-voltage on low-voltage terminals), Miswired zone control panel, Faulty low-voltage fuse on main PCB
How to Fix / Troubleshooting
Safety first: Turn off power before working on low-voltage control circuits.
- Step 1 – Verify thermostat type: Ensure that only low-voltage (24V) thermostats are connected to the Rheem boiler’s control terminals. Never connect line-voltage thermostats directly.
- Step 2 – Inspect wiring: Check thermostat and zone control wiring for shorts, loose connections, or damage. Correct any miswiring per the Rheem wiring diagram.
- Step 3 – Check low-voltage fuse: Locate the low-voltage fuse on the main PCB (if equipped). Test and replace with the same rating if blown. Investigate and correct the cause of the blown fuse before restarting.
- Step 4 – Bypass test: Temporarily jumper the thermostat terminals at the boiler (e.g., R to W) to simulate a call for heat. If the boiler responds, the issue is in the thermostat or field wiring. If not, the PCB input may be faulty.
- Step 5 – Replace faulty components: Replace damaged thermostats, zone boards, or the main PCB as indicated by testing.
Warning: Never apply 120V or 240V to low-voltage terminals. This will destroy the control board and can create a shock hazard.
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Repair Difficulty
Medium
3/5
Required Part
Low-Voltage Fuse on Main PCB
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