Safety first: Turn off power before working on low-voltage wiring.

Step 1 – Check for lockout: Look for any lockout or error indication on the Rheem display. Clear lockouts per the manual and see if the boiler responds.
Step 2 – Verify thermostat call: At the boiler’s control terminals, measure for 24V or continuity when the thermostat is calling for heat. If no signal is present, troubleshoot the thermostat and field wiring.
Step 3 – Bypass test: Temporarily jumper the call-for-heat terminals at the boiler. If the boiler starts, the issue is external (thermostat or zone controls). If it does not, the internal control may be faulty.
Step 4 – Check control mode: In the installer menu, verify that the boiler is configured for the correct type of external control (e.g., on/off thermostat vs. modulating control).
Step 5 – PCB relay: If the boiler recognizes a call but does not start the fan or ignition sequence, an internal relay or control circuit on the PCB may have failed and require replacement.

Warning: Do not leave jumpers in place as a permanent solution; this bypasses safety and control logic.

Boiler display is on but there is no response to thermostat calls (no flame icon, no fan)

Possible Causes