Weil-McLain indirect water heater causes boiler to run constantly, even when no hot water is being used.

Safety first: Turn off power before working on control wiring.

• Check tank temperature: Feel the tank shell or use a thermometer at a faucet. If water is already at or above setpoint but boiler still runs for DHW, the control is not sensing correctly.
• Inspect aquastat or sensor: For mechanical aquastats, tap the control lightly; if the boiler shuts off, the contacts may be sticking and need replacement. For sensors, inspect wiring for shorts.
• Verify wiring: Confirm that DHW call terminals on the boiler control are wired only to the tank control, not accidentally tied to space heating calls.
• Check recirculation loop: A constantly circulating hot water loop without proper controls can keep the tank calling for heat. Install or repair recirculation controls as needed.