Responded to no-heat emergency call for recurring issue. Diagnosed furnace lockout due to open limit switch. Found filter heavily contaminated with mineral deposits from customer's ultrasonic humidifier, causing extreme plenum temperatures (180°F). Replaced filter and discovered inoperative built-in furnace humidifier. Traced problem to faulty solenoid, which was replaced and tested to confirm proper system operation. Advised customer to utilize the furnace-mounted humidifier instead of ultrasonic unit, or to use distilled water if continued use is necessary.
