A thermostatic shower head may not output cold water due to blocked cold water supply, faulty temperature sensors, or pressure imbalances between hot and cold lines. These issues prevent the valve from adjusting correctly, leading to inconsistent temperature output.

The primary reasons include cold water supply blockages, often caused by mineral deposits or debris in the pipe. A 2021 study by the International Association of Plumbing and Mechanical Engineers (IAPMO) found that 30-40% of thermostatic shower failures relate to blocked inlet valves. For example, calcium and magnesium ions accumulate in pipes over time, reducing flow rates and triggering the valve to default to hot water.

Second, sensor malfunction can disrupt temperature feedback. A 2020 report by noted that 15-20% of thermostatic shower repairs involve faulty temperature sensors or microprocessors. If the sensor detects an anomaly (e.g., a hot water leak), it may override the cold supply to prevent overheating. Additionally, pressure imbalance between hot and cold lines forces the valve to favor one side. The IAPMO study cited 25-30% of cases where unequal pressure (e.g., hot water at 80 psi vs. cold at 40 psi) causes the valve to prioritize the higher-pressure line.

To resolve these issues, homeowners should inspect and clear cold supply lines, recalibrate sensors, and ensure balanced pressure. Regular maintenance, such as descaling valves annually, can mitigate 60-70% of temperature-related failures, according to plumbing industry data.