John Wood water heater makes a high-pitched whistling or hissing noise when heating

Safety: Use caution around hot pipes and fittings.

• 1. Verify valves fully open: Ensure the cold water supply valve to the heater is fully open.
• 2. Clean inlet screen: Some John Wood models have an inlet filter screen. Shut off water, remove the inlet fitting, and clean the screen.
• 3. Descale (tankless or electric): Follow manufacturer procedures to descale the heat exchanger or elements if scale is suspected.
• 4. Check for loose washers: Inspect nearby shutoff valves and flex connectors for loose washers that can vibrate and whistle.