Boiler intermittently shows F.15, especially when multiple zones call

Safety first: Allow the system to cool before adding water. Hot water can cause scalding.

• Check pressure gauge: Read the boiler's pressure gauge. For most Lochinvar hydronic systems, cold pressure should be around 12–15 psi for a two-story home.
• Add water if needed: If pressure is low and you have an automatic fill valve with a manual bypass, slowly open the bypass to raise pressure to the recommended level, then close it.
• Inspect for leaks: Look for visible leaks at boiler connections, relief valve discharge, radiators, and piping. Repair leaks promptly.
• Check expansion tank: A waterlogged expansion tank can cause pressure swings. Tap the tank; if it sounds solid and heavy, it may be full of water and need service or replacement.
• Reset boiler: After restoring proper pressure, reset the boiler and monitor for recurring F.15.

Professional help: If pressure continues to drop or the pressure sensor is suspected faulty, contact a heating technician.