System pressure rises excessively when heating, relief valve discharges water

Safety: Hot water discharge from the relief valve can cause burns. Keep clear of the discharge pipe.

Steps:

• Check cold pressure: When the system is cool, verify pressure is at the recommended level. If too high, carefully drain some water from a drain valve.
• Expansion tank test: Tap the expansion tank: a waterlogged tank sounds dull all over. With system pressure relieved, check the air charge at the Schrader valve; it should match design pressure.
• Fill valve: Ensure the automatic fill valve is not stuck open. Close the manual shutoff ahead of it if necessary.

Expansion tank replacement or recharging is best done by a heating professional.