Boiler pressure rises excessively during heating cycle and relief valve discharges water

Safety first: Hot water from the relief valve can cause burns. Keep clear of the discharge pipe when the boiler is hot.

• 1. Check pressure cold vs hot: Note system pressure when cold (should be around 12–15 psi) and when hot. If it climbs above 30 psi, the expansion tank may be failing.
• 2. Inspect expansion tank: Locate the diaphragm-type tank near the boiler. Tap it—one end should sound hollow (air side) and the other dull (water side). If it feels heavy and full of water, it may be waterlogged.
• 3. Check isolation valve: Ensure any valve between the boiler and expansion tank is fully open.
• 4. Verify fill valve setting: The pressure reducing valve should be set to the correct cold fill pressure. If it continues to feed water and pressure climbs even when cold, it may be stuck open and require replacement.