Boiler makes loud banging or knocking noises when starting or stopping

Safety first: Turn off power before working on valves or expansion tanks. Allow system to cool.

• Step 1 – Bleed air: Use air vents on the boiler and system to remove trapped air, which can cause banging. Maintain proper system pressure while bleeding.
• Step 2 – Check expansion tank: Locate the hydronic expansion tank. Tap it lightly; it should sound hollow on the air side. If it is full of water or heavy, the bladder may have failed and the tank should be replaced.
• Step 3 – Observe zone valves: Watch and listen as zone valves open and close. Fast‑closing valves can cause water hammer; consider installing water hammer arrestors or slower‑closing valves.
• Step 4 – Support piping: Add or adjust pipe hangers to prevent pipes from striking framing when expanding or contracting.

Persistent loud banging should be evaluated by a heating professional to prevent damage to piping and components.