Whether you've lived in your home for a while now or you're a new homeowner, a banging boiler can be disconcerting. Although a banging noise doesn’t necessarily mean you have a major problem, it does mean your boiler needs attention before the problem becomes a major problem.
When water gets trapped inside the boiler's steam pipe, you'll hear a banging noise. Steam and water share the same pipe, so when water puddles in the pipe and steam tries to enter the pipe, the water blocks the steam. As pressure builds up behind the blockage, the steam is forced back and forth in the pipe, which causes water hammer. If the pipes have the proper pitch, or slant, water won’t puddle in the pipe and cause water hammer.
If the boiler pressure is low and the water temperature is higher than what it should be, you'll hear a banging noise from the boiler. This indicates the water in the boiler is boiling, which is a problem. You should always know what the operating pressure (psi) should be for the boiler and what the ideal water temperature should be, so you know when the banging occurs from boiling water. Generally, the systems will run at 12 to 15 pounds psi, but the water temperature may vary, depending on the make of boiler.
Quick-closing zone valves can create a banging sound. When the valve closes quickly while the circulator is still running, pressure builds against the expansion tank. The water then bounces back to the zone valve, which has opened a little. After this cycle repeats itself for a few seconds, you'll hear a banging sound. You can replace the valve with a slow-closing zone valve.
When water flow is restricted, the boiler will start making banging noises. To operate correctly, the expansion tank must be topped off with water when the ballcock is open. Water may not be filling the expansion tank because the main supply is off. Called air lock, this means air is in the pipes or the ball cock has seized and is not functioning correctly.