Hammering return

Marek

This problem is on the 25th floor of a high rise building. Its a recent renovation job, the problem is a hammering pipe noise right before a steam coil. The hammering noise only occurs when the zone has been off for a long enough time that the coil and pipes cool off, usually more than an hour after it has beed turned off. Then the noise begins and its like clock work, the intervals between the bangs are roughly the same for each cooling off period. After a certain time the banging stops, not sure when. The coil is situated about 20 feet horizontally and 9 feet vertically from the riser in that particular floor.
Here is the senario of the last service call: system was off overnight with the supply and return shutoffs closed at the coil and both the supply and return lines were cool. No hammering noise. Opened the return shutoff, heard a vacuum/gargling noise, and the pipe started hammering about a minute after. 10 minutes in closed the shutoff but the hammering noise continued with longer intervals between hammering. 20 minuntes in opened the supply shutoff and activated the heat for the coil, about a minute after the noise is gone. What is causing this hammering noise in the return line?

Steamhead (in transit)

Vacuum!

You have vacuum forming in the coil when it shuts down, and if the water in the return is warm enough it will flash to steam under the vacuum if there is enough vacuum. The vacuum may also be holding water up in the coil.

Put a vacuum breaker on the coil's return connection before the trap, and see if that helps.

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