What is "water hammer?" When your pipes crack and pop and make noise like you're hitting them with a hammer, that's "water hammer." This can be caused by three conditions in your water supply or hydronic heating system pipes.

Before describing the three types of water hammer, let me assure you, in spite of what you may have been told, water hammer is not supposed to happen. It may have been described to you as part of the "quaint character" of the home, but, if piping has been properly installed, and/or radiator valves maintained, water hammer won't develop.

The 3 common places that "water hammer" occur are:

1. In water supply pipes, especially to sinks located some distance from the primary plumbing walls,
2. In steam heating pipes and radiators, and
3. In hot water heating system valves.

One cause of water hammer occurs when water flows in pipes and the pressure changes too quickly, (the sink faucet is suddenly shut off, as an example). Water hammer occurs if the supply lines are not secured to the framing or if there is no air-gap, (capped pipe T-ed to the water line but not used as a supply line). Without an air-gap there is no place for the water to "vent" or "relieve" the pressure.

Of course, a water supply line air-gap is something every competent plumber designs into each system to avoid water hammer. But sometimes a home inspector discovers, (while testing the fixtures), that the air-gap has been abandoned during remodeling. Repairs usually involve a retrofit with the installation of an air-gap installed as a T at the top of the water lines above the water heater -- (To see an illustration click here).

Another source of water hammer occurs in steam heating systems. If the system design allows for water to stand or pond within the steam pipes or radiators, then you hear that "bang" when the thermostat calls for heat from the boiler.

This occurs because the accumulated liquid water is cooler than the steam. When the steam passes over condensed water in the line that has not drained back to the boiler correctly, it cools, condenses, and suddenly shrinks to 1/1,700th the volume it filled as steam. That leaves a vacuum directly above the water. The water leaps up to fill the vacuum and crashes into the pipe, Clang! Bang! ... water hammer.

"Water Hammer" is a reportable condition for a home inspector. However, discovering the cause is beyond the scope of the inspector's visual evaluation and s/he will recommend that a licensed technician "evaluate" and "repair" the condition discovered.

Water hammer can also occur in "zoned" hot water heating systems. When two zones are operating at the same time, (circulating hot water through the pipes), and the thermostat turns one zone off, a valve closes. This valve has springs in it to close off the hot water supply when the thermostat indicates that the zone is warm enough.

However, if other zone thermostats are still calling for hot water flow, the circulation pump is still sending hot water through the system. With the sudden closure of the valve by one thermostat while the water is still flowing, the bang of water hammer occurs.

One expert said this can be corrected by removing one of the two springs in the zone valves so it closes more slowly. Another recommended separate circulating pumps. Whatever the solution, I know this is true: Newton's Law is still in effect: i.e., "For every action, there's an equal and opposite reaction." In your home's piping, that "reaction" can sound like you have a blacksmith's shop in the basement as your pipes prove Newton was correct.

Water hammer can be hazardous to the piping and valves of the system. It is not prudent to tolerate it as most homeowners do. Solutions, however, should be relatively inexpensive -- NOT the complete re-piping of the house that one charlatan plumber recommended to a homeowner.