Q&A / 

Water Hammer

KABOOM! CRASH! You know the sound of your water supply lines rattling and clanging. Just moments before you were running water and turned off a valve. Or, perhaps your washing machine, dishwasher or icemaker just cycled off. Any quick closing valve coupled with fast moving water in a plumbing supply line can create the noise we call water hammer.

In reality, the loud noise is actually an explosion of energy within the piping system. Liquids can't be compressed. As water travels through pipes it contains energy. If you stop this water quickly, the energy has to be absorbed somewhere. In most cases, the pipes vibrate from the reactive force and weight of the shock waves within the water. In extreme cases, pipes can burst, joints can develop leaks, and valves and meters can be damaged.

Pressure and Velocity

Water hammer is directly related to the velocity of water traveling through a pipe. Velocity of water travel is directly related to the pressure that is pushing the water. Low pressure systems simply do not develop water hammer problems. The low pressure - or energy - of the water just can't get the velocity high enough to create a sizable shock wave.

However, if you have high pressure you can still avoid water hammer. This can be done by slowing the velocity of the water flow towards the valve by upsizing the supply piping. If a fixture draws six gallons of water per minute through a 1/2 inch diameter pipe, the water will have to rush towards the fixture at a velocity of 8.6 feet per second to satisfy this demand.

If you upsize the pipe to 3/4 inch to feed the fixture, the same six gallon per minute flow rate can be achieved with a velocity of only 3.9 feet per second! This is a huge drop in speed. As you can see, the chances of water hammer with a 3/4 inch supply pipe are greatly reduced.

Pressure Reducing Valves

Pressure reducing valves can help to reduce water hammer in many instances. But they can also be the source of noise in certain cases. They must be sized properly. If your house piping is sized correctly - most residential houses should have 3/4 inch piping feeding all the main branches - then you should install a 1/2 inch size water pressure reducing valve.

If you install a 3/4 inch valve on a 3/4 inch line, the valve will not always function as designed. This can result in poor pressure control and excessive valve noise. ALWAYS READ the installation instructions to make sure you are putting the right valve in place.

Expansion Tanks

Water hammer used to be controlled by installing vertical risers near the offending fixtures. These are simply vertical pipes that are part of the supply piping setup. Basically they are a "chimney" pipe right next to the supply pipe. When originally installed, these pipes trap air in the riser pipe. The air works as a cushion for the shock wave created when the water flow is stopped abruptly. The problem, however, is that the darn riser pipes - over time - can become waterlogged. In other words, the air in the riser can actually be dissolved in the water. When this happens, you lose the ability to dampen the shock wave.

The better solution is to install an expansion tank. These devices look something like the propane cylinders for your barbecue grill. Inside the tank at its midpoint is a rubber bladder. The air on the one side of the bladder acts as the cushion for the water hammer shock wave. The reason this works is because you can compress air.

Quality expansion tanks will work for many years before they ever leak or develop burst bladders. Even when they fail, it is easy to replace one. They can be removed from a plumbing system as easy as you change a light bulb.

You would be surprised at how much air volume is required to absorb the water hammer shock wave. The simple 18 inch high risers most plumbers have installed in the past just don't get it.

For example, if your piping system is sized at 3/4 inch and you have a water velocity in the pipes of five feet per second, a pressure of just 60 PSI, and a total pipe length of 50 feet, you need an air chamber that has a volume of 60 cubic inches.

Dual Solutions

You might be able to increase flow to fixtures AND solve water hammer by simply installing 3/4 inch pipe in as many locations as possible in your house. All too often I see 1/2 inch supply piping in people's houses. The 3/4 inch pipe will reduce friction and slow velocity. Try this first before you do anything else. Make sure you install ball valves at each fixture branch for convenience.

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