What is water hammer?
Water hammer is pressure wave (or surge) caused by rapidly moving water having to change direction or stop quickly. Due to the fact that water is nearly incompressible, the kinetic energy of water moving quickly causes a backlash when it is forced to adapt quickly. An analogy would be walking around a street corner as compared to running around that same corner (figure 1). The higher the velocity (kinetic energy) the harder it is to change directions quickly. A person walking around a corner is able to control his momentum and change directions to adapt to the corner, whereas the person running cannot make rapid shifts in direction due to a large forward momentum. Similarly, water traveling in pipes builds momentum as the velocity of the water increases. Water moving slowly is like a person walking, whereas water moving quickly is like a person running. An elbow or valve acts as point of adaptation. An elbow is similar to a corner, while a valve shutting would be analogous to something like a door slamming shut. For copper piping, the recommended maximum velocity is 5 ft/sec (for hot water).
The term water hammer refers to the physical sound the water makes when this phenomenon occurs. Water moving at excessive speeds, when hitting an elbow or valve, will make a loud banging noise that sounds almost like a hammer being struck. Water hammer can also refer to the sound that the water makes when it is moving too fast through the pipes. If the water is only moving slightly too fast, the sound will often sound similar to a babbling brook. If the water is moving significantly too fast, the sound can become excessively loud, like a roaring freight train moving down the tracks.
What does this have to do with a Noritz Tankless Unit?
Flow, at its most basic level, can be traced to a differential in pressure. The larger this differential, the faster the water will flow. That is, the more pressure there is, the more potential energy there is. Water hammer is essentially the sound that the water makes when it is surging in pressure. The surge in pressure will flow from any point of relative high pressure to a relative point of low pressure. This pressure surge will cause a pressure wave to trail through the piping system until it dissipates (generally by running into an obstruction or by expansion). Noritz water heaters fire based on flow read by the spinning of an impeller wheel flow sensor. This pressure wave moving from the cold line to the hot line can induce spinning at the flow sensor, which makes the unit read flow momentarily. At this point, the unit will begin the pre fire stages. Since this pressure wave passes the flow sensor, and there is no actual flow, the flow sensor stops spinning and firing is aborted. Due to this short aborted cycle, many homeowners may hear this sound come on, and believe that the unit is not operating properly, or that their flow sensor is broken and claim the unit is defective, when in fact, the unit is operating perfectly normally, but the water system is designed improperly.
The most common noises associated with the presence of water hammer and a Noritz tankless is servo noise (described most often as a beeping type of noise), the fan blowing, or the unit clicking (the igniter sparking).
What are some solutions for eliminating this effect?
The most common effects used to prevent water hammer are the installation of a pressure reducing valve on the cold line, an expansion tank, and/or a hammer arrester.
A pressure reducing valve operates by lowering the amount of pressure that you have coming into the house. The lower amount of pressure that you have, the lower the amount of energy is available. Less energy will make for slower moving water and less significant pressure waves.
The expansion tank and hammer arrester generally work on the same basic principle. Once the water hammering starts it will continue until it is able to dissipate the excess energy. Often times, this energy is transmitted into the pipes by way of a “collision”. Think of this as similar to a collision between a car and a tree/pole (or your other favorite non-animate object): the energy gets dissipated rapidly and the car comes to a stop, but not before rending itself to pieces. This is very similar to what water hammer does to pipes over time. Enough collisions and the pipe will eventually break.
An expansion tank and hammer arrester, however, workslike the shocks on a car. They are able to absorb energy by using that energy to fight against a spring force. The expansion tank will often utilize air in a bladder. While hammer arresters, sometimes using air, may also contain gases (like nitrogen or carbon dioxide) or springs.