What is rapid pounding sound in water system when shower is turned on?

Answer 1

I can think of two causes for that noise. One is simply a defective valve seating inside the hot water knob, which can cause a noise that sounds very much like pipe pounding. This is fairly easy to fix by replacing the defective valve. If the noise is not coming from the vicinity of the knob, then the increase or decrease of water pressure in the hot water line is causing the pipe to jump or shake. The pipe may be banging into wall studs, floor joists, or other objects. If left alone, this can lead to a broken pipe or joint, which in turn can cause thousands of dollars of damage to a home. Try to identify where the sound is coming from, and look for any wiggling pipes in that area. Hardware stores and home improvement stores sell products that will support the pipe that are designed to attach to wood studs; the ultimate do-it-yourself solution is to wrap a cloth around the pipe and simply wire it to a nearby stud. OR It is probably hammer. It is caused by a pressure wave travelling through the pipework. There are various cures possible - included in an excellent and comprehensive topic on Wikipedia - "water hammer" It is very easy to tell if it is hammer. Most showers control temperature by changing flow rate and that control is what causes hammer. If you change the shower temperature, you will change the flow and the hammer should change - generally a cooler shower will reduce or eliminate hammer as the flow rate is controlled little if at all.