I have read several entries on this subject but... When lavetory and kitchen hot water faucets turned on, banging plastic pipes. Four year old house, electric water heater, 3/4 pipe to 1/2 feeders (all HC-SOR II, CPVC), pipes are very securly fastened to joists. City water with house pressure set at 55 from 65 with same problem after installing hammer arrestors on hotwater side below hotwater faucets after rising from floor. Banging quit for a very short period. Two hot water lines feeding risers still bang. Do I put one arrestor on each, or do something else? Thanks

If your house is only 4 years old you may already have air chambers installed.
Back in the 60's we were required to to put shock absorbers,(air chambers) on all bath lavatorys and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will begain to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back.
You have just recharged your air chambers and saved yourself a hefty service call bill. Hope this helps TOM