I just recently installed a 50 gallon water heater in my garage. I had a low boy in the crawlspace and decided I'm too old and fat to keep climbing under there. A plumber friend from work (I work at a hospital) helped me. We added a recirculation line. I insulated the recirculation and regular hot water lines. I made sure no pipes rubbed together. The house was plumbed with copper. We ran PEX and connected to the copper. The PEX was secured with good clamps every other joist. We have a thermal expansion tank and one way valves installed. The only thing we don't have is a relief valve on those lines. I am also on a well with a expansion tank under my house. After all was said and done I now have water hammer. My TOTO toilet upstairs and washer solenoids are the worst offenders. It even rattles the lines in the wall coming from upstairs. It did not do this before this job. What the heck? How do I solve this? Add arrestors to my washer supply? I don't understand why it happened in the first place.