All hot water tanks must have a pressure reducing valve, hot water tanks require a lower pressure to protect the temperature and pressure valve and a thermal expansion tank to prevent rust being pushed into your drinking water unless on a well system. I  recommend a thermal expansion tank just to protect rust from getting in your water- the code was made for the publics safety in municipal water systems however avoiding rust has benefits.. The thermal tank also protects the temperature n pressure valve most municipality's have a one way valve so your water cant expand back and poison the system with rust, so when water expands the 4% it does when heated the thermal expansion tank acts as a balloon cushion stopping your pressure from rising and lets the rusty water get pushed into it instead of your drinking cold water.