Hot Water Water Hammer When Cutting Off Cold Supply
Water hammer can happen with very low flows. The fact that it stopped for a few days after draining your water heater tells me that a water hammer arrestor would fix the problem as well as a “air leg”. when we built homes my dad would install vertical dead end pipe 2-3’ long pointing up on the supply line and sometimes do the same in bathrooms. This air cushion stops the water hammer like a water hammer arrestor but will last a lifetime arrestor last a long time but do leak when they wear out. The arrestor is smaller device that can be added later at a toilet or sink supply line and they cost about 15$. An air leg can be added with a T at the supply to the water heater then a section of pipe with a cap up to the ceiling less than 15$ but if copper it takes some skill to solder, CPVC is quite easy and can be done similar to copper with primer &amp; solvent for about the same cost. It’s not the water heater but a loose pipe issue combined with a resonant flow that causes the water hammer. Add a air pocket or arrestor and this usually eliminates the water hammer.