Hot Water Water Hammer When Cutting Off Cold Supply

Question

The house is 3 years old. This just started to happen. When you cut off any cold water supply the inlet line to the water heater knocks or water hammers.

The first attempt I cut the water off at the street and cut on all the faucets in the house and let it sit for 15 minutes. Water hammer came back right after.

I drained my water heater thinking there was hot air trapped after reading a couple of posts. The water hammer went away for 3 days.

I have now cut off the main again and let it sit for about 45 minutes. Water hammer came back just like attempt one.

A couple of extra things to note, the hot water heater is now dripping in the overflow pan on a regular basis. Is this just my hot water heater going bad?

Any ideas?

Answer 1

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 & 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.