Why would a faucet start water hammering?

Question

I am having a problem where the tub faucet (single handle style) in my kids bathroom recently started water hammering. Since it did not water hammer in the past I suspect that something has broken or failed. I want to properly fix the faucet or the underlying problem so that it does not water hammer instead of just tossing an arrestor on it.

The faucet water hammers when it is moved from cooler to warmer temperatures. It stops if it is flipped full cold or full hot.

What problems would cause a fixture to start water hammering?

Answer 1

There is a column of air in your plumbing that acts as a shock absorber. They may also be some devices connected to the plumbing that accomplish this, they usually contain a bladder of some sort. Commercial water hammer arrestors have air-filled cylinders that absorb the shock of the sudden rise in water pressure when a valve shuts off. I would turn off the water to your home at the main shutoff. open the lowest valve, it may be in the basement or if no basement possibly an outside hose bib. Then open all the faucets and relax a few moments until the water drains out. close all of the valves and gently open the main supply valve. Check you may have a faucet open somewhere with the water running. At this point the faucets will sputter a bit when first used then when that quits the hammering problem will be eliminated for a long time but it will eventually come back as water adsorbed air. This should take only a few moments. Do not leave the water off for an extended period of time unless the hot water is turned off.

Answer 2

Before you do anything else - replace the cartridge in the faucet.
I had that exact problem with the valve in my shower. As soon as I replaced the cartridge, the problem was solved. Calcium buildup in valves will sometimes cause turbulence that brings on the hammering.
Occam's Razor may apply here - the correct answer is often the simplest.