weird water hammer problem

My house just started to have water hammer problems. It has a pressure reducing valve and pressure is at 60 psi. I turned it lower and the 2nd floor shower is pathetic (and the hammer did not go away).

Here is what is happening. You flush any toilet, turn on the garden hose, use the washing machine, (basically anything that has high volume water flow) there is an initial hammer (and it is loud sounding almost like a jack hammer) that lasts like two seconds then silence (its like a more subdued bang bang than what is described below).

When the toilet finishes filling no sound still. 20 seconds later or so and BAM. A loud banging can be hear throughout the house. When I go to the basement the noise seems to be coming from right near the pressure reducing valve and main water shut off from the street. The banging lasts a good 5-8 seconds starting sort of loud and getting worse then dropping off quickly.

The main shut off valve is at floor level in the basement, about four feet above it is the pressure reducing valve. I think the pressure reducing valve is over ten years old (I've been in the house two).

There is a thermal expansion tank on the water heater.

I drained the system and secured pipes. Everything I've read says arrestors should be installed near the problem valve but the toilets, etc don't make the noise. The noise is by the main.

Any help would be appreciated.

Hammer stops on the ends of the lines won't help as this is not your classic water hammer. It is a bad pressure regulating valve. A hammer stop on the upstream side might help but you're best off rebuilding or replacing the PRV.

Thanks for the reply. I was guessing it might be the PRV. How does it cause this problem?

If a pressure reducer starts making noise, either repair it or replace it.
It's fairly common.

And expansion tank does nothing for water hammer.
They do recommend hammer arrestors for any "quick closing" valve.
That normally means anything with a solenoid shut off like:
Dishwasher hot
Icemaker cold
Clothes Washer hot and cold

Of course any valve that shuts off quick enough can cause it.

And Irrigation Systems have quick shutting valves.

A friend of a coworker thinks my thermal expansion tank is bad...

The PRV works. It maintains pressure. It just happens to be right near where the hammering is coming from.

I'm inclined to go with your advice.

Can a PRV cause the hammering I describe even when it does manage pressure?

Can the main shut off valve be involved?

Now I'm confused. I was hoping to replace what needed or add an arrestor. I can do that. I just need to figure out what is wrong.

Thanks for the advice!

The PRV needs to be replaced.
A Watts 25AUB is good PRV.
Yes the PRV can cause hammering even if it is reducing pressure.

A hammer when a faucet is opened is caused by an initial drop in pressure accompanied by a sudden return of pressure. Normal water hammer occurs WHEN the faucet is closed, NOT 20 seconds later. It appears that the PRV is "working" but NOT properly. A thermal expansion tank MAY control water hammer, but that is NOT its purpose, and it would have to be precisely situated to do it in the first place.

I just wanted to thank everybody. I replaced the PRV last night and my problem is solved!