Usually the popping sounds that a gas water heater makes when it's heating and shortly after are caused by lime buildup on the bottom of the tank.

My guess is you have hard water.

Expect to replace the water heater about every 12-15 years.

Have you been draining your WH annually?

Learn about thermal expansion in closed plumbing systems... http://www.watts.com/pages/learnabout/thermalExpansion.asp

Since you have a closed plumbing system (because of the PRV) do you have a thermal expansion tank at the WH?

If you do, did the plumber check the tank for proper operation?

If you don't, then get one installed ASAP.

You can check the pressure yourself with a tattletale pressure gauge that screws onto a hose bib.

Note that the sound is not coming from the hot water tank, but from the pipes behind/above the tank.

Drain annually? No, but I know I should. Couldn't get the tap open to drain it, and thought it's better to leave well enough alone.

Bought a pressure guage after hi-pressure problem after plumber installed new toilet supply valve. Pressure is now 60 psi.

We do not have a thermal exp. tank. Why didn't the plumber tell me about a thermal expansion tank?

It's just odd that one problem after another pops up after we got this plumber. I think I need a new plumber, although he is highly rated on checkbook.org.

Thanks for the advice.

This post was edited by leakinohno on Sat, Nov 8, 14 at 16:37

My home has had a pressure reducing valve since built in 1973. It had no expansion tank until three years ago. (Code Change.)

There was never an issue.

Is there a requirement for an expansion tank???

This post was edited by saltidawg on Sat, Nov 8, 14 at 16:19

Expansion devices became code in the mid 90's IIRC. Closed plumbing systems were mandated to prevent cross contamination of the water system by the home plumbing and thermal expansion devices were required..

If the T&P valve (last resort safety device) on the WH is functioning properly there will never be an issue aside from water dripping out the T&P drain line and exposing all the plumbing, fixtures, and appliances to excessive pressure and shortening their service life.

There's are videos on the net of WHs that went BOOM on a closed plumbing system when the t&P valve failed.

https://www.youtube.com/watch?v=l3i_ZGPtm68

https://www.youtube.com/watch?v=BZEIqG2RrMs

Well, I solved the knocking problem. After hours of researching online, I decided to try the easiest thing first--bleeding the lines. Too bad the plumber who did the work didn't do that or recommend it before leaving.

I turned on, and left running, all the faucets and flushed the toilets, from the top down; and then turned each off starting from the the lowest one working my way up. (I skipped the outside hose bibs, the dishwasher and the washing machine.) That must have removed any air that was trapped in the pipes and causing the knocking because I haven't heard any noise for the last week.

Also, somewhere I read that if you haven't been regularly flushing/draining your water heater for several years, you should not start as the drain valve might break, necessitating the replacement of the whole water heater. So we'll see how much longer ours lasts without the recommended draining.

I hope this info is useful to someone else. Perhaps the others who posted here assumed the lines had been bled.