Water hammer problem, I think?

[LeoB]

Just recently a strange noise began. My office is in the basement/lower level of the house adjacent to the utility room. Recently there began a random thumping/knocking sound that I traced to the utility room and subsequently to the cold water line coming into the house. The knocking sound does not seem to be tied to the use of any toilet or faucet in the house. I can be sitting at my desk working for a couple hours and I can hear the knocking begin and nobody else is in the house. The gas hot water heater is not running nor any water running in the house. The knocking lasts between 15-45 seconds and then stops and might not occur again for 12 hours or more, or it might occur twice in one hour. There does not seem to be any pattern to it. I've been in this house 12 years and have never heard it before the last week or so. The house itself was built in 1977.

After doing some searching on this forum and Googling, I found the PRV. It's a Watts N35B, which is no longer made. I have no idea how old it is, perhaps it's original to the house. When the knocking begins I can hold the water pipe and feel the vibration with each knock both before and after the PRV (although this may mean nothing). The main water shut-off is just beyond the PRV and if I shut off the water, the knocking immediately stops. When I turn it back on, the knocking seems to go away. After watching some YouTube videos on water hammer, it doesn't sound like what I hear there.

I went and bough a pressure gauge from Home Depot and tested an outside sillcock and it showed about 58 pounds. Most references say 50-60 pounds is normal. I have no idea of what the input pressure is.

A lot of people in my neighborhood, all with houses built at the same time in 1977, have had problems with the water line coming into the house. I don't know exactly what all their symptoms were, but it involved replacing the line from the water main to the house. One neighbor said that they just started losing water pressure and I assumed that the line had broken or began leaking. Since my sillcock pressure is good, I hope that's not it as most of those jobs were in the $5-7K range.

Could this just be a PRV going bad? Any other ideas from the wizards on this forum?

By the way, I'm in northern Virginia. We do get freezes here, and last year I had a pipe to a sillcock freeze and split. It was the original 1977 straight through sillcock with no freeze resistant anti-siphon feature. I replaced that myself, although it was a pain because the feeder line was only 1/4" and I had to use a hammer drill to enlarge the hole through the concrete foundation to fit the new sillcock. We had some hard freezes this last winter with about 16 days in a row below freezing.

Any ideas?

Thanks

 

[Reach4]

I would start by getting a pressure gauge. See if the pressure goes over 80 or if higher pressure corresponds to the knocking. You can get one with a garden hose thread that could go on an outside faucet or a laundry tap, etc. Some meters have a second "lazy" hand that will remember the highest pressure.

Your pressure tank may have gone bad. They are cheap enough. Turn off the water, and open a faucet. If you knock on the tank, it should sound empty. You can check the air pressure while the water is off. The air pressure should match the normal water pressure.

So if the pressure reads too high on the gauge while you trickle water from a faucet, the PRV is bad. If it is OK then but peaks high other times, the problem may well be the pressure tank.

And I am not saying that any of this should make knocking. But get the gauge and do the checks anyway.

 

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[Jadnashua]

A momentary sample water pressure reading may not show you much. Water pressure can vary through0ut the day as people use more during some time windows than others, then the utility may be pumping water into towers or reservoirs at some point in time as well. If your pressure gauge has a second, peak reading tattle tale hand, leave it attached for 24-hours or so and see what the peak reading is. While you hear the knocking, check out the pressure. Because it stops when you shut your main supply off, my guess is that the PRV internal seals are wearing out. If your expansion tank is also inadequately precharged with air, or has failed, the pressure can go up internally in the house while the water heater is running to restore the set point on the tank...no water use is required for that to happen, but is an almost certainty when you are using it for something like taking a shower or running the washing machine.

Some PRV designs have a bypass valve that can open if the house pressure rises too far. That should not happen if you've got a working expansion tank.

The last thing I can think of is if the pipes are rigidly held, the pipe temperature will correspond to expansion/contraction of the pipe and that will cause the pipe to expand and contract...but, again, if it stops immediately when you shut off the supply, it is not likely to be a source of noises as you describe.

I'd find the expansion tank...wrap on it with my knuckle, and see if it sounds and feels like it is full of air, or full of water. If the later, it likely needs to be replaced, or, if you're really lucky, recharged, but that is rarer to lose air than for the bladder to break or leak and fill with water which halts any ability to accommodate water expansion when the WH runs. FWIW, with a PRV in the system, you NEED a working expansion tank as well.