12:22PM | 05/03/05
Member Since: 05/02/05
2 lifetime posts
When I turn on the hot water for the guest bathroom sink or shower, I get several loud knocks that eventually stop. It doesn't occur when I use the cold water or flush the toilet and it doesn't occur at the other bathrooms or in the kitchen. Could this be due to excessive water pressure or do I need to attach the pipes better (the house is only four years old)?


04:41PM | 05/07/05
Member Since: 08/29/04
227 lifetime posts
There are a number of reasons why pipes knock. One of them is excessive pressure. Anything over 60 psi is enough to cause pipe knock. In addition faucets that have cartridges often cause pipe knock, because they stop the water too fast. When you get water flowing in one direction and you shut it off quickly, all the forward motion is has no place to go. It is absorbed in the piping with banging.

One way to change that is to install a pressure reducing valve on the inlet of the house to reduce pressure to 60 psi. Another way is to change out the faucets to compression type faucets. Finally installing air chambers under the sink will help to absorb the motion of the water. Since the air can be compressed and the water cannot, when the water runs up agains the closed valve, it is diverted into the air chamber compressing the air and absorbing the shock.

Good Luck

Raymond VinZant Plumbing Prof.


11:25AM | 09/22/06
Member Since: 09/21/06
1 lifetime posts
I have this very same problem in my new home when I run hot water in my kitchen sink, or even if I pour hot water from a pot down the sink, the walls in front of the sink start knocking loudly. My house is still on warranty and the maintance guys can't figure it out. Keep me posted if you find an answer and best of luck, I know how annoying it is!!


12:38PM | 09/22/06
Member Since: 01/24/06
1558 lifetime posts
RE knocking pressure on the hot water.

Why would someone suggest placing a pressure reducing valve on the main (C/W) if this phenomena is only happening in one bathroom and only on the hot water?

Seems to me if it was strictly a "pressure" problem it would be system wide and not just H/W huh?

Loud knocking sounds could be contributed to long unsupported runs of tubing / piping

failure to allow for expansion and contracting of hot water lines.

Something as simple as lose washed on a (tile stop) shut off valve that controls the shower and basin

Many times an installer wants to cut costs and does not use a four fitting swing to allow for expansion and contraction of piping movement.

If this was a hydraulic shock wave from excessive pressure it would stand to reason that when a toilet is flushed or a solenoid valve closes from a dishwasher or washing machine you would also hear the THUD which this is not the case so FORGETABOUT a pressure reducing valve as you would be adding more mechanical devices to go wrong.

Try tracing the lines to this one bathroom and make sure they are secured properly BAND IRON is not a proper support clevis hangers are the way to go.

Make sure the piping is strapped with a lie wise material to prevent galvanic or electrolytic action of dissimilar metals ferrous and non ferrous.

Shut off the supply valve to this guest room and open the valve partially as this is what a globe type of valve is supposed to do THROTTLE the water flowing though the valve.

As you slowly open it have someone run the water and see if throttling down helps.

Air chambers are on as long as you have a way to replenish the chamber and are properly sized and even they are not going to stop a chattering bib washer or piping improperly installed.

Compression type faucets have a bib washer and a lose washer sounds like machine gun firing with lots of vibration.

Try to keep it simple, do not start complicating the job by adding more devices for the sake of guessing.

Has this problem recently started of was it there from day one?

Did anyone add or change anything to this room?

Trouble shooting is not replacing or adding things to the mix Isolate then approach it logically.

If advice does not sound right then it normally isn't...


12:48PM | 09/22/06
Member Since: 01/24/06
1558 lifetime posts
"I have this very same problem in my new home when I run hot water in my kitchen sink, or even if I pour hot water from a pot down the sink, the walls in front of the sink start knocking loudly."

If you hear knocking just pouring water down the sink you have some serious issues going on.

Would you happen to have a PVC drainage system by any chance.

Not all plumbing knocking has the same cause like someone with a head ache could have a simple bad day orand the other person has a brain tumor.

A lot of folks fail to read what water heater manufacturers suggest about blow down and thus sediment build up on the bottom of of the inside of the tank can cause all kinds of noise from crackling to out and out right banging as the C/W enters the tank.

Hearing knocking in a drainage system and also from using H/W this should be interesting to trace out



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