knocking sound in the water line

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ionakana

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When I use the dishwasher there is always a knocking sound in the water line feeding it, sometimes it will do it even after the dishwasher has been cut off? I installed a new water heater about a year ago when I moved in to the house, I don't know if it made the sound before then? what could it be and how do i fix it?
 
It could be 'water-hammer' caused by entrained air in the pipes, or It could be a flexible supply or drain pipe that is tapping against the side of the cabinet.
 
it's not the flex hose, I can hear it on the other side of the house. How do I get rid of it if it's entrained air?
 
Dishwashers have a very fast closing valve that will send a "shockwave" up the supply line and it will "hammer" against the nearest bend or fitting upstream.
 
Installing a shock-arrestor near the newly installed water heater may help.
 
If your house is older and has non-mechanical air arrestors you could try to turn off the water at the street or meter, then open all faucets in and outside the house to bleed all water from the lines. Sometimes the old air arrestors would become water logged and draining the system and then turning the water back on will relieve any water that was pushed up into the air chamber and trapped.

Doesnt always work but you cant argue with a free trial.
 
Like mentioned before, the solenoid valve on the washing machine causes the water supply to the unit stop immediately once the machine decides it has enough water. This "shock-wave" that this valve causes could cause your pipes to rattle, depending on how secure your pipes are, I would first check that out, before spending money on a water hammer arrestor.
 
Other members have diagnosed `Water Hammer` - this is almost certainly the case - Just as point if you do consider installing a Water Hammer Arrestor - which is an expansion vessal which `takes` the concusive shock from for example your appliance [dishwasher] valve `shutting off very quickly -these pressurised vessals have a preset pressure and IF that pressure is too high in relatrion to your water pressure - it will have no effect in stopping the `water hammer` - concusive shock within your pipework. This also applies if the pre-set pressure within the vessal is too low compared to the water pressure - the pressure within the vessal needs to be approximately the pressure of your water so that the internal diaphram - flexible membrane will `take up` the surge of pressure when the appliance valve shuts off quickly.
With the pressure at approx./just above the water pressure - when for example the Dishwasher valve opens - the `membrane` in the water hammer arrestor dilates because of the lowering in pressure against it - when the appliance valve shuts off quickly - the water pressure `pushes` against the `membrane` which `takes` the surge and stops any `banging` noise/`water hammer`.
I have explained this because a lot of people - including some Plumbers are not aware of these facts - sometimes you will fit these Water Hammer Arrestors and they will just solve the problem - BUT - that will be just a `coincidence` that the pre-set vessal pressure WAS approx. the pressure of the WATER !!
Other times people will fit these vessals and find that they DO NOT solve the problem - and they do not know about the `relationship` between the pressure of their Water and that of the Vessal, so they do not know to `adjust` the vessal pressure.
Adjusting the `vessal pressure` CAN be a little `tricky` - as they hold so little air pressure - BUT it can be done - I am assuming that the Water Hammer Arrestor`s in the U.S. are similar to ones that I use in the U.K. ?? - in which case you can just use a Car Tyre Pump [Manual] that has a gauge - prior to this you should TRY it [fitted] first - I would ALWAYS install a VALVE to isolate it - IF you need to adjust the vessal pressure - it will be a simple matter to turn off the valve and remove the vessal for adjusting the pressure.
N.B. You MUST know the pressure of your water - ideally PRIOR to purchasing the Water Hammer Arrestor - to select one in the correct `Range` of pressure - BUT DEFINITELY PRIOR to adjusting the vessal pressure.
I hope that this has been `informative` - I tried to keep this as brief as I could - but to `fully inform` you [I hope] it turnede out this long! Chris.
 
ChrisM, the water hammer arrestors in the US that I am familiar with are a completely sealed assembly, with no means to adjust the internal pressure. Many on them are rated for inaccessible installation, meaning that there is no means of servicing them short of removing wall or ceiling coverings, so a charge valve such as you mention would be a bad idea.

What is the normal range of water pressure that you see in the UK? I don't know if plumbing pressure is figured in Bar of KPA. I am somewhat familiar with Bar pressure conversion to our pressure measurement pounds per sqaure inch (PSI). Normal pressures here range from 40 PSI (~9 Bar) to 80 PSI (~18 Bar). Above that, code requires a pressure reducing valve to moderate the pressure before it enters the dwelling. The hammer arrestors that I have examined are usually precharged somewhere in the 50 PSI range, so they service the range of pressures that we experience relatively well, in my experience.

Thanks so much for participating in the forum, I appreciate the different perspective that you bring.

Cheers!
 
Dishwashers have a very fast closing valve that will send a "shockwave" up the supply line and it will "hammer" against the nearest bend or fitting upstream.

yeah u tell him absolutely correct solution.
 
"Hi Phishfood" - Thanks for explaining that the `Hammer Arrestors` in the U.S. are NOT adjustable - here in the U.K. they are available with a `schrader` valve - the same as on a car tyre - to which we attatch a tyre pump - with a gauge - to set the vessal pressure.
The vessals are pressurised - mini versions of those used in `sealed system` Heating Systems and Hot Water Systems - with a neoprene membrane inside - and an internal pressure set at [for U.K.] 50 psi. - which in the U.K. is approximately 3.4 Bar - Bar pressure being approx. 14.7 psi.[ 1 Atmosphere] - the vessals are approx. 4" - 5" in diameter.
The Water Pressure in the U.K. varies enormously from region to region - even in London where I am based - an `average` is approx. 30 psi. -because of old supply pipework [not able to take higher pressures] - BUT I know areas where pressure reducing valves have to be used because the `Mains` pressure is 80 psi and above [mainly at night].
Our Water Pressure will be seen by the U.S. Plumber to be VERY low - BUT our `Mains` Water supply pipework in some areas is over 100 Years old - gradually being replaced - where new pipework has been installed - higher pressures exist, although MOST of our Water Heaters/ Combi Boilers etc are designed for `Low Pressure`[compared to what I read for U.S.]
When I stated that adjustment would sometimes be needed to the vessal pressure - not knowing that you do not have the same type in the U.S. - I was explaining that for example - the incoming `Mains` pressure was 30 psi and the purchased vessal pressure was pre-set at 50 psi - just installing the vessal and hoping that it would stop the `water hammer` - would not work because the 50 psi vessal pressure would NOT allow any `give`/deflection in the neoprene membrane - when the pressure surge `hit` the vessal - it would be the same as a `dead end` - as if no vessal was fitted, because the 50 psi behind the membrane would not allow any deflection.
To acheive the `deflection` of the membrane - the vessal pressure would have to be `nearer` the water pressure.
I am assuming from your description of the Water Hammer Arrestor - sometimes being installed in places which will become `inaccessable` - that in the U.S. the `Arresstors` are likely to be just a tubular form - with no membrane - which just `traps` an amount of air when the pipework is `commissioned`- and that the air `acts as a `buffer` to stop the water hammer ?? - perhaps like one could make using a larger bore of copper pipe and an end cap etc. ?? - although `sizing`this would be `trial and error`- for some situations. - We do have that type available in the U.K. - BUT I don`t think they would be used IF the purchaser KNEW about the `Pressure Vessal` type.
It was good of you to reply to my post - thanks for the information, there WILL BE plenty of situations where I will NOT be completely correct in regard to the U.S. Regulations/Codes - and I will never try to answer any question where Codes are involved - except perhaps to state what the U.K. Regulations/Codes are and make `Technical` comments.
In fact on the same day that I registered with Plumbing Forum - when I realised that it was U.S. based - I Emailed Keith to ask whether I should NOT participate - because of that.
I have `posted` on a couple of other subjects - regarding : Gas Leaks and Bathroom Vent [leaking water] - IF you can find them - I would really appreciate YOUR and other U.S. Plumbing Professionals input regarding the `relevence` of my `Posts` to the U.S. industry.
There is NOTHING `Technical` in those posts - I have not found any thing of a `VERY Technical nature to reply to - as yet. "Regards" CHRISM.
 
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A further note on hammer arrestors in use here. The type I am reffering to operate on a piston principle, with a actual physical barrier between the water and the air/gas. If there is not a physical seperation, eventually all of the air in the chamber will be dissolved into the water, and the chamber will become inoperative. We have installed "jobmade" air chambers in the past by, as you mentioned, using a section of pipe with a cap on it. These have fallen out of favor for the reason I mentioned above.

I think that you have presented your thoughts on the matter clearly and with sufficient warnings that your comments are that of a plumber from the UK. In the end, we are dealing with the same issues/problems, and your input is just as valuable as mine.
 
I would have been `surprised` if the U.S. did not have a design of `arrestor` that was more advanced than a `capped pipe` section.
When you said that the U.S. do not have the `Pressure Vessel` type that I was describing - and not having seen what you subsequently described as the `Piston type` - I had seen the `capped pipe section` used on a Canadian [?] T.V. program about correcting `Building Disasters`- including Plumbing faults - called `Holmes on Homes` - and although as you stated it is obvious that the Air which is `trapped` in the capped pipe would `dissolve`
its gasses into the water - I thought that adding those sections of pipe MIGHT have been `common practice` - I am glad to hear that it is not.

"Regards" CHRISM.
 

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