knocking sound in the water line

ionakana · 01-05-2010, 11:59 PM

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?

'Cause · 01-06-2010, 12:08 AM

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.

ionakana · 01-06-2010, 12:22 AM

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?

**Local16 Omaha** · 01-06-2010, 12:23 AM

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.

'Cause · 01-06-2010, 12:29 AM

Installing a shock-arrestor near the newly installed water heater may help.

ionakana · 01-06-2010, 12:56 AM

cool, I'll try that. Thanks.

**SilverBack4Jack** · 01-13-2010, 03:58 AM

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.

PlumbingTheCape · 03-01-2010, 06:11 AM

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.

**CHRISM** · 04-05-2010, 10:11 PM

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.

**phishfood** · 04-06-2010, 02:04 AM

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!