Knocking sound after turning on taps

it has nothing to do with the drains it is trapped air in your water pipes. When a house is piped it should have a t fitting withw/the a t facing up and another piece of pipe connected w/ a cap facing up an air chamber that allows the air to be released before it gets to the tap.Either some doit your selfer re plumbed w/out putting them in or they put to small of one in they should be about 8"s tall and u can buy at any hardware store pre maid . look under your sinks if there isnt a t w/ a pipe pointing up thats capped then thats your problem or depending on how old your place is calcium may have built up enough to close the chamber, sometimes they are in the wall like your tub shower will most likely be behind the wall consult your local plumber.

Water hammer - Wikipedia, the free encyclopedia

It could just be that the sewer pipes are tightly anchored to the frame of the house and what you are hearing is the movement of the pipe, from thermal expansion and contraction, when binds as its moving through what ever brackets are holding it to the frame.

Just a guess!

Good info.

I will apply it when I build my next home, but will stay with copper pipes.

They have already discontinued the original PEC pipe after about 10 years of use. It was guarantied for 25 years or so. Lets wait for 25 years to see if this generation is better.

The pex piping that is use now in North America has been used in Europe for over 25 years. It is a crosslinked polyethylene material. There is also one for Hydronic heating. This has an oxygen barrier in it. The material that I believe that you are thinking about it the failed poly butyl tubing. I believe that there is still a class action law suit in both America and Canada over that stuff. That said I still prefer to use copper as it makes for a neater installation.

"So what we get down stairs is a knocking sounds that starts 1 min after using water, and continues for up to 20 mins after the tap is closed."

Based on your timing statement above, it cannot be water hammer, air in the pipes, or loose pipes.

It can only be thermal expansion/contraction of the pipes (mostly lengthwise). The noise is from the pipes trying to slide through support brackets or joist holes that are too tight! Commercially made plastic supports or homemade plastic sleeves through wood or metal supports will usually solve the problem.

Once the pipes are buried behind drywall, the cure can be worse than the problem. Good Luck!

(GA Yusef1)

Check the float valve in the roof tank.

• It may be that the washer is perished. If so, then replace it.
• It may be that the arm on the valve is oscillating as the tank refills after the water has been drawn off, in which case it needs more mass.

Check that the pipes are correctly supported. Are they held in suitable clips, and are the clips correctly sized? Are there enough of them?

As other posters have stated, it is not water hammer or air in the lines. If it were air in the lines, you would observe 'spitting and sputtering' at the faucet from air releasing along with the water. It is either the supply line or the drain, or both, expanding as it is heated. If you have PEX supplies, I doubt seriously if they are making any noise as they are flexible enough to bow rather than extend as a rigid copper line will do and it is likely that any new or recently-built building is equipped with PEX rather than copper on account of the significant cost differential.

In my home, the water is supplied by copper lines run under the slab-on-grade and penetrating the floor to the fixtures. In my kitchen, the drain runs horizontally along the wall before dropping into the main. Every time I run hot water or during the wash and rinse cycles of the dishwasher, I can hear the drain making noise as it expands and contracts. PVC lines definitely respond to changes in temperature, maybe not as much as copper, but to an extent that they move within their constraints.