I live in a condominium. My hot water tank is situated in my laundry room directly across from my washing machine.
My upstairs neighbour's laundry room is alike and situated directly above mine.
My neighbour recently installed a heavy duty washing machine. When this washing machine is emptying, some of the water sounds to be splashing down our shared drain pipe located in the wall behind my washing machine, however, at the same time, some of the water sounds to be splashing into my hot water tank.
Could it be that my neighbour's washing machine drain pipe capacity is too small for the flow?
Could my neighbour's washing machine drainage actually be entering my hot water tank, or would this be clean water I hear being forced back by some kind of pressure?
I would greatly appreciate your advice.
Thank you so much.

When this washing machine is emptying, some of the water sounds to be splashing down our shared drain pipe located in the wall behind my washing machine, however, at the same time, some of the water sounds to be splashing into my hot water tank.
That's impossible since drainage and water are completely separated and isolated from one another.
Does this sound happen as the upper machine's draining or filling?
Do you have your own water meter or is the water common in your building? Let me know, Tom

When this washing machine is emptying, some of the water sounds to be splashing down our shared drain pipe located in the wall behind my washing machine, however, at the same time, some of the water sounds to be splashing into my hot water tank.
That's impossible since drainage and water are completely separated and isolated from one another.

Does this sound happen as the upper machine's draining or filling?
Do you have your own water meter or is the water common in your building? Let me know, Tom
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Thank you, Tom...
The sound happens when the upper machine is draining.
I do not have my own water meter. The water is common in our building.
What is the water I hear entering my hot water tank at time the above washing machine is draining?

Since you're on a common water system what you hear could be pressure fluction in the system. It doesn't sound like you should be concerned. Regards, Tom

Water12 sends a huge thank you to Tom (speedball1) for his kindness and expert knowledge.
My computer does not show a place to click "On the Scale."

That's cool! Your thanks are enough. Just glad we could help answer your question. Tom

As for the sound of rushing water: You are shareing ABS/PVC drain with condo above. Yes, water can be heard when rushing down the pipe. If you had Cast Iron pipe you would not hear it. It is a common problem in apartment buildings.

As for the sound of rushing water: You are shareing ABS/PVC drain with condo above. Yes, water can be heard when rushing down the pipe. If you had Cast Iron pipe you would not hear it. It is a common problem in apartment buildings.

This explains why drainage water sounds so loud as it splashes down our shared drain pipes. I have frequently felt that surely the drain pipes must be broken for the drainage to be heard so loudly behind the wall. Thank you so very much for this information.