Hi

I understand that if a house does not have a check valve or PRV on the cold main, then excess pressure is able to escape into the cold main supply line if necessary. How does that work exactly? I thought water couldn't be compressed? It would seem like the volume of the pipes is already full to the brim so I don't understand how it finds any room?

I'm also confused by those under sink pumps that clear out the now cooled down water that is sitting in the hot pipe and sending it to the cold side with a pump. It's this same issue I'm not understanding how this water finds any room when the pipes are completely of water?

if street pressure is 70 PSI and your house is 70 PSI, nothing happens.
but if your pressure tries to rise it will want to equalize with the street pressure.

OP,


you are assuming pure water with no dissolved air


the microscopic air bubbles absorb the expansion else the pipe would burst as water itself is noncompressable

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