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I had my bathroom sink valves replaced by brand new shut-off valves. Since this replacement, I started noticing pools of water just under the shut-off valves. What interesting is: most of times I checked for leaking, there was no leaking. I only witnessed leaking for once, which means the water pressure is sometimes high and sometimes low?
Is it possible to have a unstable water pressure in my apartment? What could have caused it?
Yes very possible to have erratic pressure that can cause leaks that appear during spikes.
This is why a pressure regulating valve should be installed in between municipal water tap and the house.
Ask your apartment manager to verify that such a regulator is in place and that it is functioning properly.
These valves can and do go bad over time and should be replaced to prevent the type leaks you are describing.
Typically the highest pressure in a municipal water system will occur during the time when fewest numbers of citizens are using water. Late at night when everyone is asleep.
In addition to Kris’s answer, you can have variable water pressure based on the number of residents of the apartment building using water at the same time. Or, even if a member of the same home or apartment uses water at the same time you are (someone flushes the toilet while you are in the shower for instance).
The design of the plumbing system has a huge impact and usually if there are multiple fixtures on a single 1/2” supply line it is most noticeable (at least within a single home/apartment). This is one reason many new builds are using direct lines to fixtures from a single manifold near the inlet to the home.
If you live on one of the floors near the top of the apartment building, these effects may also be more noticeable. Water has weight and you lose approximately 0.4 psi in water pressure for every foot of water column you go up, which gives a lower supply pressure than the lower apartment floors to begin with. As an example, 10 stories up (assuming a minimum story height of 10ft) you have ~40 psi less supply pressure than an apartment owner on of the first floor of the building. Having a lower supply pressure can make the same effects more noticeable.
