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The water pressure is enough, it should be okay, personal opinion.
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It is possible to share the water heater, but it is best to use a storage electric water heater, which produces a large amount of hot water and an average temperature, and it is no problem to use it together at the same time, but the direct electric water heater produces a small amount of hot water, and the water temperature of the upper and lower stairs may be unstable when used at the same time.
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It can be used, but the hot water is a bit long and the pipes are far apart, so if you mind, it's better to install a water heater.
You can now apply two layers of penetrating crystalline waterproof paint on the hallway, and then carry out the toilet construction, so that there will be no water seepage.
It is certainly not legal for a businessman on the first floor to make four holes in the balcony on the second floor of your house.
Definitely, the bare pipes are not beautiful, and it is a common practice to cover them with suspended ceilings!
From ancient times to the present, there are very few such layouts.
You can make a water pipe from the first floor to the second floor.