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Why is it ok to use tap water/toilets on Shabbos? Considering the source of the water pressure.

The water pressure that enables the water to flow out of the tap has to come from somewhere. It is almost always mechanically enhanced.

New York City in some cases might be an exception, since almost all the water pressure is entirely due to gravity. The water from the reservoirs in the mountains north of the city, literally goes downhill to the city, and is still benefiting from a gradient as it hits the city. This is apparently sufficient for even reasonably tall buildings! 10 stories or so before mechanical assistance is needed. In which case it is pumped to a tank on the roof, which then uses gravity again when needed.

So whenever you turn on the tap, flush the toilet, use the water, you deplete the current source, and if it is mechanically assisted, that needs to be replaced. Be that into a water tower, or via a pumping station.

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If you could state why you think it should be forbidden, it would enhance your question. Why should using a pump be a problem? –  msh210 Oct 6 '11 at 21:46
    
@msh210: I thought I did. When you use water, something has to compensate. Since water systems around the world differ (Gravity fed due to mountain based sources, vs water towers (Pumps) vs dams, vs simple pumping stations something is happening because you used the water. –  geoffc Oct 6 '11 at 22:03
    
Something is happening, yes. But why should it be forbidden? –  msh210 Oct 6 '11 at 22:05
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(In general, things are permitted unless there's a prohibition against them.) –  msh210 Oct 23 '11 at 20:41

2 Answers 2

up vote 8 down vote accepted

What you are talking about is combining grama (indirect effect, which is NOT pemritted by most, even for rabbinic prohibitions, with certain exceptions) and safek (uncertain result, i.e., we don't know if your water use will trigger the resupply loop).

Safek is generally permitted for rabbinic or grama situations, so long as there is a reasonable potential that the grama/rabbinic issue will not occur.

Here, we actually have a safek rabbinic grama. Pumps use electricity. By itself, electricity is considered by most authorities to be rabbinic, unless it causes heat and visible light to be produced together, as in an incandescent bulb.

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Electricity, according to Rav Shlomo Zalman Auerbach, involves neither molid, boneh, makeh b'patish, causing sparks, burning fuel, or bishul (of the wires). He said that minhag yisrael is not to use electricity and that turning on an incandescent bulb actually involves an issur d'oraita. –  Daniel Sayani Apr 22 '12 at 18:31
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Hello Dovid, welcome to Judaism.SE and thank you for this excellent answer! To make it even better, please consider adding a source, either for this specific case, or even just for the general rules. Also, please consider registering your account, which will give you access to more of the site's features. –  HodofHod Apr 22 '12 at 20:55

I learned that it is because there is not a direct connection between your flush (or use of the tap) and the pump; you're just part of an aggregate that collectively leads to the pump running. [citation needed]

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Does that assume that there are enough people that the system needing more water is pretty much constant and always running? Like I have heard for people in large Manhattan apartment buildings being ok using the hot water on Shabbos? –  geoffc Oct 6 '11 at 20:48
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@geoffc, I don't think it's about "constantly running" so much as it's about "can't tell you was you that caused it". So at the extreme, if you had your own private pump, you probably couldn't use the water because of the certain consequences (consult your rabbi). Most water systems are shared among enough people that the running of the pump is effectively unpredictable, which means you didn't cause it to pump. –  Monica Cellio Oct 6 '11 at 21:22
    
I get your point, but I suppose it is a question of how 'disconnected' does it have to be, before it stops being a concern? –  geoffc Oct 6 '11 at 22:04

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