(1) Fact: Water reaches our homes at a pressure provided by the municipal network, established by the height of the nearest water tower (after water is pumped into the tower) or by pumps in holding tanks. In high-rise buildings (sometimes even as low as five stories!) the external pressure is usually too low to reach the highest floors, and most buildings therefore have their own pump to increase the pressure. The building has a small tank that is filled from the municipal line until a desired pressure is reached. From this tank, the water is fed to the consumers – sometimes only to the top floors, sometimes to the entire building. Anybody who uses water in the home has a direct influence on the pressure in the local tank and can cause the pump to be turned on.
(2) The Problem: There is a very high probability that typical water use in a home, such as washing dishes or flushing the toilet, will cause the pump to go on. What can be done on Shabbat? The situation is much worse than that of a refrigerator, where the probability of directly causing the compressor to go on is much lower. Anybody who is willing to accept the doubt involved might be safe, but it is highly recommended to adopt the solution proposed below.
(3) The Solution: One should install a mechanism that "continues an existing situation," which can be operated on Shabbat without any qualms. Here is how it works: Once every ten minutes, the pump is turned on for a few seconds, no matter what the pressure is in the line. There is no longer any direct link between the water pressure and the pump, such that using water does not turn the pump on. The pressure is checked automatically every ten minutes. If necessary, the pump continues running for more than the usual short interval, until the water reaches the desired pressure.
For consulting and to order a system, contact The Zomet Institute.