Is there a way to refill an evacuated water supply reservoir from an elevated water supply tank, without using any valves, but only the power of gravity?

I already know that a siphon is needed to start the flow from the elevated supply tank, but I do not know how to push the air out of the lower reservoir, send water into it, and then retain the water without admitting air through the water fill tube or air communication tubes.

siphon refill of evacuated water reservoir

Also, where would be a place to look for possible solutions to these questions? I have researched the technology of intermittent dosing siphons for sewer flush systems from the 1880's before electricity was widespread, and I know that air communication tubes can be extremely complex and dynamic in operation, but there is apparently no formal way to describe it.

  • $\begingroup$ I rather doubt it, other than to match the pressure head in your "bottle" in that lower chamber to the pressure head from the refill tank. $\endgroup$ Apr 12, 2018 at 17:29
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    $\begingroup$ From that diagram no siphon would be required to start the flow. $\endgroup$
    – paparazzo
    Apr 16, 2018 at 7:00
  • $\begingroup$ Could you provide some context as to why the tank needs to be evacuated? Seems like a strange setup! $\endgroup$ Jun 12, 2020 at 14:07
  • $\begingroup$ Interesting question! The air pressure in the evacuated reservoir would go up as the water level goes down (less water column pulling vacuum), some areas covered by water would also switched to being covered by air. You'd have to trigger it with one of those factors (or one I haven't though of). Then you'd need to consider how you can even refill the reservoir. The vacuum would be zero when the reservoir is empty so you would have to use water flow to pull a new vacuum. $\endgroup$
    – Drew
    Jun 7, 2021 at 6:20
  • $\begingroup$ I think you need an opening on top of the reservoir to allow the air to escape. Also, the siphon will not start without an initial pumping, how do you purpose to handle it? $\endgroup$
    – r13
    Nov 4, 2021 at 17:41

2 Answers 2


As stated in the comments, based on your pictures, you don't need a siphon to start that flow.

Your influent line should have a cutoff valve to stop flow and the lower tank will also need an air bleed off valve so that the air can exit the tank as it's filling, but also so that you can introduce air back into the system when emptying the tank. If you don't permit air to exit and enter the system will stall either due to a vacuum or excessive air pressure. Vacuums and excessive air pressure are bad for the longevity of tanks not designed specifically for them.

Controls for the air bleed off valve can be tied to floats in the tank. As water enters the tank from the upper reservoir, the float will rise and eventually close the air release valve; once this closes, water from the upper reservoir will stop flowing down because the air pressure won't permit it to continue into the tank. At that point, manual valves can be utilized to close the influent line. With the influent line closed, you can manually open the air release valve which will permit air back into the tank and then permit the lower tank to drain. When it's time to refill the tank, simply open the valve on the influent line and let the tank fill until the float closes the air valve again.

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    $\begingroup$ As stated in the original question, NO VALVES should be used. The purpose is to make a device that is as low maintenance as possible and can run for potentially decades without replacement parts. $\endgroup$ Aug 21, 2020 at 23:19

Here's a solution that would work once (I think). But the upper tank must be sealed and everything filled just so. Maybe someone else can take it further.

water tanks diagram


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