How to keep two water tanks have maintain the same water level?

Question

My home has two water tanks (side by side), one used by the owner and one used by us. Unfortunately for us, the owner has a good supply of water and we don't. I have asked him to connect the two tanks at the bottom through a pipe so that we too would get enough water, but he doesn't want to as he thinks its too much work.

Now, I am looking for a way to connect the two tanks somehow, so that the water level in them is the same at all times. I know that I can use a pipe to suck out water and put it in my tank to level the tanks temporarily. However, this doesn't work in the long run. Any ideas would be greatly appreciated.

Edit:

These two tanks are on the top of a three storied building.

Answer 1

Note: this answer is for pressurized plumbing of two hot water tanks. Rereading the question I'm pretty sure it's an incorrect assumption so I'm only leaving it here in case it helps someone with this problem.

---

The typical way to do this is to run the two tanks in serial instead of parallel. You would connect the output of one tank to the input of the other. I've seen this with a solar and electric tank combination. The solar tank was there to preheat the water, and the electric was the second tank that would run on cloudy days. Connecting the two tanks is quite complicated if you need the ability to bypass either tank. Here's a quick ascii art of what I've seen:

         +----X------+--- Hot
         |           |
Cold --+------X----+ |
       | |         | O
       O +----O----+ |
       | |         | |
       C H         C H
       WH1         WH2

The X are closed valves, O are open valves, C is cold input to a WH, H is hot output from a WH. The valve setup that is shown here is for the cold input for the home to go to WH1, back out the hot, into WH2, and back out the hot to the hot line for the house.

You could simplify this if you don't need the ability to isolate one of the water heaters:

Cold --+             +--- Hot
       |             |
       O +---------+ |
       | |         | |
       C H         C H
       WH1         WH2

The nice thing about having the water heaters setup in serial is that you can completely turn one off and still have hot water available to the entire home, just in lesser quantity. And in times of high demand, having both heaters running can get the water back to the high temperature twice as fast.

Answer 2

BMitch's solution will only work for pressurized water systems, which, while standard in the US, are not standard elsewhere, and it sounds like you have an unpressurized system where the water pressure is created by gravity.

If this is the case, go with ratchet freak's solution. The only problem is getting all of the air out of the pipes. The only ways that I can come up with to do this is to buy a regular ol' pump (if there is an existing drain valve) or buy a self-priming pump (more expensive.)

With a non-priming pump, it will look like this:

  ____________________
 |                    |          
 |                    B          
 |            ________|________ 
 |    _______|_               _|_______
 |   |       | |             | |       |
 |   |       | |             | |       |
 |   |       | |             | |       |
 |   |       | |             | |       |
 |   |       | |             | |       |
 P   |       | |             | |       |
 '---D_________|             |_________|

P = pump
D = drain
B = ball valve

With a self-priming pump, it will look like this:

         __________
        |          |          
        |          B          
        P  ________|________  
  ______|_|_               _|________
 |      | | |             | |        |
 |      | | |             | |        |
 |      | | |             | |        |
 |      | | |             | |        |
 |      | | |             | |        |
 |      | | |             | |        |
 |__________|             |__________|

P = pump
B = ball valve

In both cases, the pumps will be pumping upward. The water that they push up the pipe will flush out any air. It may take a few minutes to manage this, but it will flush out any air bubbles in the pipes, allowing the water to siphon back and forth between the tanks.

In order to operate the systems, perform the following steps:

1. If you're connecting to a drain valve (like in the first diagram,) then open the drain valve.
2. Turn the pump on.
3. Immediately open the ball valve. (Recommended delay between steps 2 and 3: 3 seconds.)
4. Wait 2-3 minutes.
5. Close the ball valve (B).
6. Immediately turn off the pump. (Recommended delay between steps 5 and 6: 3 seconds.)
7. If you're connecting to a drain valve, then close the drain valve.

There are only two requirements for the system:

1. The pump needs enough "head" (how high it can push water)
2. The pump needs a sufficient flow rate to entrain the air bubbles (shove them out of the bottom of the pipe)

In order to make sure the pump meets these requirements, you need to measure from the bottom of the tank to the top of the pipes shown in this diagram. That's pretty close to the wost-case scenario in regards to how much head the pump will need.

Find out how much flow rate the pump needs to entrain the bubbles. I don't know this offhand, but you should be able to find it by googling or asking a pump professional. You will need to know the diameter of the pipes involved, as larger pipes will take more flow rate to entrain bubbles. (1/2" pipe should sufficient for this purpose, as it doesn't really matter if it takes an hour to equalize the water levels.)

In fact, if you ask a pump professional, giving him the amount of head required, pipe diameter, and tell him that it must entrain the air bubbles, he should be able to recommend an appropriate pump within 5 minutes.

Anyways, if you want to figure it out yourself, you should have figured out the head and flow rate requirements. All you need to do is find a store that sells pumps and ask for a pump that meets those requirements or ask for pump charts showing head/flow rate. The charts will look like this:

      20 |
 H       |...
 E    15 |    '''..
 A       |         '.
 D    10 |           '
(ft)     |            '
       5 |             '
         |_____________'____
             0   1   1   2
             .   .   .   .
             5   0   5   0        
           FLOW RATE (GAL/MIN)

The chart shows a basic characteristic of pumping: that flow rate goes down as head goes up. Find your head requirement on the left axis and trace a line horizontally to the right. Where it hits the curve is the flow rate that the pump will put out at that amount of head. I'd aim to overshoot your flow rate requirement by 10-20%.

If I had to guess, you should only need to run the pump for 2-3 minutes once a month or so to maintain the siphon. The only way for air to get into the pipes that are part of the siphon system is the tanks are allowed to get very low or (a much more gradual process) of air bubbles entrained in the water pumped from the well get into the pipes or air otherwise falls out of solution.

Answer 3

To maintain same level in both the tanks you could connect the tanks from bottom. This will mean that an equal level will always be maintained in both the tanks.