How can I automatically take water from 2 underground water tanks

[Jakebeckett]

I have an underground water tank with ball valve to control its level and whose water is pumped to a tank on the roof. Each tank has a float switch, these control the pump and the water levels. I have just built another underground tank. My question is how do I set up an automatic system so that if one underground tank becomes low the pump is switched to the other tank and vice versa if that comes low. Phew, hope that makes sense. Tks in advance

 

[Mliu]

Is there only a single pump? Or two pumps, one per underground tank?

 

[Sponsor]

 

[Jakebeckett]

Is there only a single pump? Or two pumps, one per underground tank?

There is one pump. And I want the pump to switch automatically as the tanks empty

 

[Mliu]

To do what you want, you will need either an electrically-controlled 3-way valve or a pair of solenoid valves. Then you will need a logic controller that will sense the state of the floats and operate the valve(s). This will not be simple and will likely be expensive to implement.

Two pumps, one for each underground tank, would be much simpler to engineer and may be less expensive. When the elevated tank calls for water, both pumps will operate, feeding from their respective underground tanks. They can both feed into a common discharge pipe that fills the elevated tank. When an underground tank is drained, then its pump is shut off. The other pump will continue to fill the elevated tank until it too is drained. As an underground tank is refilled, then its pump will begin operating again.

 

[Reach4]

No experience; just a couple of different ideas:
1. If tanks are close and the tops are at about the same height, how about a pipe that lets one drain into the other? No pump. Just gravity, so a bigger pipe is called for. Simplest.

2. If pump is suction pump, how about feeding the pump with a wye or tee. Have a mechanical float valve in each tank to block the path if the water level is low. Have electrical float switch in each tank. Put them in parallel, so that the pump runs unless both tanks are empty. The switches must each be set to be sure the switch opens before the water level is down to the intake pipe opening, and each float valve also must stop the flow from its respective tank too. If the pump is sucking air or if it is pumping when both intakes are blocked by the valves, expect the pump to fail.

 

[Mliu]

No experience; just a couple of different ideas:
1. If tanks are close and the tops are at about the same height, how about a pipe that lets one drain into the other? No pump. Just gravity, so a bigger pipe is called for. Simplest.

I had thought about this. But since they are underground tanks, it would require deep trenching down to the level of the tanks' bottoms to tie the two tanks together. However, you could install an above-ground siphon pipe (or one that is shallow buried) between the two tanks so they will cross feed. As long as the water level never drops below the siphon intakes, then the two tanks will maintain a common water level. But you will have to have a means to prime the siphon (or re-prime it if it ever breaks suction).

2. If pump is suction pump, how about feeding the pump with a wye or tee. Have a mechanical float valve in each tank to block the path if the water level is low. Have electrical float switch in each tank. Put them in parallel, so that the pump runs unless both tanks are empty. The switches must each be set to be sure the switch opens before the water level is down to the intake pipe opening, and each float valve also must stop the flow from its respective tank too. If the pump is sucking air or if it is pumping when both intakes are blocked by the valves, expect the pump to fail.

I had thought about this too. The problem with this idea is that it is a more complex solution and you are introducing a number of failure points into the system, some of which could result in the destruction of your pump. Then you have to buy a new pump, and it too will be subject to the same risk as the first pump.

That is why I recommended two standalone pumps, one for each tank. That system is both simple, redundant, and much less likely to fail. And if one pump does die, you still have a supply of water while you order and replace the dead pump.

 

[Murphy625]

A low level float switch in each tank and a simple double throw relay should be all you need. N.C contacts on relay cause a motorized valve to switch to Tank A.... When the level gets low, the float energizes the relay so that the N.O contacts then close causing the motorized valve to switch to the second tank.
It wouldn't matter if you used a single three way valve or two separate motor valves.. it all works out the same.

I do caution however that you'll probably want a 3rd float that would kill power to the system if both tanks happen to be empty.

This all really depends on the application... I do a lot of this kind of simple work for clients.. can't count the number of small panel boxes I've wired for this type of thing.

Hope that helps,

 

[Jakebeckett]

Thanks for all your really helpful ideas, and for taking the time to send them. Using the float switches seems cheap and practicable but I will chat with my plumber before I any final decision.
Cheers all

 

[Valveman]

If the lines from the storage tanks are even and plumbed properly, the level of both tanks should draw down evenly. You can draw from multiple tanks at the same time if the levels stay the same.