switch for two tank pumps control

Discussion in 'Pumps and Tanks Well Forum & Blog' started by GeneMN, Sep 1, 2012.

  1. GeneMN

    GeneMN New Member

    Messages:
    2
    Location:
    Minnesota
    I have a small greenhouse with a two tank cistern--graywater and rainwater with a pump in each tank--and need to switch automatically from the graywater pump when it is nearly empty to the rainwater pump. All I've been able to find are on/off switches; what I need is the equivalent of a three way electrical switch from one circuit to another.

    Does such an animal exist? Any help appreciated.

    If a moderator is here, will I be emailed if someone replies? Or do I have to keep checking? Thanks.
    Last edited: Sep 2, 2012
  2. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,841
    Location:
    NW Ontario, Canada
    If your pumps are 120V, a simple 3-way (SPDT) light switch would do it. Common as borscht.

    For 240V you would need a DPDT (double pole, double throw). Harder to find.
  3. GeneMN

    GeneMN New Member

    Messages:
    2
    Location:
    Minnesota
    Thanks. Yes, 120V. That was my first thought, and would be a good solution to do it manually. However, I need it to function on it's own. The circuit is on a timer to control the total amount of water pumped. I need a way to check and empty the graywater tank first, then switch to the rainwater for the rest of the time. When more graywater comes in, it resets to that pump. I thought of making a mechanical Rube Goldberg design using a 3 way switch and a float valve, but this is sketchy. I checked with the plumbing shops locally, and they knew of no such switch.
  4. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,841
    Location:
    NW Ontario, Canada
    Sorry, I didn't get that you wanted "automatic". I see you updated your OP since.

    I think you are not likely to find a canned package and will have to make your own from a float switch and a simple DPDT relay. A float switch in the grey water tank as used for a sump pump would work to energize the relay and pump. When the switch opens, the relay falls out and "flops" to the other pump.

    A flip flop (AKA latching) version would need energy from one source to flip it and energy from another source to flop it so two floats would be needed; a low limit and a high limit.
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