How to Choose the Right Float Switch for Your Septic Pump

Float switches are not one-size-fits-all, and selecting the right one is crucial to pump function

How to Choose the Right Float Switch for Your Septic Pump

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There was a day when you ordered a pump and you said, “and a float switch.” How times have changed. Now you have to be a float switch expert, as the selection of the float switch is extremely important to the system working and a pump functioning or not.

The two main kinds of float switches that my company uses come in different maximum amps, a 13 amp and a 15 amp. There are float switches for higher amperage pumps as well.

Recently I had a customer who needed his pump replaced. He had already stopped at a local plumbing warehouse and purchased his own float switch. After the pump was installed he called saying the pump kept tripping the breaker. After checking, the pump had a max amps of 14, and the float switch the customer provided was a 13 amp switch. Every time the switch tried activating it tripped the breaker. The float switch was changed to one that was sized for 15 amps, and the pump worked fine and the breaker stopped tripping. Amp size of the float switch must be properly sized for the pump it is operating.

Down open or closed

Not all float switches are created equal. Most of us are used to a float that is off while down and turns the pump on when raised in the up position. This is considered a DOWN (electrical contact) OPEN. The contact closes and turns the pump on when up. However there are DOWN CLOSED switches that are on when down. You obviously don’t want this float switch in the normal demand-dose system described earlier. In some cases the down closed (on) floats are used to show redundant off (override) when used in a panel, typically with time-dosed systems and other applications.

In some sand filter systems there are down closed float switches used with a time-dosing panel. This has caused some issues since when testing floats, some service providers are not aware of the requirement of down closed floats, which are not very common in typical demand dosed systems.

Tether lengths

Please note that each float switch also has limitations with minimum and maximum ranges of pumping. These limitations are based on tether length minimums and maximums that each float switch has. Tether length is the length of the cord measured from where you connect it to the float pole to the switch. Just because your design says you are going to have a 79-gallon dose doesn’t mean the float switch you are using can pump that amount based on tether length. Make sure you are reading the instructions and know the tether length minimums and maximums of the float switch you are using. 

Float poles

By now most onsite installers  are using a separate float switch pole. A float switch pole is typically a PVC pipe inserted into the dose tank with the float switches attached. Installers used to attach float switches to the force main pipe coming up from the pump. But then every time a float switch needed replacing you had to pull the entire pump. Now most use a separate pipe so the float switches can easily be removed, replaced, adjusted, etc., without removing the entire pump. 

Float switch mechanics

Most of the float switches discussed here operate by a steel ball rolling back and forth in the switch as the water level (and switch) goes up and down. The steel ball causes the electrical contact to open or close depending on whether the float switch is raised up or lowered down with the water level. Older float switches used mercury switches in place of the steel ball, but with concerns about mercury most manufacturers have moved away from those. Here in Wisconsin switches that use mercury are prohibited from use.

Newer generations of float switches use optics/light to turn the switch on and off. I have seen these used in commercial and municipal applications and the prices on these have substantially dropped. Instead of a steel ball or mercury switch, these switches use a fiber optic cord that sends down an optical light beam and turns on and off based on whether the light is continuous or the beam is interrupted due to the tilt of the switch. With the fiber optic float switches you have fewer (if any) moving parts. Some fiber optic float switches might require they be used with a control panel.

Be careful splicing float switch cords. Some float switches claim they won’t work, or will void warranty, if you cut the cords in any way. I think this is mostly prevalent in the transducer and fiber optic cords but check with your float switch manufacturer. 


A transducer works differently by hanging into the tank always in a down position, and it measures the pressure in the water. These controls require a panel and are more expensive. Transducers are typically used in municipal lift stations and commercial onsite systems and are becoming more prevalent. 

There are several benefits of using a transducer to control the dosage. 

Transducers can be controlled from the panel, and most can now be controlled from an app on a smartphone. Another benefit is by using a transducer and smartphone app you can more accurately set on and off parameters to fractions of an inch and be way more precise in dose volumes being achieved.

When using a transducer, a float switch can still be used as an alarm and high water override in case of any malfunction.

There’s not just one float that fits all systems. Make sure to match the right float switch for the right application.

About the author
Todd Stair is vice president of Herr Construction, Inc., with 34 years’ experience designing, installing, repairing, replacing and evaluating septic and mound systems in southeast Wisconsin. He is the author of The Book on Septics and Mounds and a former president of the Wisconsin Onsite Water Recycling Association.


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