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Mulitple Floats - Why, When, How

by Doug Pushard

Float animationFloats are the most common method of managing water levels in tanks. In the first article on floats, I reviewed how floats work and the role floats play in managing water levels in a tank. A previous article explored how floats can help manage water levels (i.e. Floats – Why, When and Where). This article will explore how multiple floats can be used to manage more complex systems.

One float can manage water to one specific water level. Multiple floats can be used in the same tank to manage to multiple levels (e.g. a low level so the tank is never empty and a high level so the tank does not overfill) or manage levels in multiple tanks.

Below ground tanks are more expensive than above ground tanks and consequently using both types of tanks is a good way of maximizing storage yet minimizing costs. In this configuration, it is a requirement to connect and control the water level in the two independent tanks. Floats are the way of accomplishing this easily.

In this design, the below ground tank is primary storage and the above ground tank secondary. There would be only one pump and it would be in the below ground system attached to a valve. To maximize your storage capabilities, you would drain water from the above ground tank to the below ground tank, but not overfill it. In this configuration two floats are required in the below ground tank.


The float in the below ground tank would be used to indicate there is water to be pumped. The float in the above ground tank would indicate there is room for water. Both floats would have to be in their respective ON position for the valve connecting the two tanks to open (i.e. turn on).

The above ground tank would have a float installed in the position of how much water you want to have in the tank at all times:

If near empty then the float is installed near the bottom (e.g. 6” from the bottom),

If half full then the float is installed at the mid-point, The float in the below ground tank would be a PUMP DOWN float.

The float in this tank would be in the ON position when it is up and in the OFF position when it is down.

The above ground tank gets the second float. This float is typically installed in the full position (e.g. right near the top of the tank) so it does not over-fill the tank. This float is a PUMP UP float, in that it is ON when the float is down and OFF when the float is up. When installing this second float take care that in the UP position the float is not hitting the top of the tank and therefore not able to shut off.


Please be sure when dealing with electricity that the power is off. The power supply should be either a 24v AC or 24v DC and control a valve of similar voltage. If you are unsure or not comfortable with electricity it is recommended you use a professional licensed electrician. Never install when there is water in the tanks and use extreme caution. You are dealing with electricity and water, a potentially deadly combination.

Once the floats are installed, connect their wires to the valves to be controlled. Connect one line (e.g. typically the black wire from the unplugged power supply to the valve to be controlled). Connect the other wire from the unplugged power supply (e.g. white wire) to one line of the first float. Connect the second wire of this float to one wire on the second float. Connect the second leg from the second float back to the valve being controlled.

Now you have a complete circuit. Both floats need to be in their ON position for the device (i.e. valve) to receive power. The lower tank will fill the upper tank, but not overfill it. All the electrical connections should be outside the tanks in waterproof electrical boxes that are protected from the elements and above the fill levels of the tanks. This scenario assumes that the below ground tank has an auto on/off pump installed in it and the below ground tank has the overflow line installed in it.

When the above ground tank is empty (e.g. float in the ON position), rainwater fills the below ground tank, the float in the below ground tank rises to open. So now the below ground tank float is in the ON position. Now both floats are in the ON position. This completes the electrical circuit and the valve opens and water will be pumped to the above ground tank until that tank is full (e.g. the float is in the up position and OFF). The pump goes off and any additional rainwater would then go out the overflow of the below ground tank.

Great, now you have two full tanks! But you ask, how can I get it to automatically empty the above ground tank into the below ground tank? As per the illustration above, by adding more floats of course!

In the case of an above ground tank feeding to a below ground tank it is often accomplished via gravity without use of a pump by installing a valve connecting the two tanks. When the valve opens the above tank drains to the below ground tank via gravity. Be sure to use a ball or gate-type valve versus a standard irrigation diagram valve for this type of configuration.

This case can be expanded where the two tanks will automatically fill and drain to each other based on the float locations. For example, the below ground tank could be thought of as the primary tank and the above ground tank is the secondary storage tank. The below ground tank has a pump and the above ground tank has no pump in it. Extra water is stored in the above ground (i.e. secondary) tank, but the below ground tank supplies the water to the ultimate use (i.e. irrigation, toilet flushing, or indoor water supply).

For this to work the tanks would have two pipes connecting them (i.e. one for filling and for one draining water) and two floats in each tank - a high float and low float.

The floats and configuration would be as follows:

1. The below ground tank would have a float installed near the top that would be ON when up or a PUMP DOWN type float.

2. The below ground tank would have a float installed near the middle level (i.e. or at any level you don’t want the tank to drop below) that would be ON when down or a PUMP UP type float.

3. The above ground tank would have a float installed near the top that would be ON when up or PUMP DOWN type float.

4. The above ground tank would have a float installed near the bottom (i.e. or at any level you don’t want the tank level to fall below) that would be OFF when down or a PUMP UP type float.

Floats #1 and #3 would be wired as previously described. The valve would be installed on the plumbing line that connects the pump in the below ground tank to an inlet connection in the above ground tank.

As the below ground tank fills the float moves to the ON position and if there is room in the above ground tank (i.e. both floats in the ON position), the pump turns on and the below ground tank fills the above ground tank. When the above ground tank is full, the float moves to the OFF position and the valve closes to quit filling the above ground tank.

Floats #2 and #4 would have the floats installed as described above. A valve would be installed in the second plumbing line connecting the below and the above ground tanks.

As the below ground tank empties though use, float #2 drops and it moves to the ON position (i.e. DOWN). If the above ground tank has water (i.e. float #4 is in the ON position), it opens the second valve and water will gravity feed the below ground tank until one or the other floats move to the OFF position.

LED tank depth displayIn the above cases, floats were controlling a pump or valve, but floats can also be used for other purposes as well. They can be used to turn a pump on or off with a pump relay or to turn indicator lights on and off. These lights could indicate how much water is in a tank. In this case the float would attach to a light and not a valve. This can be done by simply adding another float of the right type at the location where you want the light to turn on.

There are a few common methods of physically attaching a float with the most common being to attach it to the pump. Another common way of installing a float is to install a dedicated float pole in the tank. This allows floats to be installed in multiple positions on the pole and yet all the floats are installed in an easy place to maintain. It is critical that when installing a float, it must be securely fastened. Finding your float unattached floating in the top of a tank is never a good thing.

Some other things to be aware of when using floats:

1. Do not secure them directly with a metal clamp. Metal clamps will cut through the line covering and cause the float to fail over time.

2. Install the float in a way they can be easily tested and maintained (i.e. directly under the tank access point) with connections close by the outside of the tank so they are easy to replace when they fail.

3. Write on the float or wiring with a waterproof marker what the float is controlling.

4. No electrical connections are made inside the tank.

5. Floats are not obstructed in anyway. For example the top of the tank preventing the float from being in the full up position or the float does not lay on the bottom of the tank preventing it from being in the full down position, or other floats or tank piping prevent the float from performing correctly.

Floats, regardless if just one or many, are a great addition to any system. They can be a very, very useful way of controlling, monitoring and/or automating parts of rainwater, greywater, or blackwater systems. They are readily available, fairly inexpensive, and rarely fail. They provide a minimal type of automation or visual control that should be installed in most systems.

Floats provide some automated functionality to a system but there are other ways. Relays and dedicated control systems are another and will be covered in other articles.





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