Float switches are essential components for any whole house reverse osmosis system equipped with a storage tank. These simple mechanical devices protect both the RO system and booster pump by automatically controlling operation based on water levels—preventing overflow when the tank is full and protecting the pump from dry running when the tank empties.
This guide covers float switch function, the tools required for installation, and provides complete step-by-step instructions for mounting and wiring both high and low float switches to a whole house RO system.
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What Is a Float Switch?
A float switch is a mechanical device that controls electrical signals based on water levels. The internal mechanism consists of a ball and lever system housed within a buoyant casing. As water rises and lifts the float, the ball rolls backward onto the lever, closing the electrical contact and completing the circuit.
Whole house RO systems with storage tanks require two float switches:
- High float switch — Connected to the RO system. As the storage tank fills, this float rises with the water level. When the tank approaches capacity, the float triggers a signal that stops the RO from producing additional water.
- Low float switch — Connected to the booster pump. As tank water depletes, this float lowers with the water level. When water reaches a critical low point, the float signals the booster pump to shut off.
This guide covers tethered (cabled) float switches, which are the most common type used with residential whole house RO systems. View float switch specifications for additional technical details.
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Why Float Switches Are Necessary
Float switches serve a critical protective function in RO system installations:
- Overflow prevention — The high float switch stops the RO system from overfilling the storage tank, preventing water damage, system strain, and wasted water.
- Dry-run protection — The low float switch safeguards the booster pump motor from operating without water. Running a pump dry generates excessive heat and friction, causing premature wear and potential motor failure.
Without properly installed float switches, both the storage tank and booster pump are vulnerable to damage that can result in costly repairs or complete system replacement.
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Required Tools and Materials
Gather the following items before beginning installation:
- Drill with step bit (½" size)
- Cable glands (one per float switch—typically two total)
- Wire strippers/snippers
- Box cutter
- Flathead screwdriver
- Wire nuts (3-4)
- Tethered float switches (2)
- Small cup (to catch drill shavings)
The RO system and booster pump should already be in place before beginning float switch installation.
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Part 1: Mounting Float Switches to the Storage Tank
Step 1: Drill the Mounting Holes
Drill two holes in the top of the storage tank, each approximately ½" in diameter—one for each float switch.
Important: Position a cup inside the tank to catch any shavings during drilling. Drill incrementally and verify the hole size after each pass—the hole must not exceed the diameter of the cable gland, or the seal will be compromised. Thoroughly rinse the tank interior before filling with filtered water.
Step 2: Secure the Cable Glands
- Remove the back nut from each cable gland
- Insert the cable gland through the hole from the exterior of the tank
- Thread the back nut onto the cable gland from inside the tank
- Tighten until secure
- Repeat for the second mounting hole
Step 3: Prepare the Float Switches
- Locate the weight included with each float switch
- If present, remove the insert inside the weight using a flathead screwdriver
- Feed the wire through the weight until it rests just above the float housing
Step 4: Install the High Float Switch
- Lower the float into the tank
- Feed the wire out through one of the cable glands
- Position the float approximately 12 inches below the top of the tank
- Tighten the cable gland to secure the wire in place
Step 5: Install the Low Float Switch
- Lower the second float into the tank
- Feed the wire out through the remaining cable gland
- Position this float just above the tank bottom
- Tighten the cable gland to secure
Positioning tip: The high float should have enough clearance to rise fully before the tank overflows. The low float should sit high enough to trigger before water completely depletes, but low enough to maximize usable tank capacity.
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Part 2: Wiring the High Float Switch to the RO System
Step 1: Prepare the Float Switch Wiring
- Cut back the black outer sheathing to expose the three inner conductors (yellow/green, blue, and brown)
- Strip each wire end to expose approximately ½" of copper
Caution: Exercise care when using box cutters or wire strippers. Cut away from your body and keep fingers clear of the blade path. Wear protective gloves if available.
Step 2: Access the RO Pressure Switch
- Remove the pressure switch cover
- Identify the two wires feeding into the switch
- Critical: Locate the black wire originating from the on/off switch—not the wire from the power cord
Step 3: Complete the Wiring Connections
- Disconnect the black wire from the on/off switch terminal
- Cut off its blue connector and strip the end to expose copper
- Twist the yellow/green wire from the float switch together with this black wire from the on/off switch
- Secure the connection with a wire nut
- Cap the brown wire with a wire nut by itself (this wire remains unused)
- Connect the blue wire from the float switch to the terminal where the black wire was originally connected
Step 4: Verify and Reassemble
- Tug each wire and wire nut firmly to confirm secure connections
- Replace the pressure switch cover
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Part 3: Wiring the Low Float Switch to the Booster Pump
Step 1: Prepare the Float Switch Wiring
- Cut back the black outer sheathing to expose the three inner conductors
- Strip each wire end to expose copper
Follow the same wire preparation process shown in Part 2, Step 1 above.
Step 2: Access the Pump Pressure Switch
- Remove the pressure switch cover from the booster pump
- Identify the two cables feeding into the switch: power supply (left side) and motor (right side)
- Critical: Locate the black wire from the motor connection—this is typically the second terminal from the left
Step 3: Complete the Wiring Connections
- Disconnect the black motor wire from its terminal
- Cut off its connector and strip the wire end to expose copper
- Twist the yellow/green wire from the float switch together with this black motor wire
- Secure the connection with a wire nut
- Cap the blue wire with a wire nut by itself (unused—note this is opposite from the RO wiring)
- Connect the brown wire from the float switch to the terminal where the black motor wire was originally connected
Step 4: Verify and Reassemble
- Tug each wire and wire nut firmly to confirm secure connections
- Replace the pressure switch cover
Wiring note: The booster pump wiring is opposite from the RO wiring. On the RO, the blue wire connects to the terminal and brown is capped. On the booster pump, the brown wire connects to the terminal and blue is capped. Double-check connections before powering on the system.
Wiring Reference Chart
| Component | Yellow/Green Wire | Blue Wire | Brown Wire |
|---|---|---|---|
| RO System (high float) | Joins on/off switch black wire | Connects to terminal | Capped (unused) |
| Booster Pump (low float) | Joins motor black wire | Capped (unused) | Connects to terminal |
Download the complete wiring diagram (PDF) for print reference.
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Frequently Asked Questions
Do I need a float switch for my reverse osmosis system?
Float switches are necessary for any RO system connected to a storage tank. Without them, the RO system has no mechanism to detect tank capacity, leading to continuous operation, overflow, and potential water damage. The booster pump also requires a low float switch to prevent dry running, which can destroy the pump motor.
What happens if a float switch fails on an RO system?
A failed high float switch causes the RO system to run continuously, overfilling the storage tank and potentially causing flooding or component damage. A failed low float switch allows the booster pump to operate without water, generating excessive heat and friction that can destroy the motor. Regular inspection of float switch operation is recommended to catch failures early.
Where should I position float switches in the storage tank?
The high float switch should be positioned approximately 12 inches below the top of the tank, allowing adequate clearance before the tank reaches full capacity. The low float switch should sit just above the tank bottom—high enough to trigger before water completely depletes, but low enough to maximize usable storage capacity.
Can I use one float switch instead of two on my whole house RO system?
Two float switches are strongly recommended for complete system protection. A single high float would protect against overflow but leave the booster pump vulnerable to dry running. A single low float would protect the pump but allow tank overflow. Each switch serves a distinct protective function that the other cannot provide.
Why is my RO system not shutting off when the tank is full?
If the RO continues running after the tank fills, the high float switch may be malfunctioning, improperly positioned, or incorrectly wired. First verify the float moves freely without obstruction from the tank wall. Check that wiring connections are secure and match the configuration in this guide. If the float housing is damaged or waterlogged, replacement is necessary.
What is the difference between a float switch and a pressure switch on an RO system?
A pressure switch monitors water pressure and activates the pump when pressure drops below a set threshold. A float switch monitors the physical water level in a storage tank. Both serve important but different functions—pressure switches regulate operation based on demand, while float switches prevent overflow and dry-run conditions based on tank capacity.
How do I test if my RO float switch is working properly?
Manually lift the high float to simulate a full tank—the RO system should stop producing water. For the low float, lowering it should cause the booster pump to shut off. If either switch fails to trigger the appropriate response, inspect wiring connections and test switch continuity with a multimeter. Replace any switch that doesn't respond correctly.
What size cable gland do I need for float switch installation?
Most tethered float switches require a ½" cable gland, which accommodates the standard wire diameter while providing a watertight seal. Always verify the cable gland diameter matches the float switch wire thickness before drilling the mounting hole, as an oversized hole compromises seal integrity and can cause leaks.
How often should float switches be replaced on a whole house RO system?
Float switches typically last 3–5 years under normal operating conditions, though lifespan varies based on water quality and usage frequency. Signs of wear include sluggish response, failure to trigger at correct water levels, or visible damage to the float housing. Preventive replacement every 3–4 years helps avoid unexpected failures that could damage other system components.
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