How To Use A Hot Tub Flow Switch

As a hot tub owner, you would know by now that the flow switch is one of the important components of your hot tub. For those who do not have an idea what it is, hot tub flow switches are automatic switches that act as a safety mechanism in hot tubs against damages to the hot tub heater and other parts that have to do with water flow.

They monitor the water flow and the pressure of the water flow through the hot tub, checking if it is flowing normally or has been interrupted. If the flow is not within normal range, the hot tub flow switch turns off the hot tub water heater as a control measure against hazards and damages that are associated with faulty water flow. Some flow switches can monitor and then trigger a series of actions such as activating an alarm, powering on a pump, diverting flow, shutting off some parts of the system.

Using a hot tub flow switch requires knowing how they function in the day-to-day running of the hot tub, what causes them to malfunction or stop working and how to fix such conditions whenever they arise. Without this little knowledge about the hot tub flow switch, you might not be able to enjoy your hot tub the way you wish. Now that you know what a hot tub flow switch is and what it does, the next step is knowing how it works.

How does a flow switch work in a hot tub?

A flow switch specifically monitors the flow rate and pressure of water flowing through the hot tub piping or plumbing system. And not just that, but also can switch on and off parts of the system like the pump based on readings it can sense. These two functions of being able to monitor and control water flow are what makes it a flow switch, else it is better to refer to it as a flow indicator or meter if it only does the function of measuring the water flow rate and pressure.

From a specific part of the hot tub system, the flow switch can determine how fast the water is flowing and at what pressure through a sensor, which in most cases is usually a magnetic or paddle-like trigger placed in the part of the water flow, all other sensors like ultrasonic sensors and other non-contact sensors are used in some flow switches. This trigger is fixed on one end and free on the other just like a door, and is capable of being moved through a certain distance, rotated or displaced depending on how fast the water going through is pushing it.

The information regarding how the paddle is displaced is measured and sent back to the part of the flow switch that does the calculation of the flow rate and pressure based on already programmed data. The result is compared with inbuilt set parameters that tell it what to take when the reading is at so and so value. Actions like activating an alarm when it senses an obstructed flow, and many others depend on the make and design purpose of the flow switch.

What causes flow error in a hot tub?

Now, if your hot tub flow switch seems not to be functioning properly, it can be because of some other factors other than a faulty flow switch. Factors which can be as a result of you not maintaining good hot tub hygiene.

Filter issues: A dirty hot tub filter can restrict the flow of water in the hot tub. The hot tub filter does the work of continually removing debris and impurities from the water as it flows into the hot tub. Over time, these particles that are filtered accumulated in the filter fiber or material. When the filter is not cleaned on a regular timely basis, the filtered particles begin to pile up and get to a point where they clog the filter material, and water flow is reduced. So, a flow error can be because you have not cleaned your hot tub filter in a long time.

An old filter is also capable of restricting water flow and causing flow errors. When a filter has been used for a long time, the fibers gradually weaken over time and so does the meshing, which is the spacing in the filter through which water flows. The weak fibers can break down on the filter meshing and stick together, closing the spacing and restricting water flow. So if you have not replaced your water filter in the past 12 to 18 months, then the flow error may be a sign telling you to do so.

An incorrect water level can bring about a hot tub flow error: Most hot tubs come with a waterline indicator on the inner part of the hot tub shell. This is to indicate where the water is supposed to be for the hot tub to run perfectly well without having any issues that might arise due to inadequate water in some parts. Below such threshold level, a flow error might be indicated to direct attention to the water level. However, for some hot tubs that do not have such an indicator, the minimum waterline is the point where the water touches the bottom of the hot tub pillows, or headrest when the hot tub is not in use. So if your hot tub does not meet the minimum waterline requirement, ensure to switch off and then top up the water in the spa to avoid flow issues.

Airlock in the piping: If you have just recently changed or refilled your hot tub water and you are having a flow error, then the issue might likely be from an airlock created in the hot tub piping during refilling. An airlock is an air trapped between water in the piping, and this will cause a break in the water flow hence the flow error. When refilling your hot tub, put the hose you are using in the part of the piping or compartment where the filter is located. This will ensure the internal parts of the piping are filled first, eliminating the chances of air getting trapped in there.

How do you fix a hot tub flow error?

Now that you know what could be causing flow errors regarding water flow in your hot tub, fixing it should be easy. Just eliminate the faults and you should have the water flowing properly.

  • Increase your water level if it is low or below the waterline on the hot tub shell. Switch off from the power source, top up the water, and switch on the power. The error should be cleared if that was the issue. But if not, then it should be the filter.
  • Try running the hot tub without the filter. Switch off from the power source and wait for some minutes or half an hour for the water to settle. Switch on the hot tub and run normally except this time without the filter. Now check if the flow issue is still there. Switch off and replace the water filter, then switch on and run again observing for flow error. If the flow issue cleared when the filter was removed and returned when the filter was put back, then the filter is the reason for the flow error and should be replaced.
  • If you have tried the above two and the error does not clear, then it should probably be an airlock problem and should be removed by running water with a hose through the filter compartment.

What is a hot tub airlock?

An airlock is a large pocket of air trapped within the pipes of your hot tub. It will reduce the circulation of water as it will not properly flow through the hot tub heater. After draining and cleaning a spa, air pockets can remain in the piping and not allow the pump to push water, thus affecting the flow switch.

How do I know if my hot tub has an airlock?

In most cases, you’ll hear the jets running but you don’t see water flowing out of them. That is enough to tell you that your hot tub has an airlock problem.

How to get rid of an airlock in a hot tub

  1. One popular way of getting rid of trapped air in a hot tub is by burping. Simply turn on the hot tub jets for about 10-30 seconds and then turn it off quickly to release some amount of air bubbles in the process. This is done repeatedly or about three times. Ensure each process gets longer than the former. Then leave the jets running until all the trapped air bubbles have been completely removed. However, care should be taken to switch off the water heater when burping to avoid any danger associated with running a water heater when there is no sufficient water flow, which is the main reason why the flow switch was installed and is showing flow error. Also, ensure to stop the burping process at the third trial if the amount of water bubbles is lesser than the previous amount or any improvement is not noticed at all.
  2. Another way is to run a hose through the filter compartment to try and flush out the trapped air.
  3. Lastly, another way of getting rid of an airlock is by bleeding the pump. This is done by loosening the union nut which is the nut connecting the hot tub motor and the hot tub piping system, allowing the air trapped inside to flow out directly until only water flow is noticed. Although this is an effective way of getting rid of an airlock, it requires some technical knowledge to be able to open the hot tub service compartment and identify the union nut. Hence, it is better to contact an experienced person to check out your hot tub if you have attempted the steps above and you are not sure you can attempt this step without damaging something.

How do I know if my flow switch is bad?

A faulty flow switch can likely show a flow error when there is none. Because flow error can be caused by several reasons other than a faulty flow switch, the best way of determining if a flow switch is bad is by bypassing the flow switch and checking if the flow error clears.

How do you bypass a hot tub flow switch?

To check if a flow switch is still working properly or has a broken component, bypassing is a good way of doing so. That means, getting the electrical power to flow directly to the hot tub control panel without it having to pass through the flow switch first. To do this, the first thing first is safety, as usual, disconnect the power to the hot tub and then unplug the two wires connecting the flow switch on the control and electrical source sides.

Use another short piece of wire to connect the two wires you just pulled off that connect the spa to the main control panel. This action bypasses the flow switch and completes the electrical circuit back to the main control panel. Be careful that the exposed wire ends aren’t touching any metal. Turn on the power to the hot tub and check if it still has flow issues. If the hot tub pump and heater now work, then the flow switch is bad and should be replaced.


The hot tub flow switch is a very important safety device for you and the health of your hot tub. It serves as a preventive filter to some insufficiencies that might cause harm to major parts of your hot tub and also to you, hence proper care should be taken in keeping the hot tub clean and in good shape to ensure the flow switch does its job well.

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