Troubleshooting and Replacing a Hot Tub Pressure Switch

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In order to test a hot tub pressure switch, power off first.
There are 2 wires attached to the pressure switch that lead from the spa pack to the flow switch or pressure switch. Disconnect the 2 wires and making sure they don't touch any other component or each other.

First, the filter will be checked along with any other possible obstructions, as the warning could really be something clogging the line, or even an air pocket. If it is certain that there are no obstructions, then the pressure switch may be out of calibration.

The service technician will then power on the spa and measure continuity with a volt/ohm meter across the 2 terminals on the pressure switch from where the 2 wires were disconnected. Measure resistance (Ohms) across the terminals, and it should have none (ie. read for continuity). If it does not read for continuity, the pressure switch can be adjusted (via a black notched ring on the switch or other adjustment ring). Counter-clockwise rotation decreases pressure needed. This should always be done with power off. Adjustments should be small.

When replacing the pressure switch, always power the spa down first. Choose a replacement switch with stainless steel threads where possible and use a little teflon tape for a snug fit.

Hot Tub Pressure Switch - FLOW

Spa Rubber DuckHot Tub Pressure Switches

Many older style hot tubs use pressure switches as a safety mechanism to make sure there is enough water in the heater to avoid catastrophic failure of the heater.

When the line is obstructed, when there is not enough water in the heater manifold or when the pressure switch calibration is off, the heater is automatically shut down and an error message like FLO or FLOW usually appears on the display.

There are different pressure switches for different types of installations.



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A FLOW or FLO error can mean a dirty filter, an obstruction in the line, a leak or air pocket in the heater when first refilling, or a mis-calibrated pressure switch.

Over time, spa chemicals can also take their toll on pressure switches leading to calcium deposits and buildup or other issues.

Pressure switches can be adjusted/calibrated, or they can be replaced. Some flow switches are mounted on the plumbing next to the heater while others are mounted directly to the heater back of the heater manifold.



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The pressure switch is the safety police of the heater. It normally allows power to pass through the device and back down the second wire lead to the control pack unless there is a problem determining adequate water pressure in the heater manifold. Water flows under pressure anytime the spa pump or heater circulation pump is activated. To heat the spa water, both the pump and the heater must be on and there must be sufficient water in the flow through heater.

Gecko Flow switch for hot tubs


Be sure to replace the flow switch or pressure switch with the same type. For instance, this (DTEC-1) is a Gecko pressure switch. The connectors are usually mpt (Male pipe thread) or barb type. Pressure switches are either stainless steel or plastic.

Technically there is a difference between flow switches and pressure switches, but many technicians use the term interchangeably. So whatever safety device is on that particular spa brand should be replaced with a similarly rated device. Be careful when replacing flow switches as wires carry the full voltage (usually 120v).

Before suspecting the pressure switch or flo switch, it is important to make sure there really is not a water flow problem such as low water level, dirty or clogged filter, air pocket in the heater tube (especially after a new fill), unprimed pump, or other obstruction in the line.

If rust, calcium deposits or corrosion is found on the pressure switch or worse, on the heater assembly, then it is best to replace not only the pressure switch but the entire heater including pressure switch.

Some problems with pressure switches include corrosion or they are frozen shut. Pressure switch wiring also may have come loose on the 2 terminals on the switch. Replacing the pressure switch mounted to a heater manifold involves either draining all of the water so there is none in the heater manifold that can swamp the inside of the spa cabinet, or shutting of service valves on both sides of the heater if available. Pressure switches with stainless steel threads are more robust than their plastic counterparts, but when replacing use a little teflon tape to assure a good seal around the mpt pipe threads. Don't forget to hook the 2 wires back up to the pressure switch, open any service valves back up so water fills the heater and then turn the spa back on.