The pressure switch has nothing to do with water pressure out of the jets of the hot tub. It's sole purpose is to tell the control box whether to turn on the pump and then whether to turn on the heater. Simply stated, when you first turn on main power or when the hot tub is about to turn on the heater/circ pump, the pcboard checks the pressure switch to confirm the micro switch is open. If it is not, then the topside either says solid FLO, FL, FL2, or the heater light blinks.
If it is open, then the pcboard turns on the #1 pump. Shortly after the pcboard now looks to the pressure switch to see if the micro switch is closed. If so, it turns on the heater. If it is not, the topside either now flashes FLO, FL, Fl1, or the heater light flashes, or on some pcboards, everything looks OK but the heater light does not come.
So you can see there is a narrow adjustment on the pressure switch that will make sure it's open when the hot tub is off and close when the pump is one.
The switch shown above is simple. The bottom threaded piece screws into the heater or pipe. When the pump turns on it pushes pressure into the piece and up into a diaphram inside the main housing. That diaphram pushes a rod up into the microswitch on the top and that switch either stays open or closes.
On the above example the black adjustment will allows for a little tweaking of the switch.
If your model does not have a pressure switch, it will have a flow switch. The purpose is the same but rather than having a part with a diaphragm and micro switch, you have a simple switch that as water passes by closes the flap against the post. The flap has a magnet on it and the post has a "reed" switch inside it. When the flap is away from the post, the switch is open. When the circulation pump is turned on and water starts moving the flat goes agains the post and triggers the reed switch which closes the circuit.
There are no user serveable parts on a flow switch and no adjustments. You will need to replace if this switch fails.
