Welcome to Pressure (and Flow) Switch Tutor:

 

 First, the pressure switch has nothing to do with pressure in the spa. It's sole purpose is to tell the pcboard 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 spa 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 or FL 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 or FL, 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 spa is off and close when the pump is one. This leads to a common problem: quality of pressure switches. Our recommend switches are more expensive but reliable. The cheaper the switch, the reliability the switch lessens.

FLOW Switch:

If your spa does not have a pressure switch, it will have a flow switch. The purpose is the same as above but rather than having a part with a diaphram and micro switch, you have a simple switch that as water passes by closes the switch and completes the circuit. IF not enough water goes by or the switch is broken, it won't allow the heater to come on.