I just drained my Cal Spa hot tub for the 1st time since purchasing it ans after refilling it and replacing the box filter with a new one it keeps giving me a FL error which in the user manual says that a Pressure switch has detected a loss of water flow. The manual says to clean filter but it is a new one. Not sure what to look at . When I lift the filter up a little it clears the message and the Hot tub resumes normal filtration or heating.

Any help before calling someone out is appreciated

clean your filter and problem will go away. you say the heater comes on when filter is out, right? if this doesnt work, you must adjust pressure switch.

sorry, i misunderstood, what kind of filter do you have? is it possible that something is caught where filter sits in base? if you put new filter in and this just started, then you might also have a valve partially closed valve? ithere is a restriction somewhere. i wouldnt recomend adjusting pressure switch until you are satisfied that there are no restrictions.

Asimon, apparently you're using a filter that has a closed top (possibly with a handle molded in place), and an open bottom and it sits in a reservoir. My solution to this problem in the past has been to drill a small 1/16" vent hole in the top. What's happenening is that air is getting trapped inside the filter element.