I just replaced the Balboa 58019 heating unit on my Jacuzzi Z150 spa. It ran perfectly, heating the water up to the 100 degree setting and then shut down with FL being shown on the readout. I turned the power off and back on and the spa ran again for about 1/2 an hour. It then shutoff and the FL message came back on, which according to the owner's manual is a pressure switch problem, but the pressure switch is new.

The heater died over the winter and some of the pvc pipes and fixtures burst. I replaced the broken pipes and fixtures. I replaced one 3 foot length of pipe with 1 1/2" pipe (it was 2") because the fixtures which were sent to me had 1 1/2" fittings instead of the original 2". Could that be related to the problem? Would replacing the 3' length of pipe with 2" pipe help?

I noticed this afternoon the the pressure switch can be adjusted. What should it be set at?

Check your pressure sitch for continuity while the wires are disconnected and the power is off, you should read an "Open" circuit between the pressure switch terminals. If you read a "closed" circuit then you may need to do a minor pressure adjustment on the switch to get the circuit to open when the pump is off, if this is impossible then you may need to replace your "new" pressure switch.