In addition to my set up below I have an aqua heat heat pump. The heater has worked well twice this year already to heat up a spill over spa with no issues. Today when I turned it on, a problem that occured several times last season occurred again. The heater will not kick on into heating mode. The fan gOes on and the unit tries twice to kick into heat mode and then shuts off with the hp warning in the status window. The problem is that my pump pressure using a new guage this year has never hit 20 and it worked yesterday under identical conditiOns. The unit is only 4 years old. Any ideas? I'm stumped.