PermaHeat Bath Heater

PermaHeat Bath Heater Not Powering on.

I purchased a Permaheat m/n 935113 heater and installed in new tub installation. My problem is that the heater power indicator light is not lit and no heat is produced when pump is running. It is plugged into a seperate GFCI outlet. Also this is a vacuum side pump. Any help with where to start diagnosis would be appreciated.

:::UPDATE :::

Found that there was not enough suction pressure to trip the vacuum switch. By partially blocking the suction port the unit switches on and begins to heat.

Question now is the switch calibratable or does anyone no where I can order just a switch?