Our hercules spa, a 1.5 hp, two-speed 230 volt model will run, but the heater is off curing the circulation mode. The heater works only when the jet pump is on. What is the correct mode and do you think the problem is with the three-step air switch, a tri-delta model SAS143?

Does the pump actually run on low speed? If the pump is running and the heater is not activating it may be due to a clogged filter (remove filter to check) or an air pocket (bleed the system) or of course it could simply be a bad pressure switch. If the low pump is not running at all it is probably the stepper.