I have a 20 year old spa with original equipemtn. EM 203 Brett Aqualine unit, catalog no 25-2300A-1, serial no. 85142. It is wired for 220. After a year of non-use, I cleaned the filter and tub and refilled it. It worked like it always has, except no heat. The heat indicator light is NOT on. I removed the filter and ran it for several hours but no heat. The air and jet controls are on the top of the tub and work fine. When I turn the thermostat dial, the pump kicks on, but no heater light and no heat. Any ideas?

If when you turn the t-stat up low speed of the pump comes on, I would suspect the pressure switch, based on you description.