I have a Tiger River Siberian (230v) that only heats when the jet pumps are running. If I leave the pumps off for an extended period, the water temp will drop. The unit is about 6 years old - out of warranty at this point - and I will be responsible for fixing it myself or hiring someone. I have an electronics background and necessary tools but no spa troubleshooting experience.
In the past, a heater was replaced and, another time, a controller board was replaced.

Thanks for any suggestions,
Scott

im having the same problem with mine ! any suggestions yet?

Make sure the circulation pump is pushing water - it comes out from the bottom drain in the spa. Look for a ripple on top of the water - or stick your foot down there.
If you have flow then check the 30 amp breaker - it runs the heater.
Is either the power or ready light flashing?
Depending on which heater you have it may have a reset button on top (a long black rectangular heater) or on the base (a tri-bend heater that looks like a trumpet).
Lastly - go in the control box and look for a red LED in the upper right portion of the circuit board - if there is a "heater on" LED then unhook the heater leads (on the far left of the control box) and test voltage across the 2 terminals the leads came off. If no power there it is probably a bad heater board (the board the power wires are hooked up to). You may see burning between the relays on the board.