Hi all I have a spaform hot tub with a GS510SZ Spa Pack I keep getting the HFL trouble code, its a 2 pump system, the heater trips in (light on full) for couple of seconds, then flashes rapidly for about a minute, comes on full again, heats for around a minute or so then trips out with HFL code. Does anyone know if a pump should be running all the time when the heater is on ? or does it heat the water in the tube and periodically circulate it, then heating the next lot of water in the tube ?