Your confusing the electric and gas operations, heres an overview which should explain how the Cascade should work;
The Carver Cascade 2 is a 9 litre storage water heater, which when running on gas will heat the water to 65deg c in about 45 minutes. On 240V mains assuming it has this facility, the time can be somewhat longer or shorter depending on the wattage (630w 3amp to 840w 5amp) of the element fitted, you can use both gas and electric together for faster times.
To operate the gas there is a wall switch or a switch within a main control panel, either way both have three lights, green, amber and red. When switched on the green lights, (water tank must be full, i.e. water coming from hot taps), if it stays on after about 8 seconds then the gas has lit and all is well. If the green is joined by the red then you may have a problem, but if the gas bottle has just been changed then air in the pipes will have to be bled through by repeating the above 2 or 3 times. Once lit, and it should light without any pops and bangs, (this would indicate it needs a service), the heater looks after itself and gives constant hot water. Any problems will cause it to shut down safely and show the red light. Forget the amber light, it's to show low voltage and won't light unless the voltage is so low the heater and everything else packed up long since, though you may notice it 'flash' as the switch is turned on or off.
The 240v emersion heater if fitted is totally separate, and lies behind a white plastic box on the inboard end of the water tank. It is controlled by a switch, often close by and at floor level, but again sometimes as part of a remote control panel. The switch has a red light to show it's "on", not that it's working, this will be determined by the water getting hot. If it does not then it may have "tripped" Two types exist, early circa 1990 are non-re-settable but are repairable. Later models have a Red button on the end of the plastic box which is sometimes behind a little flap. Switch off mains, and press to reset.
Other faults concerning the gas side very often come down to the "Burner Module". This handy little unit contains the burner, gas valve and all the electronics which control it and is accessible from behind the cover outside of the van. In the event things go wrong it's a 5 minute job to replace it, with a new or serviced exchange unit. One other safety device is a wax filled 'fusible' plug, this again is behind the outer cover and shows itself as a 13 mm nut set in the fins above the burner. The wax will melt if things get too hot allowing hot water from the tank to spray over the burner and put the flame out. This will render things safe but will require a new module because it's control circuitry is faulty. However given if the fuse 'blows' without the water apparently getting to hot then replacing the plug will be sufficient. The point to note here is that over time the wax in the plug degrades or the threads leak, at the very least allowing water to seep onto the burner causing it to rust and eventually will still require replacement of the whole burner module.
Further problems that come to light in spring is the discovery of frost damage to the water tank, the non-return valve which is part of the cold water inlet and other plastic fittings. Failing to drain the heater when there is a chance of temperatures dropping below freezing can be very expensive to repair and should be avoided by removing the drain bung and allowing the heater to drain completely. Later models have a valve above the drain hole in the top left corner of the flue cowl, these have a 'toggle' showing that when turned a
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