WASTE TANK HEATER ISOLATION SWITCH

Hope that this isn't too dim a question BUT in the 2018 Symbol and probably other models there is a large round black rocker switch with a red dot/light ? on the bottom half. Sited in the cupboard under the cooker.
The label reads WASTE TANK HEATER ISOLATION 
Now I understand that its purpose is to turn off the waste tank heater when there is no liquid in the waste tank (to avoid burn out) when the tank heaters have been switched on. (Hope that I am right on that)
Having been told two different things about this, perhaps someone on here knows the definitive answer?
Do the tank heaters only work when on EHU, or will they take power from the battery? If battery - vehicle or leisure?
Should the big black button be left in the top depressed position or bottom depressed position?

We have never used the tank heaters and probably not likely to, but it would be good to know

Many thanks

i don’t know if they have changed the way this works, but I don’t have that isolation switch in the cupboard under the cooker. Nor anywhere else, that I have noticed anyway.

I think I read somewhere the heaters are mainly for use on EHU, and they turn off when you disconnect the mains. But you can start the engine and switch them back on again, and they will stay on until 15 minutes after you switch the engine off. Which would be useful when travelling at very low temperatures.

Of course I have EC700 and it seems it does work differently, removing the need for the override.

Tank Heaters – This button will be back lit blue when active. This doesn’t mean that the tank heaters will be switched on. The tank heaters will only be on while the external temperature is below 2oC and if there is over 25% of water in the tank.

Thank you -

On my Warwick the switch has a red light so if I want to use the Hesters should the switch be illuminated?

This is confusing as we have been told that this Isolator switch is to turn off the Waste Tank heater if there is no water in it but you wish to select the Tank Heater switch on the control panel above the side door (or wherever) to heat the fresh water tank. So avoiding burn-out /overheating of an empty tank.
Presumably when this switch is pushed down the  red light is on so indicating that the waste tank heater is isolated? Others have interpreted it the other way.
We shall probably do what others have done and that is to remove the relevant fuse altogether , (No13 ?) so that the Tank Heaters don't work at all, especially accidentally! The Isolator Switch is very vulnerable in some AS models.
We have never used the tank heaters, and are unlikely too.

I I intend to use my WXL during the winter months so the tank heaters would be useful to me which is why I asked the question. At the moment all o know is that I can turn them on at the control panel but as to whether they are on is a mystery I am guessing that of the red light is on there is power going through it and therefore to the heaters themselves. On the other hand I could be completely wrong. Ha ha.

Well, it's probably a matter of try and see what happens 
As long as you have water in both tanks presumably it will be safe! (Although if Isolator does turn off waste tank heater that might freeze )
It will be interesting to know how you get on with it.
Good luck! Keep us posted.

p.s Last year, having drained the water system down, we did still have a few trips away. Not having water on tap is quite manageable for a night or two....using bottles etc, and the heating works with or without water in the Truma, so that's a bonus.
We actually had to be without water for a while well into the year as the submersible water pump packed up and had to be replaced. Now fitted with a Shurflo.

Well I have the answer, I think, having read through the AS manual I found the section on the tank heaters.

"If there is no liquid in the --waste tank-- the heater for it must be turned off using the isolation switch, ensuring that the LED is NOT lit"

So it seems that turning on the tank heaters from the control panel turns on the heaters in the fresh water tank and the waste water tank. However, is there is no water in the waste tank then this should be isolated by turning the isolation switch for it off, no LED lit. This then is a way of having the fresh tank heater on when it has water in it while at the same time having the waste tank heater off. Perhaps this is intended for when the waste outlet is left open, on a serviced pitch or connected to a hose draining  into a hedge of similar, or just left open over a suitable drain etc.

Of course if the waste tank tap closed and it is known there is water in it then both heaters can be left on. This would stop it freezing overnight. 

The only question I have now is  whether the heaters work when on EHU only or if they work on battery power only but that's another story.

They only work within a certain temperature range so it will be interesting to know, when it gets cold enough, how you get on 

IanJohnson wrote:... The only question I have now is  whether the heaters work when on EHU only or if they work on battery power only... 

They're powered by the battery - we have used ours when not connected to EHU. 

Remember, they're not 'on' all the time. From memory, they come on when the tank water temperature drops to 7 degrees, then go off again when the temperature has been raised to 18 degrees.

That’s helpful thanks