This has been done, it's not very good. The lighter socket idea is worse.
if posting as comment like that at least say why it is worse so evryone else that does not know can learn from it, I am guessing myself (I dont know) but would drain battery very quick leaving a dead water heater and a van that dont move. My reason for saying this I had a working light from cig (tab up north ) lighter went to start the van later battery dead a as dodo.
Ian
I'm hardly likely to say it's been done when it hasn't am i? what's more you took part in this post. Wayne did it, heres his comment, he has since bought an omni retro.
I have fitted a 75 litre twin coil calorifier tank into the back of my van. I have it connected to the hot water outlet on my radiator so that the heat that would normally be dispersed through the radiator is instead passed through my calorifier tank and returned to the cold circulation side of the radiator hose where it is recirculated back through the engine block.
Inside my calorifier tank are two coils with a lot of fins which disperse the heat into the stored water encased in the calorifier which surrounds these two coils. The water that comes from the radiator is filled with a percentage of antifreeze/summer coolant to stop the engine block from freezing or overheating which then pass through the coils inside the calorifier tank. The water surrounding the coils I circulate around my WFP tank via a 12volt, 75psi shureflow pump.
On a long journey the water stored in the calorifier tank gets very hot. I can either leave this hot water to remain stored in the calorifier tank for 24 hours where it would remain hotter than if I circulated it through my very insulated WFP tank or I can transfer it to my WFP tank to use for the day. I have tried using the water straight from my calorifier tank straight to my WF pole but I find it tends to be too hot and the pressure builds up to quick and is likely to blow at the pressure relief valve which I fitted onto my calorifier tank. So for simplicity I use the heat from my calorifier tank by circulating it around my WFP tank instead.
To confuse you even further, in my calorifier tank I have a 2kw immersion heater element which I can connect to an electricity supply for heating the stored water in the calorifier, although it takes much longer to heat using the electric, then it would to have the engine running.
Using the immersion heater with electric cost money. Using the twin coil heat input from the engine costs me nothing if I'm travelling to and from work.
Hope this helps the DIY want to use hot water users