The best things in life are free

Who fancies trying one of THESE and letting me know how good it is before I buy one?

In all seriousness it seems like a good idea!

That's cheaper but I wouldn't have a clue how to fit it!

I have a het exchanger in my van, it was in when we got it but it only heats the van not the water. The fan blows hot air or cold depending what you need and it gets rid of the cold air behind you as you drive along.

If that is the case it is not ''Free'' Water.

You're supposed to let the engine idle for about 4 minutes to generate the "free" hot water.
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Quite right, but Chausson was asking about the morning after arrival. Here's from the site:

(emphasis mine)

I like the idea of it in winter months especially, usually Wild Camping and travelling most days, so the water would be heating each time.

Seen something along the lines of that on Grand Designs to heat up a Hot Tub.

According to the diagram on the Website the heat exchanger takes water from the cooling system. So you run the engine until the temperature reaches 35°C and that provides water hot enough for a shower - supposedly. Again according to the site that takes about 4 minutes of idling.

Will That works in reverse too, if the engine is not left running the cold water going through the heat exchanger will cool the water in your engine block and your shower will get colder the longer you use it ?

Think of it this way... The coolant in yer average car engine runs at around 90c at normal operating temperature. All their heat exchanger does is transfer that heat from the flowing engine coolant circuit to the 'domestic' circuit BUT it does not store the heated water, It simply transfers the heat from the engine coolant as the 'domestic' water passes through the heat exchanger. Turn off the engine and you would run out of 'domestic' hot water gradually. The water in the heat exchanger wouldn't "boil" when the engine shut off, it'd simply heat the standing water in the domestic side of the exchanger up to the temperature of the engine coolant (90c). I guess this is why these guys have a thermostatic mixer valve in their system. To prevent someone from getting scalded...

I'm not sure it'd be the most efficient in a camper, I guess its down to HOW you use the camper. It would be perfect in a fishing boat where the engine is nearly always running...

In our case, would a 6.6litre petrol engine be more efficient at heating water when at a campsite? Having to start & run the engine for hot water versus using the LPG furnace? I'm not so sure...

If you look at the moglanders blog, they have a different system. They have the engine coolant lines connected to a coil in their water storage tank which heats their 30 litres of 'domestic' hot water as they drive. http://moglander.com/?page_id=2107 The risk in that system is eventually getting that stored water up to 90c... A thermostatic mixer would be vital in this system too.

For a system that uses the vehicles engine to heat the domestic water, I think I prefer the moglander approach...

Out in the MH last night, it was pretty cold! thought I would see how long it takes for heat to come out of the vents, lost interest, but a damn site longer than 4 minutes.