I showed in my other Instructable http://www.instructables.com/id/HillBilly-Hot-Tub/ how to make a cheap off grid hot tub with a wood fired heater. In true Tim 'Tool Time' Taylor style the first thing we want to do is add More Power ! Yes - I want to be able to fill my hot tub direct from my cold water supply and have it come out of the heater hot enough for me to relax in - for me that is around 40c.
In order to figure out if this is possible we need to delve into the world of physics. We need to figure out what kind of energy are we talking about to heat this water up quickly enough. The rate my water comes out of the tap is 4.6 litres every minute and the temperature is 15C and we need to heat it up to around 40C so we need to raise it 25C. The formula for calculating the energy needed to raise a flow of water is (Dt x Flow)/14.4 in our case (25 x 4.6) / 14.4 or 8Kw. As a comparison the max power in an electrical appliance in the UK is around 7 KW. So we need a bit of power to do this and while wood is good there is something easily available that is better - charcoal. This has been used for many years to smelt iron and will burn at a much hotter temperature.