I showed in my other Instructable https://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. 

Step 1: Ye Cannae Change the Laws of Physics!

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. 

<p>How well does it heat in the winter time?</p>
<p>Dude, I love SCOTTY! HAHA Ya can't withstand gravametric distortions of that magnitude! <br>I too made a coil-based outdoor stove to heat my home: https://diybarrelstoveoutdoorfurnace.wordpress.com/</p>
<p>and here it stops ? .. but we want to hear how fast the thing heats up, or how fast your flow is into the tub, and/or how much charcoal is needed and how long it takes to eat up before you get this continuous flow .. and how this compares to 4 hrs it used to take .. oh oh ...aoohh (TTT) :)</p>
Thank you so much for posting this! This is exactly the type of project I need to make life easy at a primitive cabin in the mountains! You are amazing and clever and the best part is that I know I can make this! I am so excited, I can't wait to make this.

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More by richiemcb:HillBilly Hot Tub - More Power! HillBilly Hot Tub 
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