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.

Step 2: Fetch Me My Bucket

We are going to create a mini heater using a bucket to contain our charcoal but we are going to learn from those guys who perfected the charcoal furnace. The best way to create a hot furnace is to have some air being injected into the base to allow the charcoal to have plenty of oxygen to burn. We start with a standard galvanised bucket - 1st drill a hole in the side at the base to take a standard copper pipe - in the UK that is usually a 15 mm pipe. I then used a pie tin to create a base to hold the coals above the pipe. I used the same drill I used for the hole in the side to put some air holes in the pie tin base. We now have a mini blast furnace to create the heat - next we have to get that heat into our hot tub.

Step 3: Bend a Pipe

We need to get a coil of copper pipe bent to fit inside our bucket. This can be a bit tricky with the thicker 15 mm pipe I used in this project compared with the 10 mm pipe I used in my previous design since the pipe will want to collapse when you bend it. To get around this problem there is an easy fix – put a stopper in one end of the pipe and fill it up with table salt. Tap the end of the pipe to ensure the salt has settled down and then top it off – we now have a pipe full of salt which will not crush when you bend it. We then need a former to bend our pipe around – select something circular and strong that will create a circle that will fit into your bucket – I used a paint tin full of paint to stop it crushing. You can use a large screw or bolt screwed into a sheet of wood to pin one end of the pipe in place while you wind the copper pipe around the former. You will need a bit of force but at the end there will be a nice copper coil that will fit inside your bucket.

Step 4: Put It All Together

Now it is time to put it all together and get some heat going. Put your coil into the bucket with one end sticking through the hole you drilled in the base. You will connect up your water supply into this - I used a copper elbow to connect another piece of pipe to form the outlet into my hot tub. Now put some pre heated charcoal into the base of the bucket and top off with unlit charcoal so that the bucket is full up. I then insert a copper pipe into the hole below the pie tin base and this will be you air supply. When you turn the air on your charcoal will glow white hot - and that will generate the heat we need to fill our tub up directly from our water supply. Here you can see me filling up from cold to 42C continuously.