Step 9: Cut the tank to take the heat exchange coil
Mark the location on the bottom half of the tank (bearing in mind that you want to place the more of the coil at the top of the tank to get more of the heat) and cut with an angle grinder.
Insert the coil into the keyhole and then hold the top hemisphere over the bottom, balanced on top of the coil. Mark the location of the outlet pipe on the upper hemisphere then measure how deep you need to make the cuts.
Make the slot wide enough to admit the pipe without having to rotate the top half at an odd angle to account for the angle of exit of the tube.