Here are pictures of what the hp/solar coils should look like:
It is actually two coils, plumbed in parallel, one on top of the other. The spirals are 15mm copper tubes, the common input/output is 22mm.
Here is the coil fixed to the bottom cap. Notice the heating element inside the coil. Two more elements will be installed (arrows).
The chimney (before perforating the top half):
The heat exchanger for domestic hot water. Notice how the coils get closer to each other near the top.
The inside of a (partailly completed) heat buffer. Notice the how the inputs/outputs will cause the water to rotate. The direction of rotation should be tailored to the Coriolis effect in your part of the globe.