Screw-in heat exchanger
 

I once came across a picture of a HX like this
https://ecorenovator.org/forum/attac...0&d=1689616596

This would be good to screw into one of the 2" ports of my heat buffer, to connect a heat pump instead of one of the electric heaters.

Just wondering:

1. How difficult it would be to make one of these?
2. How efficient would it be, assuming it is immersed in warm water (30-50 deg C)?

That is a cool idea. I would imagine you could probably make something similar with soft copper, but I imagine it would be pretty difficult. I'm not sure how the heck they twist that without crushing the fins on it...

You can only transmit a reasonable amount of heat with a small heat exchanging surface like this, if you have a large temperature difference between the energy input side and the water around it.
So with an electrical element this works fine, but not with a heat pump, since the heatpump has to generate such a high temperature, that it's efficiency will be very poor.
The only way to transmit the heat with a lower temperature difference (over the heat exchanger) and so a more efficient operation of the heatpump, is when you will be using a much larger heat exchanging surface.