Interesting information about the defrost, I'll study it a bit more - I'm still playing with different ideas rather than just temperatures - the latest I'm investigating is using a small fan with an infra red optical link to detect the flow rate of air through the evaporator on the basis that less flow = more ice. Just waiting for the IR diodes to arrive.. The drop in pressure I tried before wasn't really reliable enough.
It's a pith the universal controller was abandoned. Yes, there are commercial pre-assembled controls out there but I doubt they give you the control and connectivity a home-made unit does (for me = no fun), for example adding a new method of controlling/monitoring remotely as above would probably be almost impossible with a commercial unit as it can't be modified.
All of my units have come from replacements, I'm currently running 4 home-made systems, 2 on my house, 1 on my greenhouse (in the process of being upgraded to run on solar as a combined solar water/heatpump system) and one in a flat where my mother-in-law lives.
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