The problem with switching a relay depending on the voltage of the system is that when the 220V comes online, the voltage changes, so it switches back. Repeat...
Gordon's idea sounds pretty good to me, depending on how the system is assembled and how easy it is to add 3 more elements.
"solar hot water direct pumped into floor. no tank"
Curious about the element assembly, and how the plumbing physically goes together.
Sounds interesting.
G-