One of the legacy sites I still do a bit of maintenance on has a large
sealed thermal store with plate heat exchangers for DHW plus an underfloor
loop. It is normally solar powered with a wood burner for high demand.

It has a 3ph delta wired immersion heater provided as back up but, due to
cock ups in communication there is only a single phase supply.

Obvious solution is to pull the 3ph immersion out and replace with 240V one
but nothing has been done about this in 4 years. Spare parts for the wood
boiler take about 3 days to source and fit so there is a need for a back
up.

I talked with the electrician and pointed out one could supply 240V across
two of the immersion terminals and neutral on the third, which would drive
~0.6 of the current and produce 1/3 of the heat of each element in the
immersion. So the 9kW 3phase immersion would derate to 2 elements at 1kW
each, still better than nothing if the 240V supply is up to it. The
suggestion was immediately condemned as unsafe and inappropriate.

I can see that it would be in defensible as it is using equipment in a way
for which it isn't designed but unsafe? After all it's using 0.6 the
voltage and current it was designed for.

AJH