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 c*ck 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