On 5 Mar, 10:47, andrew wrote:
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

I just did a fag packet calculation to check your power per element
and you are closely correct. The only thing I can see that you have
not mentioned is temperature control but for a three phase unit I
would expect it to be via a separate thermostat and contactor in the
supply. The coil voltage may be 400(415) and would require a coil
change.
Are the ends of the elements not accessible to split the delta at all?
If they are and providing you use high temperature connectors all
three elements could be used in parallel, otherwise your idea of
gaining the use of two with one leg of the delta shorted to either N
or L is sound.
Many "electricians" are sadly far from fully competent or hide behind
the "safety" mantra when in reality there is no risk once the work is
completed, tested and covers/enclosures replaced.

The question remains is 2 kW going to be enough for a meaningful
contribution or would it be a waste of time anyway?

Finally if it takes three days for spares for the wood burner to
arrive what is the cost of downtime and it may be worth keeping a set
of common parts on stock.