andrew wrote:

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.

This hasn't gone any further, boiler parts haven't arrived and the system is
running on the loaned parts.

One other thing crossed my mind, there are 12 flats so I think there is
possibly 3ph coming to the site or it would present a bad unbalance to the
local grid, or is it common in London to have one phase supply several
houses?

At work we have a 100A 3ph supply with several buildings being on separate
phases and only the workshop with 3ph. If the flats are the same and given
that there may not be enough peak capacity to supply the immersion and all
the flats at once; is there a device that would switch off the immersion if
the total load on the 3ph supply exceeded 80A and then only switch it back
in when it fell below 50A in any one phase?

AJH