Peter Jason brought next idea :
To get more heat we connect another bank of similar elements in parallel, to
> give a total of 2 x 12000 = 24000W.
>
> Of course these are totally resistive loads.
>
> But how do we connect the two banks in SERIES to get a very low heat output,
> often reqiured to heat viscous materials?
Have you thought about modifying a standard ready made star/delta contactor, as used for a motor starter?
Star would give you 240v across each element and delta 415v. Including the possibilty of switching the second set of elements would give you four different levels of heat input.