Tech support from Eco-smart say you need two inputs (two hot wires) 120 volts each but no neutral making 240 volt. Here in Australia we have 240 volts which is Active (Red) Neutral (Black) and Earth (green/yellow). So will I need a transformer? Even my electrician is scratching his head!
Apart from the question of whether it's got the appropriate certifications, etc. to be installed in Australia, which is a legal rather than practical matter...
It's a 240V-only appliance, it should be just fine. It is normal for everything operating on the US supply at 120V from ground to be rated for 250V to ground, so the insulation (electrical) should be safe with a 240V single-sided supply rather than a 120-0-120 supply. But your local electrical and/or plumbing inspector may object if it lacks the correct documentation for the local area.
If boiler is resistive type and no neutral is required on the boiler itself you sould be able to install it across live and neutral with live on L1 and neutral on L2 (it still have 240V across the two terminals) and also frequency (50 vs 60Hz) won't be an issue. If it's heat-pump type (with a motor in it) it probaly will not work.