How would you build a 240v controller that can power a 9kw element?

[sw_reijnders]

You don't have to control the voltage to the whole 9kW.

That element has three 3kW elements, the links can be removed to isolate them.

You can switch two of them on and off, on separate circuits if necessary, and control the voltage to the third, so that your controller only has to be robust enough to control one 3KW element.

That's what I was thinking also.

i would rather have 3x 1000 watt controlled on all 3 elements than 1x 3000 watt to prevent scorching.

controlling 9kw isn't difficult, just use decent materials that can handle that much power.
so no cheap junk from ebay, but a crydom ssr like https://www.galco.com/catalogsearch/res ... =&q=hd4850

this is how i make them in 1 or 3 phase configuration for European installations using an Arduino Nano
here we have 230v on L1 to N

[Sporacle]

SW, the guys are referring to having one 3kw voltage controlled and the other two 3kw elements switchable.
That gives the ability to control everything from 0kw to 9kw with only one 3kw controller and two switches

[sw_reijnders]

i know, read my first sentence

it's better to have 3 elements on low power than 1 element on high power.
that's the whole point

you can add 3000 watt to you boiler with 1 3000 watt element or with 3x 1000 watt controlled.
i would choose 3x 1000 watt

[Sporacle]

Makes sense never scorched a wash so never thought that way.
It read to me like you were putting a 1kw controller on each element, I get what you are saying now I think

[sw_reijnders]

never say never scorched
but less likely.