I have been useing a triac to pulse a 1800 watt element. The unit I built was featured in Silicon Chip magazine. It works very well this unit switches 13 times a second and allows you to set the pulse width. Be aware that this unit includes circuitry to squelch the
interference the triac gives off that would upset your neighbours TV reception. If you wish to control a water purifier this is highly desirable.
A TRIAC or SCR would do the trick nicely. I have been trying to design one that uses proportional control with a feed back loop from a thermistor mounted in the HLT. The actuall control/triger cct is proving to be less than easy, but then again it has been 16yrs since I did all that stuff at Tech.
The Triac side of things is dead simple. you just buy one.