Switching / dimming a high power mains heater
No need for phase angle modulation like on a dimmer for lighting, no need for PWM nor for switching on/off every second. It will introduce interference and the thermal capacity of the heater will be so large that you can easily switch on and off for full mains cycles and even every so many seconds (if your neighbors don't mind). 

Switching at zero crossings is good practice, and a Solid State Relay can help you with that and is safe when properly connected.