Normal dimmers for incandescent lamps doen not work with "normal" PWM, no. They work with phase angle control.

They're using a triac. A triac, once turned on, doesn't turn off untill the current thru it goes to zero - which it does each zero crossing - you can not turn it off randomly. If you run a "high frequency" PWM signal into to the gate of such a device the lamp would flicker like crazy because the PWM signal isn't sync'ed to the AC mains. Sometimes it turns on at the end of the AC cycle, some times it turns on the start and some time it turns on the middle.

The way they work is that they detect the zero-crossing and then waits for a certain amount of time untill it sends a trig pulse to the triac which then turns on and conducts untill the next zero-crossing. For 60Hz mains each half-cycle is 8.333ms. If the delay between the zero-crossing and triggering the triac is 4.16ms the lamp would be at 50% (well not really since it's not linear but you get the idea).

Your typical AC SSR contains a triac on the output-side and works the same way. Once triggered it doens't turn off untill next zero-crossing.

The dutycycle of the PWM signal feeding your SSR would not relate to the relative power sent to the heaters because the current to the load would be turned ON at the rising edge of the PWM signal but not turned OFF at the falling edge.

/Henrik.