Use zero crossing totally for the heating control and then establish a period of control that corresponds to 16 AC cycles. Apply the ON control to the heater for a greater or lesser number of cycles out of the 16 to control the amount of heat.

In the past I've used a linear control where error difference was mapped in linear manner from the 0 to 16 AC cycles per control period. (That was in an analog control circuit in the days before microcontroller controls).

More recently we worked on a heater system of another device where the error signal became the index to a lookup table with 16 entries. The 0 -> 15 index looked up a number of AC cycles to apply for that period. The table permitted the error mapping to cycles to be non-linear so that large error could use more aggressive heating whilst the lower error could be less aggressive. (MCUs with a zero crossing interrupt permit this type of control to be easily implemented via a simple state machine).

Slow, approximately quarter second, control periods work for most heater control applications because the thermal time constant is much longer than this amount. Depending on the type of temp sensor used there may even be an additional time constant imposed by that.

Michael Karas