A heater can be wired to power up only when the filter pump runs. This setup assures that the heater cannot operate if the pump is not running and is often viewed as the backup for the pressure switch and high-limit in the heater.
Of course there is a delay in the heat cycle while the heater is powering up and its control circuits are activated, but this is generally not noticeable by the user.
The low voltage control or "Fireman's Switch" is still used by the Pool Control circuit board and will control the heater based on the programmed temperature settings as soon as it is allowed to run.
