On seasonal start up, a pool heater should run:

When starting up a pool heater for the season, it is essential that the heater operates continuously until the thermostat is satisfied. This method ensures that the water temperature reaches the desired set point as efficiently and effectively as possible.

Continuously running the heater allows it to work uninterrupted to raise the water temperature without fluctuating, which can occur if the heater is turned on and off intermittently. This is particularly important when heating a pool for the first time in the season, as the water can be significantly cooler and requires a steady and sustained effort from the heater to achieve the desired warmth.

Operating in this manner also allows for the most efficient use of energy; most modern pool heaters are designed to maintain temperature quickly and then cycle down to maintain that temperature without unnecessary energy expenditure. Using this steady approach ensures that the pool can be ready for use in a timely manner.