Modulating pool or spa heater systems and associated methods

A modulating heater system for a pool or spa, comprising:

• a pool or spa heater in fluidic communication with water of a pool or spa, the heater capable of variably modulating energy output thereof to heat the water of the pool or spa;
• and
• a controller communicatively coupled to the pool or spa heater, the controller configured to:
• determine a predicted load based on a previous load, a current ambient temperature, a current water temperature, and an estimated pool or spa size;
• determine a temperature overshoot setpoint for a mode of operation;
• establish a target water temperature setpoint for the pool or spa water for the mode of operation;
• receive a current water temperature of the pool or spa water from a temperature sensor;
• determine if the temperature overshoot setpoint for the mode of operation plus the target water temperature setpoint for the mode of operation is greater than the current water temperature;
• determine an error value based on the temperature overshoot setpoint for the mode of operation, the target water temperature setpoint for the mode of operation, and the current water temperature, where the error value is determined as the temperature overshoot setpoint plus the target water temperature setpoint minus the current water temperature;
• determine if the error value is less than a target value for the mode of operation;
• determine a first modulation rate for the pool or spa heater for the mode of operation if the error value is determined to be less than the target value, the first modulation rate being determined based at least on the mode of operation;
• determine a second modulation rate for the pool or spa heater for the mode of operation if the error value is determined to be greater than the target value, the second modulation rate being a maximum modulation rate determined based on a maximum output of the pool or spa heater, an available fuel supply, and the mode of operation;
• and
• control the pool or spa heater to operate at one of the first modulation rate and the second modulation rate.