Control system for modulating water heater

A method of controlling a plurality of modulating water heaters for providing hot water to a system at a water supply temperature corresponding to a temperature setpoint, the plurality of heaters including at least a first heater and a second heater, the first heater having a modulatable output ranging from a first heater minimum output greater than zero to a first heater maximum output, the second heater having a modulatable output ranging from a second heater minimum output greater than zero to a second heater maximum output, the plurality of heaters having a combined range of output including a first modulating range extending from the minimum output of the first heater to the maximum output of the first heater, a deadband between maximum output of the first heater and the sum of the maximum output of the first heater and the minimum output of the second heater, and a second modulating range extending from the deadband to the combined maximum output of the first and second heaters, the method comprising:

• (a) when a heat demand upon the plurality of heaters falls within the first modulating range, continuously modulating the first heater output to maintain the water supply temperature at the temperature setpoint;
• (b) when the heat demand upon the plurality of heaters falls within the deadband, allowing the water supply temperature to vary at least within a defined temperature range spanning the temperature setpoint before the second heater is turned on or off;
• and
• (c) when the heat demand upon the plurality of heaters falls within the second modulating range, maintaining the first heater at its maximum output and continuously modulating the second heater output to maintain the water supply temperature at the temperature setpoint.