System and method for determining heat transfer capacity of an indirect water heater

A water heater system comprising:

• a boiler including a boiler water inlet fluidly connected to a boiler water outlet via a boiler heat exchanger internal to the boiler;
• a heat source providing heat to the boiler heat exchanger;
• an indirect water heater, separate from the boiler, including an indirect water heater water inlet fluidly connected to an indirect water heater water outlet via an indirect water heater heat exchanger internal to the indirect water heater, wherein the boiler water outlet is fluidly connected to the indirect water heater water inlet, and the indirect water heater water outlet is fluidly connected to the boiler water inlet, such that water flows between the boiler heat exchanger and the indirect water heater exchanger;
• and
• a controller configured to control the heat source by:
• activating a pump such that the water flows between the boiler heat exchanger and the indirect water heater exchanger,
• controlling the heat source to provide heat to the boiler heat exchanger at a firing rate,
• measuring temperatures of the water at the boiler water inlet and at the boiler water outlet,
• calculating an amount of heat transfer from the boiler heat exchanger to the indirect water heater exchanger based on the measured temperatures, and
• adjusting the firing rate based on the calculated amount of heat transfer to determine a heat transfer capacity of the indirect water heater exchanger.