Methods and system for demand-based control of a combination boiler

A method of controlling domestic hot water (DHW) output temperature in a combination boiler having a primary heat exchanger connected to a boiler loop, a burner configured to provide heat to the primary heat exchanger, an input fan configured to supply a fuel and air mixture to the burner, and a secondary heat exchanger configured to transfer heat energy from the boiler loop to a domestic water loop, the method comprising:

• initiating a domestic water loop flow and a boiler loop flow;
• measuring an inlet temperature and an outlet temperature of the primary heat exchanger;
• measuring a DHW output temperature of the secondary heat exchanger;
• determining a DHW flow rate based on a boiler loop flow rate, a boiler loop temperature differential based on the inlet temperature and the outlet temperature, and a DHW temperature differential between the DHW output temperature and a DHW input temperature;
• calculating a required heat output associated with the burner, the required heat output being defined as the DHW flow rate multiplied by a difference between the DHW output temperature and the DHW input temperature;
• and
• controlling the input fan at a fan rate corresponding to the required heat output.