I had a better idea about DHW vs heating demand. Make sure the piping at the output of the tank for DHW has a accesable place on it near the system. You may need a loop to stop convection. Attach a sensor to the pipe and when I gets hot from running DHW, use the sensor to disable the heating system.
A possible sensor would be a high temp limit switch. It just clamps on and has a switch and adjustable temp.