Bill,
If the coolant reservoir around the burner chamber is hot, the burner does not need to come on to heat water. The domestic hot water is heated by circulating through a heat exchanger inside that reservoir, and as the fresh water cools the fluid in that reservoir, the burner will fire up to maintain the proper temperature. So if the burner has run long enough to heat the reservoir of boiler fluid that surrounds the burner, you will have hot water unless your mixing valve is not operating properly.
Gerald