Abstract: The invention is an improved method for controlling a boiler used in a building space heating system. Certain control parameters such as outdoor ambient temperature and return line water temperature are monitored and boiler firing and quiescence are controlled as a function of a combination of water and ambient temperatures. This helps limit the time required to bring the system up to operating temperature. In general, the lower the ambient, the higher will be the return line water temperature which will initiate firing. And there are combinations of such temperatures at which the boiler is maintained quiescent notwithstanding a call for heat and other combinations at which the boiler is fired in the absence of a call for heat. Several control routines are disclosed.