Modulating condensing boiler's (Mod-Con boiler's) can increase combustion efficiency to about 96% up from the 80% efficiency of a non-condensing boiler at sea level, they provide a range of firing rate's and variable supply water temperature's to match the variable heating load of a building space or domestic hot water demand. The required boiler water supply temperature to the heating system can be predetermined from the use of an out-door temperature sensor/thermistor (Outdoor Reset Control Function) that automatically adjusts the temperature based on outdoor temperature to provide even more efficiency over the course of the entire heating season. The lower the required water supply temperature is to the heating system the more efficient a mod-con boiler can perform, upwards of 98% @ 65F-104F. The overall efficiency is dependent on the return water temperature and can range between 86%-98.5%. With a Non-Condensing boiler, condensation occurs during the combustion process within the boilers heat exchanger and flue piping when the boilers return water temperature is roughly below 134 F.  Non-Condensing boilers need to maintain a minimum return water temperature of 134F to prevent sustained flue gas condensation from occurring inside the flue vent piping and the boilers heat exchanger. Sustained flue gas condensation can lead to types of corrosion inside the flue piping and heat exchanger causing premature damage to a Non-Condensing atmospheric vented boiler. With a Mod-Con boiler the acidic condensation drains from the boilers heat exchanger combustion chamber to a condensate drain neutralizer with neutralizing media, limestone or other type's, then disposed of through a floor drain or indirect waste line. A Non-Condensing atmospheric vented boilers flue gas maintains a high enough temperature to prevent vaporization and condensation, the flue gas retains the acidic flue exhaust which is released through flue vent piping into the atmosphere instead of being recaptured and neutralized partly by a condensation neutralizer. A Modulating Condensing boilers heat exchanger and associated component's including flue vent piping are constructed out of materials that are resistant to acidic condensation like stainless steel and types of plastic's. Non- condensing boilers are designed to operate at higher supply water temperature's 155-180 degrees. The one other contributing factor with contributing to increased efficiency with a modulating condensing boiler is forced combustion meaning the gas is mixed with outside air/oxygen and forced into the burner to produce higher flame temperatures, which reduces emissions.