Mod-Con or modulating-condensing boilers are the current standard for high efficiency natural or propane-fired space heating boilers.
Modulation then allows for less-than-full-fired operation at thermal efficiencies (the burner flame actually goes up and down controlled by water temperature demand) in the mid to high 90s% (many reaching 95% AFUE!). Old-fashioned low efficiency (88% or lower) boilers are full-fire (a few actually fire in two stages) but can’t be controlled to the same extent or effect as a true modulating boiler.
A condensing boiler recovers the latent heat from combustion gases (mostly water vapor) adding at least 10% thermal efficiency and real fuel savings of 25% or more in a typical installation (either new construction or retrofit). You can’t burn any fossil fuel any cleaner than with condensing technology.
The vast majority of Mod-Con boilers are sealed combustion and direct (PVC plastic) vented, eliminating old energy-wasting chimneys all together and saving conditioned air, while eliminating the risk of spillage (the byproducts of combustion entering the living space). They also incorporate outdoor reset which reads outdoor temperature and “resets” the water temperature delivered to the radiation; the colder it gets outdoors the hotter the boiler water and vise versa. This coupled with the modulating burner creates system efficiencies and comfort unattainable with conventional “dumb boilers” or scorched air heating systems.
System design is critical when specifying a Mod-Con boiler, as each is custom to the house its serves. The Mod-Con high efficiency boiler is the perfect match for radiant floor heating and old cast iron radiator systems also. However, I have installed many on fin-tube baseboard systems with 15% fuel savings.
Don’t forget the second biggest energy load on any home; the domestic hot water heater. A Mod-Con boiler coupled with an indirect-fired water heater, will give you unmatched efficiency and performance, making more hot water, while using less fuel, than any other water heater - tankless water heaters notwithstanding.
AFUE can be a misleading number as over-sizing and misapplication can drive the numbers down dramatically. Most boilers in service today will be 65-80 percent AFUE and 3/4 of the boilers sold today just barely meet the federal minimum of 82% AFUE. The reason for this terrible fact is based in tradition, as most boiler contractors still shun technology.
http://www.pmmag.com/CDA/Articles/Latest_News/BNP_GUID_9-5-2006_A_10000000000000221146
As with any boiler installation-new or retrofit; a heat load analysis is a must. Asking a qualified heating contractor if he will produce a heat load analysis (delivered after you sign a contract mentioning this requirement) is a good way to distinguish the handyman from the true professional.
