Atmospheric Boilers are vented into a chimney. If your boiler is connected directly into your chimney, this is what you have. Atmospheric boilers produce waste gases which are hot enough to be less dense than air. The products of combustion move up through the chimney because of their own buoyancy. Air for combustion is usually drawn from inside the house. This is the least efficient type of boiler, but also the least expensive to buy and install. Atmospheric boilers approach, at best, 85 percent efficiency.
Sealed Combustion Boilers are designed to draw a bit more combustion heat from the flue gas, leaving the gas too cold to move up a chimney on its own, requiring a fan which pushes the products of combustion out of the house through special stainless steel or plastic vent pipe. Air is drawn from the outside to provide for combustion. They are slightly more efficient, and therefore less expensive to operate but more expensive to buy and install than the atmospheric type boiler.
Because the boiler does not use the existing chimney, and because the type of venting it uses has severe length limitations, it can and should be installed near an outside wall in the area of its vent penetration. This type of boiler requires a carbon monoxide detector to be installed in the basement and plug-in detectors to be installed on the floors that have bedrooms. Sealed combustion boilers have 85 percent plus efficiencies.
Modulating-Condensing Boilers capture more heat from their combustion gases than conventional boilers, and they are also designed to condense the water vapor in the combustion gas, which captures the latent heat in the vapor. They have modulating burners which can ramp up and down in response to the needs of the space heating or domestic water heating system. These boilers have efficiencies in the 90 percents. Because they purposefully condense water vapor, they require a way to dispose of the condensate which includes neutralization (the condensate is acidic and dangerous to the drainage system). Condensing boilers can be floor mounted or hung on a wall. They are mechanically vented, usually through a side wall of the house. Condensing boilers have the same carbon monoxide detection requirements as sealed combustion boilers. They are the least expensive to operate but the most expensive to buy and install.