How a Non-Condensing Boiler Works

A non-condensing boiler burns gas and exhausts the combustion gases (including water vapor) out through a metal flue. The exhaust temperature is kept high enough — typically 300–400°F — to prevent the water vapor from condensing inside the flue, which would cause corrosion. This means a significant amount of heat energy goes up the flue unused. AFUE: typically 80–87%.

How a Condensing Boiler Works

A condensing boiler extracts additional heat from the exhaust gases by cooling them enough that the water vapor condenses into liquid. This condensate is drained away, and the extra heat that would have been lost goes into the heating system instead. The exhaust temperature drops to 100–130°F, which is why PVC pipe can be used for venting instead of metal. AFUE: typically 90–98%.

Installation Differences

Condensing boilers require a condensate drain — a small pipe that carries the acidic condensate to a floor drain or utility sink. They also use PVC or CPVC venting, which is less expensive than metal flue but requires a different installation approach. If you're replacing a non-condensing boiler with a condensing model, the venting will need to be changed.

Which Is Right for Chicago?

For hot water (hydronic) systems, a condensing boiler is almost always the right choice for a new installation or replacement. The efficiency advantage is real, the payback period is reasonable, and the installation requirements are manageable.

For steam systems, condensing boilers are generally not compatible — steam systems require non-condensing boilers.