Condensing Technology Reduces the Cost of Heating

Technology reduces home heating costs.

furnace efficiency

Most everyone who pays to heat a home or business shares a similar goal: to reduce the cost of heating. There are several ways to bring down the cost, such as buying less expensive fuel and improving the building’s outer shell to prevent heat loss. The single most cost-effective strategy, however, is to improve the efficiency of the boiler or furnace that is heating the building.

Manufacturers are constantly developing new heating appliances that convert energy to heat more efficiently. One technology that is finding its way into many high-efficiency boilers and furnaces is fuel condensing. Condensing is a complex technology that delivers a simple improvement: More of the heat that is generated by burning the fuel goes into the building, with less heat going up the chimney.

A non-condensing boiler or furnace might send 80 percent of the heat from the burner into the home or building, with 20 percent escaping through the chimney as waste. With a condensing appliance, only 5 to 10 percent of the heat is lost up the chimney, with 90 to 95 percent of the flame’s heat being transferred to the living space.

The effect on heating costs can be profound. If a home needs 1,000 gallons of heating oil a year with an 80 percent efficient traditional boiler or furnace, it would require only 889 gallons with a 90 percent efficient condensing appliance and only 842 gallons at 95 percent efficiency.

A condensing appliance increases efficiency by extracting heat two times from the “flue gas,” which is the air heated by the burner flame. A non-condensing appliance, by contrast, extracts heat only once, when the flue gas travels through the heat exchanger. That heat exchanger absorbs the heat and transfers it to the water (boiler) or air (furnace) that heats the building’s indoor space.

In a condensing appliance, the flue gas travels first through a primary heat exchanger, as described above, and then into a secondary heat exchanger, where more heat is absorbed. The term “condensing” refers to the fact that some of the flue gas “condenses” into liquid form during the secondary heat exchange process. This liquid, known as “condensate,” collects in the secondary heat exchanger, and condensing appliances are built with drainage systems to remove it safely so it neither leaks nor stays put to corrode the heat exchanger.

We mentioned above that a condensing appliance puts more heat into the building and less heat into the chimney. This means that the flue gas that exits a condensing appliance is dramatically cooler than with a non-condensing appliance. The gas leaving a condensing appliance might be as cool as 100 degrees, versus 400 degrees in a non-condensing appliance. Where did the missing heat go? Into the building as heat for the indoor space, instead of going up the chimney as wasted heat. That is the difference that the secondary heat exchange creates: it extracts more usable heat from the flue gas.

One interesting side effect relates to the use of chimneys. It is a simple fact of physics that “heat rises.” When 400-degree exhaust exits a traditional boiler or furnace, it easily rises through a brick chimney, even if it is tall and wide. The 100-degree exhaust from a condensing appliance, on the other hand, cannot make that climb because it lacks sufficient heat to rise vigorously. An installer will often vent a condensing appliance into a narrower pipe made of polypropylene plastic.