Gas Heat Efficiency Comparison

Conventional Efficiency Furnaces

• Conventional efficiency furnaces were designed and built at a time when energy was relatively inexpensive. Although when in operation, they convert 80 percent of the natural gas burned to heat, they also lose 20 percent more energy when not in use. This energy is consumed by constantly burning a pilot light and leaving the exhaust flue open, letting warm house air escape the building. The season efficiency of a conventional furnace is at the highest 60 percent. Season efficiency is a percentage of the total fuel consumed over an entire heating season in comparison to the heat delivered to the home.

Mid-Efficiency Furnaces

• The oil crisis of the 1970s led engineers to look more closely at how furnaces burned natural gas. This lead to several changes in the mechanics of the furnace. Some of the biggest gains in the mid-efficiency furnace were made by controlling its energy use when not heating the home. An automatic vent dampener was installed in the furnace's flue to close when the unit was not running. This increased efficiency by around 15 percent, because it stopped warm house air from escaping the building envelop. The pilot light was replaced with an automatic ignition source that operated only when the furnace was lit. This gave the furnace an additional 5 percent efficiency. The season efficiency of a mid-efficiency furnace is around 75 to 85 percent.

High-Efficiency Furnace

• The high-efficiency or condensing furnace increases the efficiency of the gas furnace even more. Furnace engineers wanted to extract more heat from the exhaust gases as they left the furnace. This was accomplished by adding a second heat exchanger to slow down the movement of the gases and allow the house air to extract the extra energy. The exhaust gases became so cool that it would convert from a gas to a liquid. The energy released during this change of state increased furnace efficiency into the low to mid 90 percent range. While this increase was significant, engineers had to invent a system to remove the acidic moisture generated by the process, making the system even more complex.

Gas Furnace Maintenace

• A properly maintained gas furnace can operate from 15 to 25 years depending on the local climate. This maintenance includes seasonal cleaning and inspection by a certified technician and changing the furnace's air filter on regular basis. Never store flammable materials near the furnace cabinet and repair problems quickly when they arise.