The forced-air gas furnace is the most common heating system in North America. It has undergone many improvements over the past few years making them efficient, quiet and reliable.


Most of the heat generated when a gas furnace burns goes into the house, but some of the heat goes up the chimney. Furnace efficiency refers to the amount of heat delivered into the house relative to the total amount of fuel energy used. Another way to look at it: if you burn $1 worth of gas and you get 80 cents worth of heat into the house, your furnace is operating at 80% efficiency. This quotient is often called AFUE, or annual fuel utilization efficiency.


A conventional furnace is the oldest type and is generally 55% to 65% efficient. In other words, a great deal of heat is lost up the chimney during the operation of the furnace. Conventional furnaces are no longer made but many still exist in homes.


Improvements in design led to the mid-efficiency furnace, operating at around 80% AFUE. The big development, the induced draft fan, sucks the combustion products through the furnace and discharges them in to the flue. No longer reliant on natural draft to run, the heat exchanger design was optimized in order to extract more heat before combustion gasses went up the chimney.


Further developments in furnace design led to the modern high-efficiency furnace, operating at an AFUE of 90% to 97%. A high-efficiency furnace has two heat exchangers, the second's job being to condense the gases, thus extracting most of the heat that would otherwise have been lost up the chimney.

Other benefits of a high-efficiency furnace:

  • Does not require a chimney: since most of the combustion gasses are condensed and trickle down the drain, the remaining (fairly cool) gasses can be vented through a plastic pipe directly through the wall of the house.
  • Doesn't burn house-hold air: combustion air is drawn directly from the outside through one plastic pipe and a second plastic pipe discharges the remaining combustion gas to the exterior.


Clean or change the filter: The filter protects the furnace as well as cleaning the air. This is especially important for a high efficiency furnace. The secondary heat exchanger can clog with dirt, preventing air from properly flowing through the furnace. Without good air flow, a high-efficiency furnace cannot operate within it's normal operating temperature, becoming as a result, less efficient, more expensive to run, and a risk for potential premature failure to the heat exchanger.

Do not block air vents: It is uncommon to see mats, shoes and piece of furniture blocking return air vents. Proper air flow though the furnace is very important.

Look for leaks: Water can leak down into your furnace several ways, ultimately causing failure. If you discover these leaks early it is an easy fix for your furnace technician.

  • Check for water leaks from the air-conditioner evaporator, located in the supply duct on top of the furnace.
  • Humidifiers can also leak and damage the furnace.
  • High-efficiency furnaces create a steady stream of condensation (water). If the drain tubes get disconnected, water leaks in around the furnace. Since this condensate is also slightly acidic, it will corrode steel very fast.

Have your furnace serviced regularly: all furnaces require regular check ups for optimal operation.

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