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.
Efficiency
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.
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.
Furnaces are classified into three efficiency categories, each correlating to a specific design: conventional, mid, and high efficiency designs.
Conventional
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.
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.
Mid
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 into the flue. No longer reliant on natural draft to run, the heat exchanger design was optimized in order to extract more heat before the combustion gasses went up the chimney.
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 into the flue. No longer reliant on natural draft to run, the heat exchanger design was optimized in order to extract more heat before the combustion gasses went up the chimney.
High
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.
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 gases are condensed and trickle down the drain, the remaining (fairly cool) gases 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.
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