Spencer Heating and Air Conditioning Heating 101

Gas Furnace | Electric Heat Pump | Electric Gas Dual Fuel

Heating 101

Gas Furnace

In a gas furnace heat is supplied by burning gas.  Burner flames heat up the heat exchanger.  The heat exchanger has alternating chambers which separate the flame exhaust from the air you breathe. The blower moves air across the heat exchanger, through the duct system and into your living space.  The by-products of combustion pass harmlessly through the venting system to the outdoors.

Heat Exchanger Cracks  

The heat exchanger can develop cracks or holes and the potential exists for dangerous fumes to mix with indoor air.  According to:

  • The American Gas Association (AGA) publication 'Fact Sheet On Heat Exchangers' "Any visible crack or hole discovered is reason for requiring replacement of the heat exchanger or furnace."
  • The Society of Professional Real Estate Inspectors manual 'Heating Systems Inspection' states: "...the committee consulted with all of the major furnace manufacturers, as well as the American Gas Association (AGA), and The Gas Research Institute (GRI).  The consensus of opinion is that THERE IS NO ACCEPTABLE CRACK, OPENING OR DAMAGE IN A FURNACE HEAT EXCHANGER" (capitalization theirs).  When you attempt to sell your home a home inspector will likely see the crack and require a furnace replacement.
  • The Atlanta Gas Light Company's policy is to "Red Tag" and refuse to light the pilot on any furnace with any visible crack or hole.


Gas furnaces are rated by AFUE (Annual Fuel Utilization Efficiency).  The AFUE indicates how much of the furnace's heat goes into the house vs. how much heat goes up the exhaust flue.  Prior to 1992 most furnaces were rated at about 60% AFUE.  Today the minimum efficiency rating is 80%.  A high efficiency furnace is 90% and above.  The added cost of a 90% furnace is about $1,300 - $1,500.

Single-stage vs. two-stage 

A single-stage furnace works on "high stage" all the time.  You get the full blast of heat when less might be better.  The furnace might short cycle because it satisfies the thermostat faster.  A two-stage furnace operates on low fire unless high fire is called for by the thermostat.  This will give you longer heating cycles and more even heating temperatures.


The variable speed blower motor adjusts airflow for greater comfort.  This motor uses about half the electricity of a traditional motor.  It also increases the efficiency of the air conditioner by about 7% and offers better humidity control.  Where blower noise is a factor this blower offers the quietest operation.

Electric Heat-Pump

A heat-pump is basically an air conditioner that cools and heats.  By manipulating refrigerant pressures it transfers heat from indoors to outdoors in the summer, then reverses refrigerant flow and transfers heat from outdoors to indoors in the winter.  Heat-pumps will often have electric heat strips that come on when the thermostat is set for emergency heat.  The heat strips will also come on to aid the heat-pump if the weather is so cold it falls behind.  The heat-pump efficiency is rated according to its air conditioning mode.

Electric/Gas Dual-Fuel

A dual-fuel system utilizes both a heat-pump and a gas furnace.  The heat-pump works at its greatest efficiency when the temperature outside is mild.  When the outside temperature gets cold enough the thermostat turns off the heat-pump and turns on the gas furnace. The dual-fuel system also turns the heat-pump off and the gas furnace on when the "emergency heat" function at the thermostat is utilized.

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