Angie's LIST Guide to
Home heating systems

Although natural gas is the most common energy source for home furnaces, there are several other options, including the relatively new trend of geothermal heat.


Regular maintenance keeps a furnace working efficiently, according to HVAC experts like Dave Mejean of B&W Plumbing & Heating. (Angie's List photo by Brandon Smith)
Regular maintenance keeps a furnace working efficiently, according to HVAC experts like Dave Mejean of B&W Plumbing & Heating. (Angie's List photo by Brandon Smith)

Measuring heat efficiency

When comparing energy sources for home heating, the scientific unit of measurement is known as the "AFUE rating." This refers to a measurement of annual fuel use efficiency -- the ratio of how much heat output a furnace system will produce compared to the amount of energy it consumes.

For example, a system that carries an 80% AFUE rating means that 80% of the energy it uses translates directly to heat output, while the remaining 20% of energy is lost to inefficiencies like heat loss or escape. Federal regulations now require that this efficiency indicator be included with every new furnace sold so consumers can compare energy efficiency.

Although they're more expensive than their non-label counterparts, Energy Star heating products have achieved high energy efficiency standards, saving you money in utility bills in the long run.

Natural gas furnaces

The most common type of home heating system in the United States burns natural gas or propane to produce heat. According to the DOE, older natural gas systems typically achieve a 68% to 72% AFUE, while newer, highly efficient natural gas furnaces can achieve AFUE ratings as high as 90% to 97%.

How a gas furnace works

1. Fuel supply
Upon construction or installation of a new gas line, permanently installed gas plumbing supplies natural gas to the furnace. Since natural gas is combustible and leaks in gas pipe may result in a fire or explosion, this piping should only be repaired or replaced by a licensed professional HVAC company or, in some areas, a licensed pipefitter or plumber.

2. Furnace startup and fuel ignition
When your home's thermostat falls below a certain preprogrammed temperature, it sends a signal to the furnace to switch on and begin supplying heat to the home. A permanently lit pilot light or an electronic ignition will now ignite natural gas that's been supplied to a burner. Lighting and combusting the natural gas at the burner is how heat is supplied to the rest of the home.

3. Heating the home
Once the furnace senses that the burner has reached a high enough temperature, a blower fan will begin to introduce air to a heat exchanger. The heat exchanger allows air to be heated without coming in contact with the actual flame or burner area. Once it reaches a sufficient temperature, the air warmed by the heat exchanger is then forced through the home's duct work, supplying warm air to individual rooms like bedrooms, living rooms and kitchens through the vents or registers.

4. Shutdown
When a thermostat senses that the home's temperature has reached the desired level, it sends a signal to the furnace to end the heating cycle. The gas valve that supplies heat to the burner shuts off, extinguishing the flames. The blower will likely continue to operate for a short time to ensure that all the available heat has been retrieved from the system.

Electric heat

In areas of the country where electricity rates are cheap, electrical resistance furnaces are common. Rather than moving air over a flame as with combustion-based forced-air heating, air is moved over a hot electrical coil.

Electric-based heating can achieve 100% AFUE, since all the electrical power is converted directly to heat. However, because the electricity supplied is produced primarily by burning coal, natural gas or fossil fuels, electric furnaces aren't necessarily an environmentally friendly heating choice.

Alternative heat systems

Boilers - More common in relatively older homes, a boiler-based heating system uses gas, oil or electricity to heat water and produce steam that moves through pipes to heat radiators in individual rooms. All-electric boilers can achieve up to 95% to 100% AFUE, according to the DOE. Newer gas boilers must achieve 80% AFUE and newer fossil-fueled boilers must achieve a 75% AFUE.

Air-source heat pumps - If your home relies on cheap electricity rates, a high-efficiency heat pump may be a better choice. Used for both heating and cooling, an air-source heat pump condenses warm air and moves it either indoors or outdoors. In warm weather, it condenses and extracts heat from inside, cooling a home, and in cool weather, it condenses and extracts heat from outdoor air, heating a home.

According to the DOE, the heating efficiency of an air-source heat pump is determined by its heating season performance factor, or HSPF. It's a measure of the total space heating required during the heating season, expressed in BTUs, divided by the total electrical energy consumed during the same season, expressed in watt-hours.

Geothermal heat pumps - A geothermal heat pump is one of the most energy efficient methods of heating or cooling a home. Relying on a series of liquid-filled pipes, a home can be heated in cooler months by transferring and condensing energy from the 50 to 60 temperatures just a few feet below ground. In the summer months, the process is reversed, thereby cooling a home by removing heat.

Geothermal offers maximum efficiency while costing less to operate and lasting longer. Upfront installation costs are high, ranging from $10,000 to $30,000 depending on the site conditions, but some of these costs may be recouped through local, state or federal tax incentives.


My furnace will not come on.
the thermostate was replaced at the begining of the cooling season.

need air conditioner install or repaired as soon as possible.

thank you

my blows hot air than cool when it blows cool it keep blowing until I reset it lately when I turn it of and on it stay cool

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