Furnace, understand the basics

Your furnace is designed to automatically heat your home. By understanding its basic functions you can help increase efficiency. Efficient operation means your money stays in your wallet.

A furnace is described by the type of fuel it uses. The most common fuel types are gas, oil, and electric.

A special type of furnace is used in mobile homes. They use special venting and duct systems and are certified for use in manufactured housing.


Size & Efficiency; Why Does It Matter?

Size: A gas furnace has an input rating that is expressed in British Thermal Units (BTU) per hour. This is how the size of the unit is expressed. Common sizes are 40000, 60000, 80000, 100000, and 120000 btu/hr. It is absolutely critical that your unit is the proper size for your home. A load calculation is performed to find the right unit for your specific home. (The size of an electric furnace is expressed in kilowatts instead of BTU/hr.)

Efficiency: Not all of the heat that is available from the gas is put into the home’s air. The ratio of the heat output of the unit to the heat content of the gas supplied is the unit’s efficiency or AFUE. AFUE is short for Annual Fuel Utilization Efficiency and is expressed as a percentage. Some common efficiency ratings are 80%, 90%, etc. A higher efficiency unit requires less gas to heat the home and therefore your utility bill is lower.


A special type is used in mobile homes. They use special venting and duct systems and are certified for use in manufactured homes.

Understanding a Gas Furnace

These are the typical residential gas-fired forced air heating systems. These units burn either natural gas or propane.

The heating cycle starts when the thermostat senses that the room temperature is below your selected temperature. On newer units, this causes an inducer fan to pull air through the unit, establishing adequate combustion airflow. After proper airflow is verified, the gas valve is opened. The gas flows through the burners where it is ignited by a spark or heat from the igniter. Older units have what is called a standing pilot which is a small flame that burns constantly to light the gas.

After the unit senses the flame is lit, the spark or igniter is turned off. Normally, the gas will burn for approximately two minutes before the blower is started. This prevents blowing cold air out the vents at the start.

After the preset time or at a set temperature, the blower motor is energized and air is blown over the heat exchanger. The air is heated and supplied to the duct system which delivers the air to the registers in your house.

When the thermostat senses that the room temperature is the same as your selected temperature it sends a signal that shuts the gas valve which then stops the furnace operation. The blower motor usually runs for another few minutes to cool off the heat exchanger. This can be controlled by either a timer or a temperature switch.


Understanding an Oil Furnace

The heating cycle starts when the thermostat senses that the room temperature is below your selected temperature. This activates a burner motor which pulls fuel oil from a tank and combines it with the proper amount of air. At the same time a spark is generated by an ignition transformer. The oil passes through a nozzle which has a very small hole and a tiny screen in it. This breaks up the oil and causes it to vaporize so it can be ignited by the spark.

After a delay to allow the heat exchanger to warm up, the blower motor starts up. This draws air into the unit and passes it over the heat exchanger where it is heated. The air is then sent into the duct system to be distributed throughout the home. The blower is normally turned on and off by a temperature switch. This switch also will shut the unit off if the unit overheats.

After the thermostat is satisfied, the burner motor turns off and oil flow is stopped. The blower motor continues to run until the heat exchanger is sufficiently cooled.


Understanding an Electric Furnace

These units produce heat from electricity flowing through special wires called elements. Unlike the other units, these are 100% efficient. All of the electricity is used to produce heat. The downside is that electricity is a significantly more expensive heat source than gas or oil. When the thermostat senses a need for heat, a relay energizes the element and the blower motor. Some units use more than one heating element. These units have other relays to turn on the other elements.

When the thermostat is satisfied, the relays are shut off. After a short time delay, the elements and fan are shut off as well.

These are the products commonly recommended by Lincoln Heating & Air Conditioning


These are the products commonly used by Lincoln Heating & Air Conditioning.


Rheem Gas Furnace

Gas Furnace Rheem

Intertherm Gas Furnace (Manufactured Housing)

Manufactured Housing Intertherm

Coleman Electric Furnace

Coleman Electric Furnace

Thermo Pride Oil Furnace

Thermo Pride Oil Furnace