How does a Gas Furnace Work? | What are the Types of Gas Furnaces?

The furnaces are most commonly used in different applications. The gas furnace is one of the most commonly used furnaces. This article explains the gas furnace working, types, applications, and replacement cost.

What is a Gas Furnace?

A furnace that uses natural gas to produce heat is known as a gas furnace.

In the gas furnace, the gas ignition process occurs inside the furnace. This gas is provided by the municipal pipelines. The ignition of the gas generates heat which transfers to the desired room or area. A thermostat is installed to control the heat produced by the combusted gas.

Gas Furnace

Gas furnaces are the most commonly used in HVAC systems. In an HVAC system, the furnace is connected to a central AC unit.

This furnace provides heat faster than the electric furnace. One of the main advantages of the gas furnace is that it consumes less energy and is more efficient than the electric furnace.

The latest gas furnaces have become one of the most efficient methods to heat up your desired area at a low cost. A well-maintained gas furnace has a service life of 15 to 20 years.

Working of Gas Furnace

The gas is delivered from the gas source to the furnace through a pipeline. The LPG burner requires an external storage cylinder, while the natural gas model has an underground connection to the natural gas grid.

Working of Gas Furnace

  • First of all, the furnace takes the desired amount of gas from the gas storage tank through a gas valve.
  • The indoor cold air is introduced into the burner, which mixes with gas.
  • As the indoor cold air enters the furnace, the heat exchanger’s hot gases heat it.
  • The exhaust gases leave the furnace through a vent and discharge into the environment through the flue pipe.
  • Depending on where the thermostat detects heat demand, the blower fan will send hot air to different areas of the building.
  • When the hot air exits the duct and enters the room, the duct pushes cold air back into the furnace, leaving only hot air in the room.
  • As the hot air is exhausted, the room’s internal temperature gradually rises. This process keeps you and your family warm and comfortable.
  • As the old hot air cools, the new hot air replaces it, and the process continues until the heater is turned off.
  • When the thermostat detects that the set temperature has been reached, it closes the gas valve to stop hot airflow.

Components of Gas Furnace

The gas furnace has the following major parts:

  • Thermostat
  • Transformer
  • Draft Fan/Hood
  • Air Filter
  • Combustion Chamber
  • Heat Exchanger
  • Plenum
  • Blower
  • Gas Valve
  • Chimney

1) Thermostat

A thermostat is a temperature-controlled switch. It controls the ignition switch. It turns on the ignition switch when heat is needed (room temperature is lower than the set temperature) and sends a signal to the control panel in the furnace to start the ignition process.

2) Transformer

The transformer of the gas furnace ensures proper delivery of power to the furnace and sets it to the correct voltage.

3) Draft Fan/Hood

It is one of the most important components of the gas furnace. It introduces air into the combustion chamber and mixes it with gas fuel to start the ignition process.

In highly efficient systems, the induced draft fan ensures more economical and thinner combustion. The draft fan is also utilized to direct the generated heat from the combustion chamber to the heat exchanger, where the useless gases are discharged into the environment.

4) Air Filter

An air filter is the most important part of the furnace. It separates the solid contamination from the cold air and stops them from going into the room with cold air.  The proper cleaning, maintenance, and replacement of the air filters are very important if you want to prevent your furnace from any damage.

5) Combustion Chamber

The combustion chamber is the place where gas is ignited.  It contains a series of tubes through which gas is combusted and directed to the desired area.

During the combustion process, a flame sensor, igniter, and gas valve all work together to handle the flame.

When the thermostat sends the signal to the furnace control panel, the control panel opens the gas valve and ignites the fuel-air mixture. The flame sensor works as a safety measure that turns off the arriving gas flow if no flame is detected.

6) Heat Exchanger

The heat exchanger contains multiple tubes. The fresh air flows inside the tubes. The combustion process of the gas heats the inner air.

As the gas burns in the combustion chamber, the heat exchanger metal tubes heat up and transmit their radiant to the air flowing by them.

In case of a damaged heat exchanger, exhaust gas can enter your room and mix with the room air. In high-efficiency furnaces, curved surface (slow airflow) heat exchangers can be added to increase the heat transfer coefficient.

7) Plenum

The supply-air plenum delivers hot air to the ducts. The return-air plenum directs air from the room back to the furnace. 

8) Blower

The blower transfers air from the return duct to the heat exchanger. As the heat exchanger heats up the air, the heated air is transferred through the ducts of the house. Some types of furnaces contain a “multi-speed” blower.

A more efficient furnace always has a “multi-speed” blower that automatically adjusts the fan speed according to the room temperature requirements.

9) Gas Valve

It controls the pressure of the gas that enters the furnace from an external source (natural gas supply line or LP gas tank).

10) Chimney

The chimney collects the flue gas used to produce heat and discharges it into the atmosphere. Galvanized steel is most commonly employed for standard efficiency units with polypropylene vent units that are easy to install.

Types of Gas Furnaces

The gas furnace has the following major types:

  1. Single-stage furnace
  2. Double-stage furnace
  3. Modulation furnace

1) Single-stage Furnace

This furnace contains a gas valve that closes and opens, allowing gas to flow into the unit at a (high) rate. The annual fuel utilization efficiency of many single-stage gas furnaces is around 80%.

2) Two-stage Furnace

The two-stage furnaces have the capability to set the gas flow into the valve at a high or low flow speed. It takes the home’s load condition by the thermostat and adjusts the gas flow rate according to that.

3) Modulating Furnace

Modulation furnaces regulate the heat output more efficiently. The most tuned power is increased by 1%. This permits the system to regulate the amount of gas that the house needs every minute. It is ideal for cool climates and consistently regulates the home temperature.

Read More: Induced Draft Cooling Tower Working and Parts

FAQ Section

What is a furnace used for?

A furnace delivers heat to a building by heating air and then distributing it through a network of ducts.

How much does it cost to replace a Gas Furnace?

The replacement or installation cost of a gas furnace depends on many factors, such as your living area, type of brand, and service cost of the local labor.

The average installation or replacement cost of the gas furnace is from $2,790 to $6,750. The average cost of $4,647 includes labor, equipment, and materials costs.

What are the types of the gas furnaces?

  • Double-stage furnace
  • Single-stage furnace
  • Modulation furnace

Is furnace important?

Furnaces are employed to maintain indoor warmth during the winter by heating the air inside. In the summer, furnaces also aid in circulating conditioned air from the cooling system. They are an excellent choice when consistent air circulation is required to ensure comfort and warmth within the home.

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