Your General Gas Furnace Parts

Generally speaking gas furnaces are made out of metal, a casing that will enclose and protect all of the internal parts of the machine. The system contains a main gas line. This gas line is connected to the ignition system.

The ignition point is the point at which the gas is lit and the furnace begins to heat your home. When the air warms up it travels through a plenum or air duct exhaust and travels through the damper to the rooms of your home. The main gas furnace parts that we will spend some time looking at today are:

  • Thermostats
  • Thermocouples
  • Gas Furnace Igniters
  • Gas Valves
  • Blower Motors
  • Freeze Alarms

The operation of your gas furnace depends on air flow in an out of it. To achieve this you will have air ducts installed in the rooms of your home. Cooler air will be sucked in and transported along sealed air system of duct work to the furnace to begin the warming process. The warming process is repeated over and over again until the air is warm enough.

It is important that the size of your gas furnace matches the size of the space that you want to heat. If it is too big it will turn on and off all the time and create spikes in electricity, while if it is too small it will run constantly when it is cold and cost you a lot of money.


The first gas furnace part we will consider is the thermostat. It should be considered first because it is basically the main centre of your entire system. This is because it controls the amount of heat that is conducted through your home. However, despite its importance it is a very simple part. In effect it is merely a switch that is sensitive to temperature and that reacts to different temperatures in different ways. There are two main types of thermostats:
Gas Furnace Thermostat

Low Voltage Thermostats

Low voltage thermostats are produced mainly by the following manufacturers (as well as a number of others that are not as well known):

  • Honeywell
  • White-Rodgers
  • Lux
  • Robertshaw

A low voltage thermostat runs of a transformer which reduces the amount of incoming line voltage from 120 volts down to between 12 and 24 volts. The exact reduction amount depends largely on the type of furnace that you have. There are four main different types of low voltage thermostats that are currently in existence:

  • Mercury Contact
  • Mechanical Contact
  • Digital
  • Electronic Programmable

Line Voltage Thermostats

Line voltage thermostats are hooked up to 120 volt or 240 volt electric resistance heating systems and, to put it simply, functions in a very similar way to a light switch dimmer in that you can control the amount of heat that comes through.

Low voltage thermostats are more common than line voltage thermostats and this is more likely to be what you have in your gas furnace.


The thermocouple could be described as the pilot light’s co-pilot in that it assists the pilot light by sensing whether or not it is hot enough to ignite natural gas or propane fuel to the burner. In some ways this I a safety feature. The thermocouple will determine whether or not it is safe to keep the gas valve open. If it is safe, it will do so, thereby ensuring the right amount of heat comes through all the time. If the thermocouple does not sense enough heat from the pilot, like what will happen at times when the pilot light is switched off, it will turn off the gas valve.

A thermocouple consists of:

  • Two metal wires welded at the ends
  • A protective metal case
  • A thermocouple sensor (which is specifically designed to withstand the hottest part of the flame as this is where it will be placed within your gas furnace)

The sensor is placed in the hottest part of the furnace while the other end is connected to the pilot valve. When the pilot is switched on the thermocouple will slowly start to heat up, sending a very small amount of electricity to the gas valve, signaling it to open. It essentially controls whether the gas valve will be open or closed. To do this is converts heat into electrical energy which is used to determine and to power the opening and closing of the gas valve. It is therefore an essential one of the gas furnace parts.

Gas Furnace Igniters

There are two main types of gas furnace igniters:

  • Electronic ignition system (more modern)
  • Standing pilot furnaces (older, low AFUE, unreliable, wastes energy)

The first option is generally considered toGas Furnace Ignitor be the better options for a number of reasons. For one thing it allows for more reliable performance. In addition it is also far more cost effective than the standing pilot option. It also tends to result in a furnace that is more energy efficient and that has a higher AFUE. Electric ignition switches are ideal in that they easily exceed the governmental AFUE standard of 78% and result in higher savings all round.

There are two primary ways in which electronic ignition of a gas furnace can occur. This is through:

  • Intermittent Pilot, which uses an “electronically controlled high voltage electrical spark to ignite the gas pilot and then subsequently the main burners, when the thermostat calls for heat” – , or
  • Hot Surface Ignition, which uses an “electronically controlled resistance heating element not unlike a light bulb filament, to ignite the gas burner” –

If you have a furnace that uses a standing pilot you should probably consider replacing it with one that uses an electronic ignition system as this will be more cost effective for you all round. It is also easier to maintain an electronic ignition system, which is important as this is clearly an essential part of the gas furnace system.

Gas Valves

The gas valve of your furnace will be referred to as a furnace gas valve or a pilot gas valve. The gas valve:

  • Supplies the furnace’s burners with flammable gas once they’ve heated up enough to allow for combustion.
  • Shuts off the supply of gas when the burners go out to avoid a fire hazard.

The gas valve works together with the thermostat and the thermocouples that have already been discussed.

How it Functions:

  1. The pilot light is engaged
  2. The pilot light produces a flame
  3. This flame warms the furnace’s burners
  4. This happens when the thermostat has sent an electrical signal to the standard 24-volt solenoid in the gas valve, or
  5. The thermocouple that sits in the flame of the pilot has heated up enough for the metals within to react
  6. This weak electrical signal powers the small magnetized spring-driven piston in the valve’s solenoid
  7. This prevents it from closing off the pilot’s gas output line
  8. The thermocouple becomes hot enough, or the thermostat is activated
  9. This causes an even stronger electrical current to pass to the valve’s solenoid
  10. This in turn pulls the magnetized piston out of the way of both the pilot output line and the burner output line
  11. Gas is passed through the burners and is ignited
  12. The opposite occurs in cases where the valve needs to be shut off

As an essential part of the system your gas valve must be very carefully maintained.

Blower MotorsGas Furnace Blower Motor

Your gas furnace will be equipped with blower motor. The function of the blower motor is to move air through the furnace’s duct system. There are two types of blowers that you get at the moment, and the difference between them lies in the motors that they use. The two different motors are:

  • Permanent Split Capacitor (PSC)
  • Brushless Permanent Magnet (BPM)

It is important that you know about your blower motor because this is the gas furnace part that accounts for the electricity consumption that your gas furnace will be responsible for. The most popular blower motor is the PSC option and the vast majority of gas furnaces in existence today contain this type of motor. You need to be careful with a motor like this, however, because when these motors operate at a low speed their efficiency is extremely low as well, dropping to as low as 20%. Because of how a gas furnace is set up, the blower motor will work at its slowest, and therefore at its least efficient, rate when the furnace is actually in operation, so when your furnace is on you will be using a fair amount of power. It is therefore a better idea to get a blower motor that operates with power electronics as these have been shown to provide dramatic improvements in efficiency, thereby saving you time and money when it comes to your gas furnace bills. The type of blower motor used is an important bit of information to know when choosing a gas furnace.

Freeze Alarms

In winter it can get cold enough for water to freeze in your pipes thereby damaging them. freeze alarms are gas furnace parts that are aimed at giving you fair warning that this is likely to occur, thereby saving you time and trouble by preventing you form experiencing burst pipes. Burst pipes due to freezing tend t occur because of:

  • Furnace failures
  • Prolonged power failures

The first can be prevented by carefully maintaining your furnace. However there are still things that may go wrong in winter with your furnace, so in those cases a freeze alarm is a good thing to have to use in conjunction with your gas furnace system to make sure that everything runs smoothly. In addition to protecting you against burst pipes, freeze alarms are also designed to protect the following against freezing winter temperatures:

  • Homes
  • Cabins
  • Businesses

There are some models of freeze alarms that will heat up your home or cabin for you before you get there. This function can be activated quite simply through a phone call. Other models will call up to three separate numbers if the temperature in the house drops below a certain level or f the power cuts out. This means that three separate people will be informed that there is a high chance that freezing and damage will occur in your home. These alarms are often part of your gas furnace and should therefore be carefully maintained along with the rest of it.

All of the gas furnace parts mentioned above are essential parts and need to be well maintained when and if it is possible for you to do so yourself. Regular checkups from experts are also recommended and you need to keep a close eye on the system as a whole in order to ensure that it runs smoothly are all times.

It is important to remember that you need to match your gas furnace is terms of size to the space that you would like to heat, or else the following problems may occur:

  • The gas furnace is too big for the space: it will cycle on and off over and over again causing spikes in electricity and damage to the system.
  • The gas furnace is too small for the space: it will be required to run all the time in cold weather and this will have a major impact on the amount of money that you will be required to pay in energy bills each month.

The insulation of your home is also important to take into account. If your home is well insulated you will require a smaller unit as the chances that warm air will escape from the home are smaller. Likewise if your home is not well insulated you will need a larger gas furnace to account for the heat that is lost.