How does a Furnace Works?

We will discuss in this article a complete cycle of heating from beginning to the end, just to give you a proper understanding of how does a furnace works? and what is the structure of furnace heating your houses.

How does a furnace works?

I am really sure that many people don’t know the full heating procedures of the furnaces. Only few house owners know the complete procedure of heating in furnace which placed many times in their houses. Some question arises that, is it necessary to have the knowledge of heating process. Basically by getting the information of heating procedure, you will be able to know how does a furnace works.

Also, you are able to get extra benefits in the many ways such as:

  • If anything is damaged. So, in this case you are having a proper and complete understanding why this problem will occur.
  • By knowing the proper and complete understanding of heating procedure, you will be able to choose that HVAC contractors who is committed for doing the proper maintenance and installation.
  • Now, you got a proper knowledge for variety of HVAC equipment’s which are already present in market. Also, you know the advantages and disadvantages of each furnace.

Well, information about anything is not a bad thing for you. In the best case scenario, the information about heating procedure help you in selecting the best furnace for your house.

Heating Procedure that how does a furnace works?:

There are many furnaces and every furnace have a various of operations. As time pass, the technology getting advanced just because of this some operation has been changed and some remains same.

 The heating procedure will start when your furnace thermostat

Heating procedure

the temperature differences between your house and its settings. . If there’s any differences, then thermostat initiate an order to the furnace unit for more heat.

 Here we describe two system older and modern systems for better understanding that how does a furnace works?.

Older Systems:

 In the old system, there is a thing call pilot which is constantly lighted. Also, this will provide the fuel to the furnace for burning. It’s also having a system for the direct ignition which is similar to a spark plug, which is present in the cars.

Modern systems:

In the modern HVAC systems, many furnaces have a hot surface of ignition which is some kind of similar to the lighter for cigarettes in the cars. If this is an electric furnace rather than propane or gas and oil furnace, then heating coils which is present in major cabinet starts heating.

Ignition should be occurring only on that stage, which will start produce more heat. At this stage, in the old system the heat will normally increases only inside heat exchanger. this method was not working efficiently. There is vendor motor in the modern systems which pushes the heat from burners to the heat exchanger.

Only that amount heat will be transferred to the heat exchanger which is according to the heat meter, not less nor more then that. So, this venter motor will run for a specific time, depending on how much amount of heat you need.

Also in gas furnace, the valve of gas remains opened which provide the fuel to burners. There is a flame sensor on the other side of burner which is used to monitor the functioning of burner accurately. A signal with low voltage passes towards the flame sensor which indicates the accurate operation.

Theres a little bit delay between the air flowing in your house and these stages. Meanwhile, heat exchanger start heating. Blower motor or you can say in simple words circulating fan will starts. Through this way, the air will move farward towards vents and then to ductwork then in your house.

Is heat transferred direct in the ductwork?

The heat was not transfered directly towards the ductwork, all this depends on your furnace efficiency. The excess amount of heat should be vented through the pvc pipe or from chimney flue that move towards the central cabinet from the outside of your house.

The heat was not transfered directly towards the ductwork, all this depends on your furnace efficiency. The excess amount of heat should be vented through the pvc pipe or from chimney flue that move towards the central cabinet from the outside of your house.

furnace ductwork

Well in the end of this procedure, thermostat again send the signals towards the circuit board of your furnace, which begins to shutdown. The furnace unit will start cooling, until the preparation of next operation for heating from your thermostat. Now, you understand that how does a furnace works.

Also Read: Furnace Installation Process Step by Step (Complete Guide) 

Safety Measures:

Old furnaces having very minimum devices which is used for safety puposes. But modern furnaces are design to correct all these errors.

here is an example like, rollout switches will become active and shutdown the furnace in case of heat exchanger breaches. If there’s any cracks on heat exchanger then it occurs just because of rare reasons like your furnace need to be

Safety Measure

repaired or it is the time to replace your furnace. The sensor on heat exchanger will become active and shutdown the system.

Similarly, we discussed about the heat sensor earlier in this article is the warning system which was not present in the old furnaces.

Just to make things more clear, after all this functions and operations many things remains go wrong. For that purpose, schedule maintenance is very important. These new features in the modern furnaces will give the more clarity and relaxation to house owners.

Parts of furnaces and system:

It is not crucial to get the knowledge of every part of furnace. But its very important, just for better understanding that how does a furnace works. Because if you know that how HVAC system components work together and gives you a proper understanding of your furnace. That is very beneficial for you.

Some major components used in the furnaces such as:

Blower Motor:

This Blower motor is used to distribute warm air all over your house by using the ductwork. This same blower motor is also used for cooling air as well. That is why, it is very important for having a compatible system of heating or cooling.

Heat Exchanger:

There are number of metal tubes that absorb heat which is coming from the furnace heating source. The air inside of your house is getting warm now, and it happens when the air moves from heat exchanger and then pass in your ductwork.

Heat Exchanger

Combustion Chamber:

This is the area where magic take place with the heat exchanger, and also it is the area which remains open where air move around it. It is necessary to keep this area clean and among other several areas this area needs to clean during regular maintenance.


How is this possible that we haven’t talk about the filters yet? Basically filter is important for both the air conditioning and for your furnace. It plays very important role because it filters the air that is returning towards the furnace after being circulated all around your house. Filter is necessary because for better efficiency and for safety purpose it keeps the dirt and the dust particles away from the parts of furnace. If you used inefficient dirty filter which cease your whole HVAC system.

We covered a lot of other parts before when we were talking about the heating procedure. But there are some parts which we have not discussed yet. I hope now, you understand that how every parts work with each other as a whole.

Also Read: How much does a new furnace replacement cost in 2023

Difference in the model of furnaces:

Many people focus on different types of brands in furnace. Yes, there are many different constructions or brands in furnaces. But the basic heating procedure is same for every brand of furnaces. Well, there is big differences in the furnace technology. Normally, this will be change when the furnace is of single stage and two stage, variable speed.

One Stage furnace:

Actually, in single stage furnace it has only the “on”, “off” option in settings. Its means, the furnace is at 100% or 0%.

Two Stage furnace:

It has one additional stage in the furnace of two stage, normally 60% to 70%. So, it is having few flexibilities in the heating option, through this it reduces the energy consumption and increase your furnace efficiency, gives you more comfort and also decreases your system damages.

 Variable speed furnace:

In the furnace of variable speed, there are various stages. It has 5 to 6 stages all are above 100s. So, the house owners having more controls on energy consumption and in this way you get more comfort.

In these types of furnaces, there is huge differences in the functions of blower motor and the thermostat. To control your system properly and accurately, your furnace thermostat must be higher sophisticated rather than one stage furnace and two stage furnace.

Likely, the blower motor usually operates at 60 to 70%. But not on 100% with the furnace of two stages. But with the variable speed of furnace, blower motor must be modulating on a tons of stages, just for matching the heating output.

Also Read: Popular furnace brands

Difference in the Energy Sources:

The other main difference related to the energy sources. All the thing which we talked about regarding the burner and the gas valve is not suitable for the electric furnace. Just because of this that safety checks in the gas furnace only related to the gas itself. These safety measure are totally different. Here is an example, in case of system overheats the electric furnace having fusible link which will start burning and this thing will trigger an emergency shutdown.

Other little expectation also exists such as, the delivery method of oil for oil furnaces. Its atomize and disperse before the consumption of fuel again. But the heating procedure remain same regardless of energy sources and distinct configurations.

 Furnace Power

How you measure the efficiency of furnace:

Efficiency of furnace is calculated by AFUE. AFUE stands for Annual fuel utilization efficiency. Basically it is calculated in percentages and it is measured in this way that what amount of heat is generated by your furnace which is directly goes for heating your house.

You have to think like this just for more and proper understanding. For heating purpose you spend one dollar and you are having 80% of efficiency. Out of that dollar, 20 cents are used to exhaust air from your house instead of heating it.

Normally a standard furnaces usually efficient about 80%. Something which considers as a higher efficiency means unit of furnace will be of 90% or more than this. The more and better furnaces will be of 98% efficient.

If you are having and electric furnace, then in house you have a heat pump rather than having an air conditioner which provide heat in winters season. This is calculated in HSPF, HSPF stands for heating Seasonal performance factors. Like AFUE, it also serves as a calculation just to measure the efficiency.

Modern system having HSPF rating more than 12 and old system having lower like 5 to 6.

Maintenance, Repair or lifespan of furnace:

When you have to change your furnace? Well, I have seen many furnaces which has been in working for more than 30 years. But you know, the average life of any furnace is 8 to 10 years.

For few special reasons we aren’t be able to give you the guarantee of 30 years. But it’s not right to accept 8 to 10 years of lifespan from the modern furnaces. Why this happens? Normally, if you place your furnace with proper maintenance, you can accept 15 years of lifespan from the furnace.

There are reasons through which you can easily understand how does a furnace works and these reasons you see are predictable, but also preventable:


If your furnace undergoes in maintenance once in a year, then you are adding more years of lifespan by normally doing this.

Changing your filter:

If you forget to change to filter according to scheduled and it becomes your habit, then by doing this you decrease your furnace average life. Because filter does not affect directly to the quality of air, but the clogged filter put the strains on your furnace which leads to sudden breakdown.

Best practices of furnace installation:

Basically, the department of energy tells that in accurate installation of furnace leads to decrease the efficiency more than 30%. Well, from our professional experience that number is too high.

We have seen such that bad installation which results in replacement of your furnace and also leads to large repairs after some years of furnace installation. Well, it is very rare that someone did such that bad installation for your furnace, cutting corners, which can cost you in the long run, is far more frequently done.

Also Read: Everything you know about wall furnace

Enjoy comfort from your new furnace:

Now, you know basics of the construction and the functions of furnace and you understand properly that how does a furnace works and what are the reasons which is required in system installation. So, through this way you become eligible to take further new steps.

Well, I have mentioned few more resources that will be very beneficial for you. It’s better to get information before you are ready to do next HVAC task.

Buy knowing all these reasons you are able to take better decision by just avoiding the common risks in the furnace installation, ac installation and on its maintenance. Also, it is very helpful for you in choosing the best HVAC partner for your new task.

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