Which way does air flow in furnace?

What’s the relation between the air flow in furnace and the position of furnace filters? Well, the relation of air flow in furnace depends on that how your furnace filters oriented and what its location. You are able to find your furnace air filters and when you know everything about the air flow in furnace than you are able to determine that how to set air filters position.

air flow in furnace

Basically, the air flow in a particular direction through your HVAC system and all the system components process the flow of air in the particular order. 

In every furnace, the direction of air flow is totally different. But when we discussed about the processing steps, it remains same in every furnace. Before your furnace heat up, all the house air flow through the filters.

In order to find and giving correct positions to the filters. Firstly, you have to check the air flow in furnace that whether the flow of air is up and down or horizontally.

How the air flow in furnace work?

At first, just to give you clear this thing that furnace is not able to generate air. Basically, it uses the house cold air which moves from furnace and the temperature in the furnace increases then the air send back towards the house.

This process starts when the outside temperature decreases below the thermostat temperature settings at that time your thermostat sends signals to the furnace in order to start the process.

For clear understanding that how furnace collect cold air of the house. Basically, fan and the blower of furnace pushes the air through supply ducts and starts creating the negative pressure which pulls the cold air from your rooms to returning ducts and vents.

So, in this order the cold air flow through the returning ducts to your furnace. As I said before, your furnace heat up all the cold air passes through air filters. And the air filters catch all the contaminants like dust, debris, hair and pollen etc. Issues in the flow of air mostly happens when you are using dirty or clogged filters.

After the filtration process, that filtered air starts entering in the furnace and then passing through the heat exchanger and in result the temperature of air increases. After all the process the blower starts sending the warm air in supply ducts and from supply ducts the air moves towards the house through vents of the room. So, this is the way that how air flow in furnace.

Down-flow furnace:

Up-flow furnace:

Horizontal-flow of furnace:

Air flows in downward direction from the down-flow furnace. Basically, down-flow furnace is normally installed on top of the building like in attic because it’s one of the best and good option for those houses who is not having basements. Down-flow furnace is very less efficient instead of up-flow furnace. 

Because the airflow in furnace is in opposite direction instead the heat rises normally and naturally. That’s why the down flow furnace is mostly available in larger number in such countries where hot season is shorter instead of cold season.

Down-flow furnace

How does a down-flow furnace work?

Cold air which is coming from returning ducts flow inside the blower section which is present near to the furnace top. The air flow in furnace moves down through the heat exchanger and the temperature of air rises. Then blower pushes all the hot air out of the bottom of the furnace and then it moves through the house supply vents and ducts.


  • Best option for houses without basement.
  • Suitable for that regions where shorter heating season instead of cold season.
  • Gives you efficiency cooling in summer.
  • It has lower energy cost when using for cooling purpose.

Up flow furnace moves the air in upward direction. Up-flow furnace is normally installed in the lower level of the building like in the basements. Up-flow furnace is very efficient in heating but not efficient for cooling purpose. Such furnace is best where cold climate longer with longer heating season instead of cooling season.

up-flow furnace

How does the up flow furnaces work?

Cold air is coming from the returning ducts which flows in the blower section from the furnace bottom. So, air flows in upwards direction through the heat exchanger and in result temperature of the air rises. After that warm air extracts from the furnace top and moves through the house vents and ducts.


  • Best option for the houses with basement.
  • Good option for the house which is present in cold climates and having longer heating season.
  • It gives efficient heating cycle.
  • Lower the cost of energy in the winters.

Horizontal-flow of furnace moves air in the horizontal direction. Its look like that your furnace lying on the side. All furnace components are placed in a row inside horizontal flow furnace. These furnaces are typically installed in the attics or in crawl spaces which has low ceilings which could not accommodate the upright furnace. These furnaces are usually come in various styles like in horizontal left or right and horizontal up and down.

Also come in multipose too. Hot air comes out from the left side of the horizontal left furnace and in right for horizontal right furnace. Up flow and down flow horizontal furnaces are easily configured in the two ways: horizontally and up flow or down flow. Multipose and multipositional furnace configured in the four ways: up-flow, down-flow, horizontal-right and horizontal-left.

Horizontal flow of furnace

How does the horizontal furnaces work?

Cold air is coming out from the returning ducts and then flow in the blower section at only one side of your furnace. Air flows in furnace sideways through heat exchanger and in result the temperature of air rises. After that blower pushes out all the hot air from other-side of your furnace and then moves through the house vents and with ducts.


  • It’s very compact and its design is very space saving. Such furnaces installed very efficiently in that rooms with the low ceilings.
  • It has flexible configuration just to match various house styles.

Where the filters of furnace located?

Normally, the air filters are located in that place where central returning duct meet with the furnace in order to prevent all the dust particles from entering inside your furnace. It’s also possible that filter is present inside the blower section of furnace or behind the returning vent in your house wall/ceiling. Few systems are having more filters instead of one filter.

It’s necessary for you to know the location or filter. Because you need to change filters in order to maintain good air flow in furnace and to prevent all the dust particles from the entering of your HVAC system. Air flow in furnace maintained by filters and filters should be replaced after one to 3 months.

Also Read: Why is my furnace blowing cold air?

For the horizontal flow of air units:

The filters are present on the returning ducts of your furnace, inside your furnace blower section or present behind the returning vent in the wall/ceiling.

For the vertical up-flow units:

Filter is present on the returning ducts side which nearer to the furnace bottom, inside your furnace blower section or present behind the returning vent in the wall/ceiling.

For the vertical down-flow units:

Filter is present on the returning ducts side which nearer to the furnace top, inside your furnace blower section or present behind the returning vent in the wall/ceiling. Such units need 2 filters.

For the air handlers:

Few houses are having air handlers in the warm climates with the electric coils instead of the furnace. Filter is in built in form in the filter rack which is present behind your furnace air handler access panel or present behind the returning vent in the wall/ceiling.

Which ways does your filter of furnace go?

Filters are having arrows which point the air flow in furnace direction when filter is inserted in the filters housing. In simple words, arrows in the filters point the blower in furnace. Furnaces are having arrows which indicates the direction of air flow stamped on the metal panels which present near the filter housing.

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

If the filters have a metal mesh the mesh have to face the blower. Arrows have to point towards the ceilings and walls for filters which is present behind returning vents in the ceilings or walls, also the metal mesh must have to face the walls and ceilings.

Pro tip:

Draw arrow with the help of permanent marker near to the filters slot which is pointing the direction of air flows. When you replace such filter with the new draw the same arrow on the new filter in the same direction as you drew earlier.

How to get more flow of air from the furnace?

To increase the flow of air:

  • Replace your air filters.
  • Unblock, clean and open all your furnace heating vents.
  • Clean or unblock all the returning vents.
  • Seal any air ducts leakage with the help of metal tape.

Can your furnace make you sick?

Poor furnace maintenance causes many problems like sneezing, headaches and itchy eyes and also sore throats. All such symptoms are happening just because of dust, pollens and various other allergens stuck in your furnace air filters.

Leakage of carbon monoxide in the components are very rare but it’s very dangerous. Symptoms are headaches, confusion and nausea etc.

The importance of knowing the air flow in furnace:

Understanding the air flow in furnace. Check the location of returning ducts or vents and from where the air is entering in the blower section of furnace from the central returning duct will help you to find the furnace filters and install the filters correctly.

Air flow in furnace is in particular direction like up and down, or horizontal left and horizontal right. Understanding of flow of air in furnace is very useful especially when you are performing maintenance of your HVAC system like replacing your furnace air filters.

Basically, air filters play a vital role because it protects your furnace from many dust particles and keep your system working efficiently. Also it improves your house air quality. Filters must be changed regularly after 1 to 3 months just in order to perform its function efficiently and maintain the proper flow of air.

Air filters cleans your house cold air which is coming from the returning ducts and is found in 3 locations like where central returning air ducts meet with the furnace or behind the returning air vents in the wall/ceiling. Third one is inside your furnace blower’s section.

Read the owner’s manual. If you are not having owner’s manual find the one online. If you are not able to find the air filters or you need any additional information regarding filters replacing. Then, contact with your professional HVAC contactor. I have to mention this again that you have to change your air filter in 60 to 90 days even if you don’t find your air filters dirty. For further problem contact with professional and perform a schedule maintenance of furnace.

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