How to Tell the Way Air Flows in Your Furnace


Determining the direction of airflow in your furnace is crucial for correctly installing and replacing air filters and ensuring the efficient operation of your HVAC (Heating, Ventilation, and Air Conditioning) system.

Here’s a summary of how to tell the way air flows in your furnace:

  • Check the Filter Slot: One of the easiest ways to determine airflow direction is to inspect the filter slot or housing. Look for directional arrows or labels indicating the correct orientation of the air filter.
  • Examine the Filter Itself: If you have an old filter, examine it for directional markings or an arrow indicating the airflow direction. Filters typically have this information printed on the frame.
  • Consult the Furnace Manual: The manufacturer’s manual for your furnace system often provides information about airflow direction and filter placement. Refer to the manual if you have one.
  • Look at the Blower Motor: The blower motor within your furnace is typically positioned downstream from the filter. Air is drawn in through the filter and then pushed out by the blower motor. This can help you determine the airflow direction.
  • Observe the Ductwork: If you have access to the ductwork connected to your furnace, you can observe the direction of airflow by feeling for air movement or using lightweight items like tissue paper or a feather to detect the air’s direction.
  • Use a Professional HVAC Technician: If you’re unsure about the airflow direction or if your HVAC system is complex, it’s advisable to consult a professional HVAC technician. They can inspect the system and provide guidance on filter installation and airflow direction.

Which Way Does Air Flow in a Furnace?


So, how can you tell which way air flows in a furnace or AC unit? When you find out how, you will automatically know which way an air filter goes in a furnace.

In an HVAC system, this is how air flows:

  • From the house to the furnace through the return duct
  • From the house to the outside through the exhaust vent
  • From the outside to the furnace through the ventilation vent
  • From the furnace to the house through the supply duct.

For the sake of this guide, we will focus on the air that flows from the house to the furnace through the return air duct and from the furnace back to the house through the supply duct.

Air filters in HVAC systems are always installed in the return air duct. That is because the return air ducts are the one carrying the air containing dust, lint, pollen and other particles that need to be filtered out before the air can be heated or cooled in the furnace.

You therefore don’t expect to find a filter in the supply air duct. Now that you know that, you only need to focus on identifying the return air duct, especially where it connects to the furnace.

Dash to where your furnace is located (basement, crawlspace, attic or in a utility closet) and see if you can identify the return air duct. You can identify the return air duct by virtue of where it connects to the furnace.

  • In horizontal HVAC systems, the return air connects to the furnace from the left-hand side while the supply duct exits from the right-hand side.
  • If you have a vertical HVAC system, the return air duct connects to the furnace from the bottom while the supply duct exits from the top.

The furnace filter slot will be located very close to where the return air duct connects to the furnace.

furnace filter
Furnace filter (Notice the arrow pointing towards the furnace)

Since the return air duct brings air from the house, the direction of airflow in a furnace is towards the furnace and away from the return duct. Consequently, that is the direction the airflow arrow on the furnace filter should point to.

In some instances, the filter is not located near the furnace itself but just behind the return air grill. A house will have a return air grill and a supply air grill on the ceiling, wall or in a closet.

The first step will be to identify which grill is the return air grill and which is the supply air grill. Usually, the return air grill is bigger in size and when you place your palm near it when the furnace is running you can feel a suction force.

To replace the filter in this case you only need to remove the grill by pulling the tabs then pulling out the old filter. It can be a little hard to in this case to know exactly which direction to put in the filter.

Since the direction of airflow is from the house to the furnace, the filter airflow arrow should as well point away from the house and towards the furnace.

What Happens if You Put a Furnace Air Filter in Backwards?

When a furnace filter is put in backwards, the flow of air to the furnace will be restricted. That will force the furnace blower to work extra hard to pull in air which will greatly reduce its efficiency and ultimately its lifespan.

In short, putting in an air filter backwards has the same effects as having an old and clogged filter. It becomes very hard for air to pass through the filter because the pores are blocked and hence the furnace blower will keep running in a bid to offset the deficit.

Having a furnace filter installed backwards is like driving a car up a hill while having it installed properly is like driving it on a flat road. The car will consume more gas driving uphill than when it is being driven on a flat surface.

The first effect of a furnace filter installed backwards is short cycling from the furnace. Short cycling is when the unit keeps coming on and off.

Since the unit is fitted with overheating sensors, it will keep running (to force air through the badly installed filter) and only turning off to let the blower motor cool off and then turn on again.

Another impact of a filter installed backwards is high utility bills. Since the motor will be working harder than it should, you will notice that your power bills will shoot through the roof.

The quality of air inside the house will also be greatly affected. To start with, there will be reduced airflow and secondly there will be more dust, pollen, dander and other impurities since the filter will not be working efficiently.

In the long run, a furnace filter installed backwards will reduce the lifespan of your furnace. The extra strain put on the motor will cause it to wear out prematurely necessitating a replacement or a repair which is never cheap.

How Do I Change a Furnace Filter?

Changing a furnace filter as I mentioned is very simple. The first step is to locate it. Check if yours is just behind the return air grill or next/near to the furnace.

After locating the filter, turn of the furnace. Since the old furnace will be full of dust and other particles, attempting to remove it with the blower still on will have the dust being sucked inside the furnace, which is what you have been avoiding all that time.

If your filter is located behind the return air grill, remove the grill by pulling the tabs then pull out the filter. You do not need any tools to do that.

After that, check that you have the right size of the filter. Never force a filter in. The size is always printed clearly on the side of the filter.

When that is confirmed the only thing you need to do next is put in the new filter, making sure that its airflow arrow is pointing away from return duct and towards the furnace.

If the filter is located next to the furnace, simply remove the slot cover and pull out the old filter. Slide in the new filter, making sure that the airflow arrow is pointing towards the furnace and away from the return air duct.

What is Furnace Filter MERV Rating?

A furnace filter’s Minimum Efficiency Reporting Value (MERV) rating is an industry standard (from 1 to 20) used to indicate how effective a filter is in trapping particles. The higher the rating the more effective the filter is.

Filters with a higher MERV rating are not necessarily the best filters to use. The problem with filters with a higher MERV rating is that they are expensive and clog quickly hence needs to be changed regularly. They are used in sensitive industries like labs and pharmaceuticals.

Although furnace filters with a lower MERV rating are cheaper and lasts long, they only trap the big particles which makes them unsuitable for use in residential homes. They are used in commercial industries.

The best furnace filters to use for homes are those with a MERV rating of between 8 and 13. These filters will remove most particles from the air and are not expensive. They also don’t clog easily and hence do not restrict flow of air to the furnace.

Signs you need to Change your Furnace Filter

The following are the tell-tale signs that your air filter is dirty and that it needs to be changed as soon as possible:

  • High utility bills
  • Furnace has a long runtime
  • Reduce airflow
  • Increase in headaches, asthma and allergies
  • Burning smell in the house (caused by an overheating blower motor)
  • The indoor spaces are dusty (generally not clean).
  • Black filter (after inspection)

Note: Although it is recommended that you change your furnace filter after 3 months, I would also recommend that you inspect it monthly. That way, you will be in a position to know when it needs to be replaced instead of waiting until it is completely clogged.


And basically that is how to tell which way air flows in a furnace. Understanding the direction of airflow in a furnace helps you to correctly change your furnace filter therefore having not only heated but clean air at all time. It also makes sure that the furnace is not straining to do its job.