Can the wrong filter damage a furnace?

Author: Dominic Heller  |  Last update: Saturday, October 4, 2025

A filter that blocks airflow will raise the air pressure in the HVAC ventilation system, which can damage ducts and even the AC and heater. It will also mean lower comfort around the house, since there will be reduced airflow coming from the room vents.

What happens if I put my furnace filter in wrong?

The most common problem you will face with a backwards facing filter is simple inefficiency. If your furnace is forced to blow air through the non-porous end of a filter, it will take more energy to do so. The blower will be overworked and you will pay more money for your heating.

What happens if you use the wrong air filter?

Yes, using the wrong size filter can lead to improper airflow, causing condensation to form in unwanted areas. This excess moisture can lead to leaks, particularly around the evaporator coils.

Can I use a slightly bigger air filter?

While you can technically use a different size air filter, it's not recommended. An ill-fitting filter can allow unfiltered air to pass, reducing your system's efficiency and potentially causing damage.

Why is my furnace blowing cold air?

With a furnace blowing cold air, the first question you should ask is if the furnace has been turned ON. Two thermostat issues that could cause your furnace to blow cold air include (1) your thermostat is still set to COOL, not HEAT and (2) your thermostat fan is set to ON, not AUTO.

Why “Good” filters will ruin HVAC

What happens if your furnace filter is too big?

If the air filter is too large, it will cause the system to run slowly and reduce the efficiency of the filtration process. This requires more energy to run during the day and will increase your bills.

Does a furnace filter need to be the exact size?

actual size of an air filter can affect filter compatibility if it doesn't fit properly into the HVAC system filter slot. Ensuring the actual size aligns with your HVAC system's requirements, even when the nominal size matches, is important if you want system efficiency, proper filtration, and the best performance.

Can I use a 4-inch furnace filter instead of 5?

Going down in thickness by an inch shouldn't be a big deal - for example, you should be able to use a 4-inch filter instead of a 5-inch filter. If you are considering a thicker air filter in order to improve air quality it may be better to focus on the MERV rating.

Will Merv 8 hurt my furnace?

MERV 8 filters allow good airflow and are not taxing on residential HVAC systems.

Is it okay if my air filter is a little small?

If the filter is too small, it won't cover the entire space and may allow dust and dirt to flow past. If you're not sure what size filter you need, check the manufacturer's specifications or ask your HVAC technician for help choosing the right product.

Can you put an air filter in the wrong way?

If you install your filter in the wrong direction, air will have a hard time flowing through the filter's material and your air handler will have to work overtime to make up for the loss of airflow. This is not only a waste of energy, but it can also possibly damage your air conditioner or heating system.

What happens if you use the wrong size air filter?

Using the wrong size filter reduces efficiency, air quality, and can even damage your HVAC system. You can learn more about the difference between furnace and AC filter sizes here.

Is it OK to run furnace without filter?

Your furnace can, technically, run without a filter. However, we strongly recommend you don't run your furnace without a filter for any length of time. With air constantly moving through your furnace and your home, dust and debris can quickly build up, even in a matter of minutes.

Can you damage a furnace by not changing the filter?

The danger of not changing the air filter in the house for a long time is that it will eventually lead to furnace damage. This means that the entire system will get destroyed by the excess heat. Replacing the entire system can cost you enormous amounts of money, and it's almost the same as buying a new HVAC unit.

Is there a right or wrong way to put in a furnace filter?

Replacing your air filter is not difficult. Remember that air flows from your ducts, through the filter, and into the furnace. The arrow on the filter should always point toward the furnace.

What is the most popular furnace filter size?

Currently, the most common size furnace filters are:
  • 20 x 20 x 1.
  • 16 x 25 x 1.
  • 20 x 25 x 1.
  • 20 x 25 x 4.
  • 16 x 20 x 1.
  • 30 x 30 x 1.
  • 16 x 25 x 5.
  • 14 x 20 x 1.

Why won't my furnace filter fit?

A filter that's too small will leave space where debris and particles can enter; a filter that's too large won't fit in the slot. You can't trim it down, either, since that will create gaps that allow things like dust, dander, pollen, and indoor air contaminants to enter.

What happens when you put a furnace filter in backwards?

Installing an air filter backward can damage your furnace or HVAC system, as the airflow will be decreased dramatically. A backwards filter prevents air from flowing into the furnace, which prevents air from flowing out of the furnace.

Do furnace filters have to be exact size?

The reason for that is air filters need to be slightly smaller than the slots that they fit into so that they can be easily replaced. If your filter was the same size as that slot that it fits in, it would be very difficult to install and remove it.

Can I use a 16x25x4 instead of 16x25x5?

UNDERSTANDING FILTER SIZES

Therefore, for example, a 16x25x4 filter is 16 inches long, 25 inches wide, and 4 inches thick. It's important to note that filters with different dimensions are not interchangeable due to variations in frame size and media area.

Should I turn my furnace off if it's blowing cold air?

Typically, there's a 10 to 15 minutes allowance for this. However, if after this time it's still producing cold air, then there's an underlying problem that needs to be addressed. It would be wise to turn the furnace off and get in touch with a pro immediately for help dealing with the issue.

How often should I change my furnace filter?

For 1- to 2-inch filters, replace them every 1 to 3 months. For 3- to 4-inch filters, replace them every 6 to 9 months. For 5- to 6-inch filters, replace them every 9 to 12 months.

Why is my furnace running but still cold?

If your furnace is blowing cold air in your home, this is a sign of an issue with the flame detector, gas supply, air filter, pilot light, or condensate lines. Many of these issues are able to be easily solved on your own by sanitizing or replacing components in the gas furnace.

Previous article
Where to drain a dishwasher?
Next article
Can you clean and reuse a cabin air filter?