Can You Run A Furnace Without A Filter Temporarily?

Are you curious whether you can run a furnace without a filter?

Most home HVAC systems will need a filter to protect the equipment, but there is an exception.

In this guide, I will over...

  • When you don't absolutely need a filter
  • The potential damage of not using a filter
  • How to temporarily cut a filter to size

Let's get started with this guide!

can you run a furnace without filter temporarily

Can You Run A Furnace Without A Filter Temporarily?

Yes, you can run a furnace without a filter for a short period without damage, such as an hour or less. And some may frown upon it, but if you have an older furnace, one that doesn't have an A/C evaporator coil, then you can run it safely without a filter — but I still wouldn't recommend it.

However, if your system has an evaporator coil (the main part of the A/C or heat pump) then you need to always run it with a filter otherwise the evaporator coil will act as the filter—and it will cause problems.

Contrary to popular belief, the first and primary purpose of a furnace filter is to protect your HVAC equipment. When various HVAC components get caked in dust, they run less efficiently, and may cause your A/C or furnace to overheat and short cycle.

Read Also: What Are The Best Furnace Filters for Allergies?

Short Cycling And Overheating

Short cycling is when the HVAC turns automatically on and off repeatedly due to overheating. And since your HVAC didn't reach its target temperature, as soon as it cools down, it starts back up.

The constant turning on and off of a furnace can quickly wear it out, reduce it's life expectancy, decreases the efficiency, and even blow the motor or compressor.

Basically, maintenance of the furnace filter and changing it every 1-3 months will help prevent a system failure.

Read Also: How Often To Change 1-Inch HVAC Filter?

can you run a furnace without a filter temporarily

Fire Hazard

In addition to potentially damaging expensive HVAC components, and lowering its life expectancy — it can also be a fire hazard.

If your HVAC system has a missing or defective overheat sensor switch, and temperature rises due to dust on electrical wiring or on the motor, it may lead to a home fire.

HVAC Efficiency

When your evaporator coil, blower fan, motor, and even air ducts are caked in dust — it will also greatly reduce its efficiency This means it requires more work and more energy, just to reach the target temperature (if it reaches it at all).

And especially during peak seasonal months, such as in the heat of summer or the bitter cold of winter — the HVAC system will be taxed much more than normal.

The evaporator coil is the serpentine aluminum tubing that is part of the A/C systems—it is designed to remove heat from the air. And if you don't have a filter in the system, this serpentine tubing will act like a filter because it has many metallic fins and tubing.

Even just a small coating on dust of the evaporator coil affects its performance significantly — even reducing its efficiency by 50% or more.

Read Also: What Are The Best HVAC Filters For Dust Control?

Reduced Air Flow

Running your furnace without a filter can significantly reduce air flow over time.

A common complaint of my clients during inspections is that they put there hand up against an air vent—and there isn't much air flow. And one of the most common reasons is that the HVAC is caked in dust, including the air ducts and vent covers.

Allergies And Dust Control

In addition to all of the problems that it can cause with your HVAC, not running your furnace with a filter can also increase allergies and household dust.

Since many modern homes are built to new energy standards such as being air tight, these indoor air pollutants get trapped inside the home — decreasing air quality.

If there isn't a filter, then all of the air in your home is getting re-circulated without any filtration. You may begin to notice increased dusting requirements on your furniture and floor. And if anyone in your household has allergic sensitivities, then not having a filter can make allergies much worse.

But just remember that HVAC systems are designed to use filters firstly to protect the equipment—and reducing household dust and allergies is only secondary.

Read Also: How To Clean AC & Furnace Filters?

Short Term Fix

And if you can't find the correct size of the filter, you can buy an oversized filter (or even two) and cut it to fit, using duct tape to keep it together and in the filter slot. It's better than nothing.

Just remember that you may be able to buy a filter at your local grocery store (my local grocer has standard sized filters).

Mold

As a home inspector, I have taken off a lot of covers from HVAC systems, and quite often, discover mold growth.

The primary reason for mold growth is excess dust that is caked around the furnace, the blower fan, and other components. Dust only needs three things to grow: darkness, moisture, and a food source. Well, HVAC systems have all three. 

And dust is the main food source. So if you are continuously running your furnace without a filter, you are running the risk of mold growth in your system.

Read Also: What Are The Best Furnace Filters For Odors?

Final Thoughts: Should You Do It?

Its little known that you can run an older furnace that doesn' t have an evaporator coil in the air stream without a problem.

But for the vast majority of homeowners, there will be an evaporator coil as part of the HVAC system that is critical for proper air conditioning.

You may run it for a short period without problems, such as an hour or two, but you are better off just buying an oversized filter, cutting it size, and forcing it to fit — if you have to custom order a filter and just buying time. The main purpose of the HVAC filter is to protect your equipment, and controlling dust and reducing allergens is only a secondary benefit.

Read Also: MERV Vs. MPR Filter Ratings

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Arie Van Tuijl

Arie Van Tuijl

Arie is the founder of Home Inspector Secrets, an online resource dedicated to helping people understand how homes work. He is a licensed home inspector in two U.S states and owns a residential and commercial inspection company (read his full bio on the About page). To ask Arie a question, please use the comment box at the bottom of the relevant article.

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About Home Inspector Secrets

Home Inspector Secrets is an online resource for owners, buyers, and sellers to understand all aspects of home maintenance. We have detailed home guides, product reviews, inspection advice, and much more.