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Six Side Effects of a Dirty Air Filter

An air filter sits between the return air ducts and the furnace. When it gets too dirty several things can happen, and none of them are good.

Six Side Effects of a Dirty Air Filter

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Roger Bakies


February 22nd, 2023

Buy Your Filter

If you don’t give your car proper maintenance, sooner or later it will let you know. And it won’t be a love letter.

Same with your furnace. Out of sight, out of mind.

You remember it. It probably sits in your basement humming away, providing heat in the middle of winter and beyond. And next to that piece of HVAC equipment is a simple object that can shut down your entire system: the air filter (a.k.a. the furnace filter).

When was the last time you paid attention to it? A month? Six months? Longer?

I asked one customer when was the last time he changed the filter, and he said, “What filter?”

It has no moving parts and no warranty, and costs between 10-20 dollars for the cheapest type. And if that air filter doesn’t get replaced (or washed, in the case of some models) in a timely fashion, it costs you money without you realizing it.

How? We’ll describe each of the ways an air filter could be robbing you in multiple ways.

1. Clogged Air Filters Causes Airflow Issues

With the help of a blower, your duct system moves air from your furnace throughout the house before it mixes with unconditioned air. It then finds its way to the return ducts and heads back to the furnace, where the process starts all over again.

Static pressure is the resistance a cooling and heating unit faces when trying to push air throughout your house. You want the right static pressure balance in your system because too much or too little can cause problems, including premature system failure and energy inefficiency.

Take, for example, the register (or as it’s sometimes called, the vent) in a room that supplies warm and cool air. If you close it, that air coming to it has nowhere to go. The air backs up, creating excessive static pressure on the supply side.

A few closed registers are not a huge problem. Too many causes stress on the system because the blower will have to work harder and harder to move the air.

Read more: Is it OK to Close Air Vents in Your House to Redirect Airflow?

The same phenomenon happens when there are clogged filters. Sometimes we see customers’ filters get so dirty, very little air can be forced through. When it’s that stopped up, the air backs up with no good place to go.

As the filter gets dirtier, the static pressure increases on the supply side, and the furnace will have to work harder. The harder it works, the more electricity (or gas) the system uses to circulate the air. If left unattended, it can be stressed to the point of failure.

The return air will be dirty because it’s pulling air from your home. Think about what you see behind your refrigerator. Dust mites and lint and cat hair and dead skin cells. That filter is the checkpoint.

You can clean the dust from every surface, but dust is inevitable. Changing the filter restores better air flow.

2. A Dirty Filter Means a Dirty Furnace

All that dirty air coming from the return air ducts has to go somewhere. Some of it will stop cold and back up, depositing dust in the ducts. Some air will take the path of least resistance and go around the filter. This is especially true with the thin, fiberglass filters that might not fit snugly in the furnace compartment.

And sometimes the filter can bow inward.

This means that the dust that should be on the filter winds up in the furnace. We’ve been in houses where the blower motor and blower wheel are covered in dust. It can also cover the heat exchanger and evaporator coil.

I’ve been on jobs where the furnace wasn’t doing its job during cold weather and the customer tells me, “It’s not working, It’s not working” even though it is running constantly. I open up the panel and see that the high-temperature limit switch has been tripped because it’s getting too hot.

The limit switch is a safety device on a furnace that is designed to cut the gas supply when the burner temperature exceeds the safe operating range.

Why? It’s simply a matter of a clogged evaporator coil or a clogged blower wheel, or it’s a clogged secondary heat exchanger. Or it’s all the above.

All of that is due to a plugged filter.

3. The Air Conditioner and Furnace Will Have to Work Harder

A clean system is an efficient one. A dirty system is…You can figure it out.

As that filter collects more and more debris, the static pressure increases. Once you go above the manufacturer’s specifications, you’ll have a premature motor failure. In the meantime, you’ll use excess electricity and you have reduced airflow.

The gas will be running too hot, activating the safety limit switch, which turns the furnace off when it’s too hot. It’s like blocking the vents in your car in the summertime. That heat’s going to go back to the engine, so it will keep shutting off. The same is true for the furnace. It will turn on and off more frequently, which is hard on the machine and does a substandard job of heating.

On one job, the evaporator coil looked like it had a sweater on it. Once I peeled that off, magically there was heat again.

And it’s not just the furnace that will suffer.

The air conditioner doesn’t know it’s not getting enough return air. If you set your gas pedal so the car is going 60 miles per hour on flat ground, cruise control will compensate when you start going up a hill. The car has to work harder to get the same result. You’re going to lose some speed.

In the case of the reduced return air, the AC has to speed up.

Heat transfers across the evaporator coil. If that coil gets dirty, when you disrupt the airflow from what it’s supposed to be, then the air conditioner is going to work harder. It’s set up to have a certain amount of air going across it, this much refrigerant, this much static pressure. When you knock one of those things out of balance, it can upset the whole apple cart.

4. The Indoor Air Quality Decreases

On sunny days, when the light is streaming in through the windows, you can see how dirty the air is. According to one report, indoor air is 2-5 times dirtier than outside air. Couple that with the fact that you probably spend most of your time indoors, and you can see how unhealthy your home can be.

And it’s not just dust in the air. House dust is a mix of sloughed-off skin cells, hair, clothing fibers, bacteria, dust mites, bits of dead bugs, soil particles, pollen, and microscopic specks of plastic. 

Allergy sufferers and those unfortunate folks with respiratory problems will feel it the worst. It will make for poor indoor air quality. You’ll be breathing in that dust that should have been caught by the filter.

When dust fills the filter, it gets inside the furnace. When the furnace or the air conditioner run, air will flow over the dirty blower wheel. It’s then in the ducts and out the vents. It’s a vicious cycle of dust that gets recirculated.

We talk about all of the benefits of cleaning ductwork. Well, if the filter is doing its job, the ducts won’t get dirty in the first place.

5. The Energy Bill Will Increase

Keeping track of your energy usage is difficult because there are so many factors that can influence it. Are you running your HVAC system more? Are the outside temperatures more (or less) extreme? Is the thermostat set at the same temperature as last year? Are there more occupants in the house (thus increasing the inside temperature)?

Even so, an uptick in energy bills is almost certain when the filter is dirty. It makes key components of your furnace and air conditioner work harder.

6. The Evaporator Coil Will Freeze

You can reach a point where the airflow is diminished to the extent that the evaporator coil will freeze. When it’s operating normally, the evaporator coil is where the evaporation occurs. The temperature in it will be in the low 40s or so. As the airflow decreases, it can go below the freezing point. The evaporator coil sweats. The air conditioning removes humidity. So it’s sweating and it’s below freezing, and pretty soon you have a block of ice.

When the evaporator coil freezes, you take the chance that the refrigerant will back up into the compressor. This will cause the AC to run and run and not cool, increasing the energy bill.

The best way to get the evaporator coil to thaw is to turn off your system.

Keeping an Eye on Your Air Filter in Columbus, Ohio

If you’re uncomfortable changing your own filter, HVAC technicians would be more than happy to replace your dirty air filters with similar or more expensive ones. They can also recommend higher-end filters, including HEPA varieties and air purification systems.

If you have a smart thermostat, it can keep track of when the filter needs to be changed for you.

What’s most important is that filters are as essential to your heating and cooling system as your furnace, heat pump, or air conditioner, but they are much easier to take care of. Filters are relatively cheap, especially compared to the cost of repairing your HVAC.

Check below to see if you’re in our service area. If you’re in Columbus, Ohio, or the surrounding areas, we’d love to have a Fire & Ice sales professional walk you through any and all HVAC options to help you choose the best equipment for your needs.

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