I live near a busy road and have noticed an increasing amount of dust as traffic increases. I change the furnace filter regularly and have multiple Winix purifiers running. Is it coming in from the fresh air intake? Is there a way to filter it, while allowing enough air to enter?

In all homes I have lived in, the air intake for the furnace recirculates the indoor air to heat it. I have never heard of a heater pulling in fresh air.

That said, my neighbor in Washington added a fan and a pipe to his ductwork in the attic. He could flip a switch and fill the house with cold humid air.

I live in AZ and it’s pretty dry and dusty here. My house is on a pretty busy street. I change the filter monthly. Every time I pull it out, it’s gray and nasty looking. The dust comes from the dust in the air. It just gets in to everything. Might be a little different where you are. I know that the dust/sand in Utah is superfine compared to what I have here.

Michigan has mandatory fresh air intakes to prevent CO poisoning if the furnace is malfunctioning. I think it may have started for commercial buildings to cause frequent enough air exchanges to counter "sick building syndrome" and then got expanded to residential.

But, sucking in subzero air in the winter and heating it up in the furnace will increase your energy usage vs. using 100% recirculated air. My Indiana homes are 100% recirculated and I have a CO detecter and it has never shown any CO in the house.

My understanding is that houses don't typically have vents for incoming fresh air. At least not for the purposes of providing clean air to the living spaces.

The fresh air vents you see are for providing air for combustion. Gas furnaces, hot water heater, etc must force combustion gases out of the house, which causes negative pressure in the rest of the house. Having a fresh air vent near the combustion source alleviates this negative pressure, which reduces the drafts and heating/cooling costs in the rest of the house, and prevents nasty air from crawlspaces and attics from being pulled into the house.

Newer houses here are built so tight that they actually have an exhaust fan that is supposed to be run all the time. This creates negative pressure inside the house, and intentionally pulls fresh air in past the doors and windows.

I've not seen a house with a forced air intake that you could put a filter in front of. Doesn't mean it doesn't exist, just nothing I've seen around here

Ackks, has a HVAC professional taken a look at your concern? There is some needed questions and insight to give an adequate response. Without going into a discussion about the ventilation and energy codes, I'll stick with the principle concern. My thoughts with the information given is it is possible that the house envelope is in a negative pressure condition. This will cause the outside air to infiltrate the exterior envelope drawing in the traffic dust. Exhaust fans can cause this. The furnace air delivery ( duct work ) can also cause this and can be the primary cause, as the volume of air moving is the largest fan in the home.
IF the duct work is sufficiently leaking air and cannot return to the fan inlet, the fan will cause the home to become negative.
What efficiency rating air filter are you using? Higher MERV rating will filter better, but WILL impact the air flow. Higher resistance to the air flow with higher efficiency filters can cause air flow to fall below recommended minimums that will cause operational issues and damage. To negate this pressure drop, you need to go with a deeper ( higher surface area ) filter, such as a 4" filter that will reduce the pressure drop.
Get a professional to look at your issue.
If in fact you have a fresh air intake connected to the return duct, filtering that air will obviously increase the furnace filter life.

My sister had an issue with dust in her condo and it turned out to be a poor joint in the clothes dryer exhaust leaking into her laundry closet and thus in the apt. Sharing in case that is a contributory issue in your place.

I bought my current home 20 years ago. Early on I disconnected the 6 inch fresh air intake and capped it off. We like fresh air but the concept of hot humid air to cool in the summer or cold dry air to warm & humidify in the winter just didn't make sense to me.
I live in Michigan and it is code now to have the fresh air intake.

I also have a CO detector and have never had an issue.

This house is certainly not air tight and with the dog and me going in & out all day long we don"t need the fresh air intake.

I have a 5 year old house and in the cold air return line to one of my units (it's Texas so demand is too much for one unit) there is a fresh air intake with a 12x12 filter on it. The other two air sources for that unit each have their own 20x25 filter. Modern houses are sealed so tight that most have a fresh air duct on the cold air return.

I have one neighbor who was great at changing the air filters on the 20x25s, but had no idea there was a 12x12 in the attic. When we had both lived here 4 years old, I mentioned hating getting in the attic in July to change that filter and he had a deer in the headlights look. He went and checked and of course had one and it was nasty.

The reason why I mention all of the above is to inquire if you've looked to see if there is a dedicated filter on the fresh air line?

I do not have fresh air in my system. I do, place filters on the interior returns as an added plus fro allergies. Work great.

No ductwork here; radiant heating. Still lots of dust (dogs, open windows, etc.)

I see them advertise cleaning for all duct work ( I have forced air)
Anybody say its worth doing? Looks like about $300.

Mine looks to be a 10" round duct that sticks out 12" in from the wall in the ceiling of my furnace room. That's why I'm wondering if it's acting like an open window sucking in all the dust from the outside. Can that combustion air be filtered without causing a balance issue with the system? Also, the return goes into the furnace through an Aprilaire 213 which is a MERV13.

I'll check some of the other things that were mentioned.

Do you notice more dust in any one particular spot? I grew up with a house that was only a sidewalk away from a main street and dust would come in through where the two window sashes on the old windows wouldn't make a perfect seal.

Also are you sure you have a dedicated combustion air intake? If not and your house isn't completely sealed it'll pull combustion air in through the cracks and holes in various locations of the home.

My furnace was having a hard time making complete combustion due to the home being too tight. So my HVAC contractor installed a "fan in a can" to automatically bring in new combustion air into the furnace from the outside whenever the furnace kicks on.

Perhaps the solution is to *add* a fresh air intake, well-filtered. If the house is over-pressure, exterior dust won't creep in through cracks around doors, etc.

Heating and cooling costs would increase - it would be a cost/benefit decision.