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Question DetailsAsked on 5/22/2015

We recently had our Air Ducts cleaned by a reputable company. Why is my house still dusty?

We had our Air Ducts cleaned in January because our house was so dusty all the time. They came in and cleaned them, we change out filters once a month, but the house is still just as dusty as before they came to clean them. Is there something else that could be causing this dust?

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1 Answer


Couple of common reasons below - if HAVING to change filters monthly because they are starting to clog up then you certainly have a dust source issue.

1) incoming outside air is unduly dirty - a common problem in dustbowl and desert and sandy/silty lake or ocean beach areas, also in spring in areas that sand the roads a lot.

2) air is getting past the filters - many brands they just slip in without a rubber seal or gasket to provide a true airtight seal, so much of the dusty airflow goes around the filter element. Many brands also have a very sloppy fit in the filter slot, which sometimes can be solved at least partly by putting self-adhesive foam weatherstripping or duct tape on the mating surface of the duct slot to provide a slight friction fit.

3) cheap filters that are letting most of the dust through

4) you have a lot of carpets with a lot of dust/dirt in them already that needs to be cleaned out because it is continually kicking up into the air.

5) children (of all ages)/pets tracking a lot of dirt in because you do not have clean entry surface outside the doors (wood or pavers or concrete or long grass or such) to cause the dust/dirt to rub off before they come in the house, and it is not being vacuumed up before it gets into the air

6) dust generation in garage or workshop or craft activities or such

7) dryer duct not exhausting lint outside, so it is building up in house

8) your house has negative air pressure (commonly because the "fresh air" inlet on HVAC draws from inside the house instead of from outdoors, or because indoor air intake vent (say in basement or garage) does not have sufficient opening area to the outside to let in fresh air without creating a vacuum in the house, so because it is short of intake air it is drawing outside air in around windows and through wall outlets and under doors and such, which bypasses the filters as the air has first circulated in the house so most of the dust drops out before it gets to a return duct to go back to the filters. Commonly pretty easy to diagnose - by taping thin plastic sheeting over entry door to see if the airflow is from outside to inside when HVAC fan is running (vacuum or negative pressure in house) or from inside to outside (positive pressure). You mauy need to check several places, because commonly the doewnstairs may be negative pressure but upstairs positive because of the natural temperature difference causing chimney effect in the house. Also, if outside air infiltration is the main source, the return air ducts will be nearly or as dirty as the feed ducts, whereas normally they are substantially cleaner unless you have badly shedding carpets.

Significant positive or negative pressure are both bad, because a strong vacuum means outside air is being drawn in without passing through the filter and also is pulling hot or cold air into the house which partly negates the air conditioning or heating respectively, depending on season. High positive pressure means you are pushing a lot of conditioned air out through the walls and to attic and such, both losing the energy used to condition the air and also pushing moist household air into walls and attic and such where it can cause moisture condensation problems. Most house have slight negative pressure except maybe when strong winds are pushing air in through permeable windows and doors, but even then natural attic and chimney effect drafting will commonly result in an increase in negative pressure on the upper floors.

IF you can take off a few register vents and look inside, if the outside air to the HVAC unit being dirty is the primary problem, the incoming air vents will commonly be much dustier than the return ducts and there will be much more dust right behind the filter than further along, if outside air infiltration into the house in general is the issue than commonly the return ducts will also be very dusty whereas otherwise lint is the primary buildup material in the return ducts. Heavy lint throughout the supply ducts means the filters are not working right, or the return air mixing air is entering the supply ducts downstream of the filter, so is recirculating in the house and bypassing the filter, which is a common but improper ducting error.

Answered 5 years ago by LCD

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