Ask an HVAC Expert: Tom in Ripon, CA, “Why Is My New Furnace Heating Unevenly, Blowing Dust?”

Tom in Ripon, CA, just bought a new furnace—but problems are already boiling up. He says:

I had a gas furnace and a new central AC unit put in back in 2014. My wife and I have pets and allergies, so after the installation we had the ductwork cleaned. We were really looking forward to cleaner air in our house but, unfortunately, we haven’t had much luck with that. In the past two years, we’ve noticed accumulations of gray dust all over our house, almost like lint from a dryer. They seem to be coming from our vents. I’ve tried all different kinds of filters in my furnace, but nothing’s worked.

Around the same time these gray dust clumps started to show up, some rooms in my house began to feel much colder than others even when the furnace was running, which never happened with the old model. Why is my new furnace blowing out all that dust and heating unevenly? I really don’t want to have to buy another new furnace, since I just invested in one a couple of years back. Please help!

That’s a great question, Tom. It probably won’t make you feel better, but I actually hear similar questions about air quality and uneven heating quite a bit, especially in places like Ripon where many of the homes are a bit older and all the nearby farmlands create an increase in allergens and dust. On the surface, these might seem like two separate problems but, chances are, they’re related.

Dust Clumps in Your Ducts

Let’s start with the harder-to-miss part of Tom’s question: those unsightly dust clumps that are falling out of his vents and mucking up the air in his home. I’m glad he tried a number of different filter types because he essentially ruled that out as the cause of his problem. With that in mind, I’m almost certain the problem’s in the ductwork. Even though he bought a new furnace just two years ago, the ductwork may be much older.

Ripon is an older farming town, and almost every home I’ve been to there has a conventional foundation, complete with a crawlspace or basement underneath. They also generally have an attic above. Now, these are nice touches in old homes, and I’m sure Tom enjoys having that storage space, but it also means that if there’s even a small hole in the ductwork that runs through them, the HVAC system could be sucking dust or lint in from those seldom-used areas and pushing it into the home. I’ve even seen deteriorating insulation and carpet pulled into ducts and then out into houses this way.

There’s another potential cause other than holes or leaks, but it still has to do with ductwork, and that’s imbalance. This is actually an even stronger possibility because Tom recently had that new furnace installed. A new furnace or air conditioner needs to be expertly calibrated for the size of a house and its existing ducts. See, every new furnace produces a certain amount of air pressure when it’s attached to ductwork. A contractor should test and evaluate existing ducts while installing a furnace to ensure the pressure from the new unit is identical to that of the old. A change in pressure can result in the furnace beginning to suck lint and dust from a crawl space, basement, or attic through the return vents, spitting the debris back out into other parts of a home.

Uneven Heating: Double Check Your Ducts

Uneven heating from room to room is one of the most common issues I hear about from homeowners, probably because going from freezing toes to breaking a sweat just by changing rooms is a problem folks want to get fixed ASAP. There are a few different reasons behind the uneven temperatures. See if you notice a trend:

• A dirty or overly efficient filter: With a new furnace like Tom’s, the filter is the first thing I’d check when evaluating whether there’s an airflow problem causing some rooms to be colder than others. If a furnace isn’t installed properly, the fan speed needed to push air through certain types of filters may not be set correctly, meaning the air will pass through the ducts unevenly. You may have a really efficient filter in place, but because the furnace isn’t calibrated to mesh with it, airflow will be restricted. Conversely, a clogged filter can do the same thing. But, like Tom said, he already tried several different filters, so we can rule this one out.

• Crushed or blocked ducts: When a duct is obscured or bent into improper shapes, it’s much harder for air to pass through it. Ducts are designed a specific way—any change to their shape will make it much harder for air to move. If heated air can’t pass freely through a duct, it can’t get to the home evenly. However, I’m not sure this is a likely problem for Tom, since these issues didn’t start until he had a new furnace installed.

• Duct leakage: According to the Department of Energy, the typical duct system leaks 30 percent or more of the air that passes through it. You read that right—at least 30 percent of the heated air that’s headed into a home gets lost somewhere along the way. Hot air intended for a bedroom could be escaping through a hole in the ductwork above a home office, making the office hot while the bedroom stays cold. As you can see from the DOE’s numbers, duct leaks are very common, one of the first things I check for.

• Not enough return vents: HVAC systems are closed loops, which means they produce a certain amount of air to be distributed throughout a home. That same amount of air is then returned to the system, reheated, and pushed back out. For this to work, every room in a house needs to be returning as much air as it’s getting. It’s like blowing into a straw. If both ends are clear, I can blow as much air as I want into it, but if I put my finger over one end, the straw fills up with air and I can’t blow any more into it. This same thing happens if there’s already too much air in a room not being returned to the system.

• Improperly sized ducts: As I mentioned above, the ducts have to be sized properly for your furnace. If not, the pressure will drop and the ducts may start sucking in air in some places, and not pushing out enough air in others.

Did you see the pattern? Many of the problems above involved your HVAC system’s ducts. With that in mind, my guess is that Tom’s new furnace has not been properly calibrated or sized to work well with the ducts in his house. Because of this, some of the vents in the home are sucking in dusty, linty air and pushing it out into the house, while the heat created in the furnace is not able to pass freely and equally into all of the rooms.

I know Tom just paid a whole bunch of money for a new furnace and might not be too keen on spending more soon, but a trained HVAC professional is really the only way to know for sure if imbalanced or leaky ducts are causing this problem. They can run a test to find out, and can also calibrate a furnace to fit ducts, find leaks in ductwork, or balance them for the home. Obviously, no one wants to be breathing in dust balls all the time. I know I wouldn’t. But don’t go thinking that new furnace is a dud. It’s far more likely that the problem is with the ducts.

Does Tom’s new furnace plight sound a little too familiar? At Bell Brothers, we’d be happy to help solve your furnace problem too. Contact our trained HVAC professionals to get expert advice on dusty ducts and finicky furnaces, old or new.