Low airflow, or air flow, through vents can be caused by several problems that can be categorized into air restriction, friction, turbulence and mechanical problems with the air handler.
This Pick HVAC guide to DIY furnace and AC maintenance explains each potential cause and provides solutions to the problem – low airflow in ductwork and how to increase it.
List of Low Air Flow Causes Through Vents
The list goes from the easiest DIY fixes to the most difficult that an HVAC technician will need to evaluate in order to provide you with the solution.
Start Here: Check to see if the vent is closed – If so, just open it, and we’ll forget we had this discussion.
OK, if it’s open, as it likely is, then start working through these reasons for low air flow through vents.
- The air filter is really dirty or too restrictive
- A zone damper is closed
- Blower wheel issues
- Wrong blower motor
- The ductwork has leaks
- Flex ductwork isn’t stretched out
- Dirty flex ductwork
- Ductwork turbulence at bends
Reasons for Low Airflow and How to Solve Them
Depending on your familiarity with HVAC equipment and ductwork, you might be able to resolve most of these issues.
If not, and low airflow in ductwork persists, use the toll-free 800 number on this page to get answers and, if you want them, repair estimates from local pros. There’s no charge to you.
OK, here are the causes of low airflow through vents and how to resolve the issue.
Diagnostic questions, typed in italics, are asked that are designed to help you pinpoint the problem.
The Air Filter is Very Dirty and/or It is Too Restrictive
Has the problem become steadily worse?
The first thing to check is the condition of your air filter. If it is really dirty, it will definitely restrict air flow. Try a clean/new air filter.
Did the problem start when you switched to a thicker or higher MERV filter?
Bingo! Filters with a MERV rating above 8 help with allergens and pollutants, but they can hurt the performance of the blower motor in your furnace. Take out the filter and turn the thermostat option to Fan. Does the air flow more freely? You might have to choose a filter that is less restrictive to increase airflow and to prevent potential mechanical issues from the equipment working too hard to move air.
Our “What is MERV Rating & What Rating Should I Use?” guide has all the answers to air filter ratings questions.
Equipment Tip: It’s OK to leave the filter out for a few minutes, but replace a dirty or overly restrictive air filter with a clean, proper one ASAP.
Info Tip: MERV is the rating developed by 3M. But other brands use similar ratings MPR and FPR.
Be an informed shopper with our MERV vs MPR vs FPR guide!
A Damper is Closed
Do you have a zoned AC system?
Do you have dampers in some ducts leading to rooms you sometimes don’t use?
If you said “yes” to either question, check the dampers to make sure they are opening. By design, a closed damper causes low airflow, or no airflow, to the vents fed by that section of ductwork.
If you don’t know, call an HVAC technician or inspect exposed ducts for small motors on the outside or for manual damper-closing handles on the bottom of the ducts.
Damper Tip: Make sure the handles run parallel to the ductwork to ensure they’re open, as shown in the labeling of this duct damper. You can see that the damper inside the duct runs parallel with the handle.
Blower Wheel Issues to Consider
Did you recently have the furnace serviced? Is it rattling or making other noise when the blower is on?
If a technician was there just before you noticed the issue, it is possible that the blower wasn’t fully reassembled after cleaning or service – technicians do overlook things sometimes when putting the pieces back together.
And if it hasn’t been serviced but is making noise, whatever is loose or wearing out could reduce its ability to push and pull air in the ductwork and through the vents.
Has the furnace been cleaned lately?
If it hasn’t, a really dirty blower pushes less air, so the result will be low air flow to vents.
Is there almost no airflow to AC vents / heat vents?
It’s possible the blower motor has stopped working or it might even be running backward – as happens on occasion when an ECM motor fails (strange but true).
The Wrong Blower Motor Was Installed
Was the blower motor recently replaced?
If so, you probably need a different technician, because the wrong size motor – one that isn’t powerful enough – might have been installed. Call them back, and ask them to send out a competent person to determine whether you received the proper motor.
The Ductwork Leaks
This is a common problem that slows airflow to some vents and wastes money.
Is the airflow to some vents worse than to others? Does the low airflow create hot/cold spots?
Those leaks need to be found and sealed. The pros at Pick HVAC have created two useful guides:
With the information you research there, you’ll be able to DIY duct repair or have a knowledgeable discussion with an HVAC contractor about repairing or replacing the ductwork. And while we’re on the subject of improving ductwork, you might want to read the Pick HVAC guide called: Insulate Old Ductwork vs Replace it with Insulated Ductwork?
$$$ Tip: We definitely recommend sealing and insulating your ducts for maximum efficiency and minimal energy bills. Ductwork insulation has a high Roi – you’ll recoup your cost in a couple years of lower energy bills and save money every month after.
Flexible Ductwork Isn’t Stretched Tight
Another strange-but-true fact is that friction in ductwork slows airflow by increasing static pressure.
Do you have flexible ductwork? It is typically round and looks like a large dryer vent. It could be wrapped in insulation.
If flexible ductwork isn’t pulled tight, all those crevices between wire loops create pockets of resistance and friction. The result will be low airflow.
If you just had the ductwork installed – or you’re new to the house – and the problem has been constant, this could be the cause.
If you can’t find an obvious cause of poor airflow to heat and AC vents, then the ductwork should be inspected. Look at exposed ducts, if possible, leading to the vents where airflow is low. If the ductwork looks compressed, this issue is at least part of the problem.
Dirty Flex Ductwork
Even if flexible ductwork is stretched tight, it still catches more dirt than sheet metal and spiral ductwork.
If you remove a vent and look into the duct, does it look dirty? Do you have pets that shed a lot (because the blower can suck pet hair and dander into ductwork)?
While we do not recommend duct cleaning for no good reason, this might be one time it makes sense.
Service Tip: Contact an HVAC company first!
If you call a duct cleaning company first, whoever is sent out will tell you that, “yes, of course you need your ductwork cleaned!” They likely won’t have the skills to accurately diagnose the true cause of the problem. Let an HVAC pro figure out what is wrong. If the tech says it’s dirty ducts, then call the cleaner.
Ductwork Turbulence at Bends called Throats
This might be the trickiest cause of low airflow through vents, and it isn’t easy to solve.
Have you ever seen where a river goes around a bend? Often in the inside curve of the river, there is backflow and turbulence. Airflow can be the same in the “throat” or inside bend (smaller radius side) of the elbow.
Some turbulence is expected – it can’t be completely eliminated. But an experienced HVAC pro should be able to find a solution to your problem of low airflow to ductwork vents.
Bottom Line
If the blower motor is undersized, there’s nothing that can be done but replace it with the right motor. If the airflow has never been good, and all else checks out, your HVAC system might have the wrong blower motor.
However, if the airflow used to be good, and now it isn’t, the checklist above points to the majority of things that can be wrong.
Start with what seems most likely or obvious to you. Work your way through the checklist.
Tip for Best Results: Poor airflow might be caused by several problems. For example, if the air filter is dirty, it’s possible the ducts could be dirty too, or flexible ductwork isn’t stretched tight. Go through the entire list, and solve all the problems. Or have an HVAC pro do it.
- The result will be that your AC, furnace or heat pump will:
- Run most efficiently and minimize utility bills
- Give you the best durability with fewer mechanical breakdowns
- Produce optimal indoor climate comfort
If you run into something you can’t resolve, use the handy contact form on this page to get help from experienced, pre-screened and licensed HVAC professionals from your local area. Or simply call the toll-free 888 number to get help.
