Furnace Fan Running But No Air Coming Out Of Vents


Furnaces or air handlers, as the industry calls them, are limited by the amount of air they can deliver through the duct work. The amount of air they can deliver is determined by their size. Any of the items in the list below can exceed your air handlers limit causing a reduction in the air flow at the vents.

  • Plugged air filter or too restrictive
  • Duct work plugged or falling apart
  • Insulation loose inside duct work
  • Dirty indoor air conditioning coil
  • Low on refrigerant causing coil to freeze over
  • Squirrel cage fan blade dirty
  • Indoor fan motor or capacitor failing

These tips are a guide for home owners to solve air flow problems. If you had good air flow at one time and you don’t now, these are the most common reasons that air flow has decreased through the duct work in your home. Your filter is at the top of the list because it is most often the reason behind no or diminished air flow.

Without proper air flow nothing will work well or efficiently. Nearly everything you add to a furnace has resistance to air flow.

Filters, AC coils, duct work, registers all contribute to what the industry refers to as static pressure.

And high static pressure will result in weak airflow from the vents in your house.

Just like high blood pressure is an indication of poor blood flow, high static pressure says you got low air flow in heating system.

And if your system was marginal on static pressure when it was new then you have very little wiggle room for dirty filters, dirty coils and closed registers.

The first 3 things they taught us in refrigeration school is air flow, air flow and air flow.

Just recently a customer came to me and said he “had no air flow”. Having only lived in the house a short time he then said “I don’t even know if I have a filter”.

Even after looking for some time we located a filter, plugged solid, like a board in the hole.

I wish you could have heard the big sigh of relief that furnace felt when we removed that blockage.

It’s true if you close one register that more air will come out of all the others, but at a cost of higher static pressure.

If you have checked or changed your filter and you replaced it with the same type you used when you had good air flow then you are good to go to the next step.

Be aware, if you have changed the type of filter you originally used, to something like an electrostatic filter, or a high MERV rated filter, they can have a lot of static pressure for a furnace fan motor and blade to overcome and can damage your system.

Look for air filter static pressure drop when buying filters or search on the web before buying.

For a little technical information go to hvacschool.com and you can read about cfm and static pressure.

Also, filters with very low MERV ratings will allow larger particles through the filter into the coil, fan and duct system.

Take a low MERV filter, one of those with fiberglass media. Hold it over a table and sprinkle some salt or sugar over it while looking at the table. Surprising how much falls through.

So you can start to see the battle between wanting a better filter for cleaner air and needing more air flow.

Their can be a happy medium. One that keeps your system clean and allows the proper amount of air to keep your heating equipment nearer to the SEER, HSPF or percentage ratings it had when new.

See my post on what filter you should use in you furnace and you can gain a lot of incite regarding how to choose the proper size and type of filter for your system.

How Strong Should Air Be Coming Out Of My Vents?

Because most of us want to solve these type of problems without investing in expensive tools that some service techs may have, to measure air volume, lets use a rule of thumb to determine air flow.

And then see if we can compare the proper amount of air flow to something we are more familiar with.

A good rule of thumb for how much air flow should be coming out of any single duct should be one cfm per square foot of the room.

So a 12′ X 14″ bedroom would be 168 sq. ft. or 168 cfm coming out of the duct.

Some of these vary but given a range you should be able to at least guesstimate how the airflow should feel.

  • Cloths dryer = 100 to 225 cfm
  • Hair dryer = 10 to 20 cfm
  • Shop vac = 175 cfm
  • Window AC = 125 to 200 cfm
  • 6″ clip fan + 190 cfm

It’s Quite Possible Your Duct Work Could Be Plugged Or Falling Apart.

Over the years I’ve seen a lot of variety come out of duct work. From the contractors garbage to dead animals and things we don’t talk about.

The best way to find these is with a camera. If you have one or even a few ducts plugged it’s a sure thing something has come undone or something is in the way of airflow.

A customer had an ongoing issue with no air flow to some upstairs bedrooms and finally decided that we better find out why.

After some investigation we found a likely place to cut open the floor and found that the duct was incomplete. It had been covered up without being connected.

Everyday you wait to find the problem will cost you in extra heating and cooling bills.

Search for a heating company with a camera or crawl into the access and find the culprit.

Here’s just a general tip from someone who has experience. If you call a technician to make a repair and he has to access your crawl space, have the access clear of your stuff so you’re not paying him to watch you do that.

Why Are My Upstairs Vents Not Blowing Air?

Two story houses present more than one problem when trying to condition a house.

Duct work and air flow usually has to travel farther and ultimately that duct run goes up into the attic and then each duct is terminated in the ceiling.

Path of least resistance is the main floor and those supply ducts going into the attic become the most restrictive.

A good test to see if duct work is intact would be to turn your system to the off position on your thermostat and then turn the fan to “on”.

Close all the vents on the main floor and any other vents except those on the second floor.

Now check to see if you have any air flow upstairs.

If you notice more air then the problem probably exists in the way air is distributed to the duct that feeds the second floor.

It could be the size of the duct feeding the upper level.

A scoop sometimes fixes this issue. It would involve accessing the duct where it attaches to the plenum or source of air.

Placing some sheet metal in the air stream forming a scoop in a way that forces the air up that duct.

How Do I Increase Airflow In My Vents

Another answer can involve a little sheet metal work is a booster fan. Measure the diameter of the duct feeding the attic and match that to the booster.

Amazon has many models available. The cheapest runs about $21.00 but beware that it may not deliver enough for a 2nd story problem.

Hurricane has a nice inline model. 780 cfm should give a pretty good boost. Check it out on Amazon.

That was a 10″ booster. Be sure if you order to get the proper duct size for your system.

Is It Possible Your Return Air Vents Not Working Or Is Plugged?

Yes it is, and combine that with the most common mistake or error made by contractors, not enough or insufficient return air to the furnace.

The different types of construction make it necessary for a home owner or a technician if necessary to dig pretty deep sometimes in order to find return air problems.

Removing the return grill is a good start.

With a flashlight, get inside the duct with the grill off, as far as possible, and take a good look. You may be surprised to see years of accumulation stuck to the sides of the duct.

It can be an unhealthy place. Wear a dust mask.

Flex duct can sag and kink, blocking air flow if improperly hung. Hanger can come loose with trade people or cable guys crawling around is attics and crawl spaces.

Ducts can even fall off of improperly attached fittings.

While you’re looking, check for duct work that may not be connected properly. Loose connections or seams in sheet metal that have come apart.

These need to be repaired and sealed.

This is a good place to imagine what this duct could be like if it had a filter grill unless it already does.

Filters located at the air handler don’t protect the return duct so over time the return will fill with dust and can look pretty disgusting.

For now, forget about the dust in the return, we are looking for things that have blocked the air flow.

You can address a good duct cleaning later.

Next you will have to remove panels from the air handler. Turn off the breakers. Remove the panel closest to the return duct.

Again with a flashlight take a gander into the duct. Look for loose insulation or old filters installed by others that got left in the duct. You never know.

A true story, when a filter got so dirty that it got sucked into a system and then someone else added a filter thinking no filter was there.

Could A Dirty Indoor Air Conditioning Coil Be Blocking My Air Flow?

If proper filtering has ever been an issue with your system and it’s more than a few years old then yes, your coil needs cleaning.

Vacuuming the surface of the coil will do more harm than good.

The coils are fragile and the aluminum fins will smash and distort blocking more airflow.

Their are some good products that home owner can purchase. Again, Amazon has these. Nu-Calgon makes a no rinse application for less than $20.00 and it has a four and a half star rating.

Even if your coil is not the whole air flow problem cleaning it will save so many things from high electric bills to compressor replacements.

A System Low On Refrigerant Will Cause Coils To Freeze Blocking Air Flow

It’s hard to expound on this topic any more than what is said in the title.

Their is a certain point in a ac system that air flow and Freon charge will cause a coil to ice over and airflow will be at zero.

I called it Freon to bring it to your attention that Freon is a brand name. One of the many names for the refrigerant in your ac or heat pump system.

It’s time to bite the bullet and get a service technician for this one.

Handling refrigerant takes special licensing and training.

Their Is Dirt On Your Blower Wheel

Don’t ask me how I know. I just know.

Dust, dirt or whatever that accumulates on your blower wheel can reduce efficiency and cause vibration.

Care needs to be taken when cleaning.

Blower wheels have balancing weights just like a tire on a car has lead weights to keep it from vibrating.

If one of those weights on the fan blade gets knocked off it means a new wheel or a lot of vibration.

Your Indoor Blower Motor Or Capacitor Could Be Failing

So many of the things we have covered here should be checked by a qualified service technician. This is one of them.

A good amp probe and a capacitor checker will be needed to properly diagnose this problem.

Without getting too technical a run capacitor is necessary to energize the winding in the motor.

Years ago it seems that capacitors rarely failed compared to the ones manufactured today. Not to blame those countries that are making most of them now. No I shouldn’t do that.

When a capacitor fails the motor can spin slow or completely stop.

A fan motor failing under certain circumstances can even run backwards.

Related Topic: Can A Duct Blaster Or A Blower Door Find Air Flow Problems

Duct leakage tests involve pressurizing the duct work and the home to find areas that allow the conditioned air in your home to escape.

Blower doors are exactly what they say. A curtain so to speak that is placed in the place of an outside door that has a powerful fan that either pressurized or depressurizes a home.

When the home is under pressure then artificial smoke can be used to locate leaks in door and window seals or any other openings that your home may have.

Leaks in duct work can also be detected.

A blower door technician can even set up his test in a way to isolate the duct work and get a report that states the percentage of total leakage in the house that is duct related.

If you ever do this, one great followup to a blower door report that says you have significant duct leakage is a product called Aeroseal.

When the product first came out we tested it on my own house.

My system wasn’t particularly noisy. At least I had no idea how noisy it was until the product was applied.

Wow! What a difference. Our system was so quiet you could hardly tell when it was on.

In a video it looks like dirt being blown into the duct. It’s a polymer of some type. Their is no debris. Just that polymer that knows how to find leaks and seal them. Weird huh?

It is a very good product, but pricey at the same time. Would it be worth it? I don’t have a dog in this fight. Get a bid and decide for yourself.

Over and out.

Chad Peterson

Chad Peterson is near 40 year veteran of the HVAC industry. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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