FAQs on defects in HVAC duct systems:

This article series explains how to inspect and diagnose trouble with heating or cooling air flow, air ducts, air registers and zone components of heating and air conditioning (HVAC) duct systems.

Diagnostic FAQS: Heating & Cooling Air Ducts

These questions & answers about air ducts on HVAC systems were posted originally at DUCT SYSTEM DESIGN SIZE & DEFECTS - home. That's a good place to start reading if you're trying to diagnose a duct system problem or if you want to know what problems to look for.

Question: We have 10 8-inch round air supply ducts in the ceiling but we don't feel enough air coming out

I bought this building and the central system has 1 16x25 in return. they have 10 8in round supply ducts in the ceiling. It won't cool right and you can barely feel any air coming from the ducts. Do they have to many supply ducts so the air won't circulate right?

Reply:

Before thinking about adding more ducts or even questioning the number installed, let's figure out what's going on with your air conditioning system that is causing "hardly any air flow"

Look for problems such as

• an iced-over cooling coil
• a blower unit that is not turning on
• disconnected, squashed, crimped, or leaky air ducts
• a dirty air filter that is blocking air flow
• a dirty air handler blower that spins but doesn't move much air

Each of those problems as well as other causes for reduced airflow are here at InspectApedia - use the links listed at the ARTICLE INDEX the bottom of this article or the search box you find at page top to find more details.

Or start diagnosing the trouble

at LOST COOLING CAPACITY.

Question: why does cool air flow on my second floor but not my first floor

Single zone air conditioning system unit in attic second floor is fine first floor two ducts not air flow - Dan 7/1/12

Reply:

Dan, if your air conditioning air handler unit is located in the home's attic, the closest rooms to the unit will be those on the second floor.

Unless balancing dampers were used in the duct work, you may be getting more air flow thorough the shorter closer ducts and registers (the second floor ceilings) than to the first floor.

If your air conditioner air handler is located in the home's first floor or basement but the first floor is receiving warmer or less cooling than the upper floor, again look for and adjust duct balancing dampers.

Question: Why is air coming out of one of my air conditioning ducts much warmer than air from the others?

I am in Phoenix az. There are three ducts coming from the cool air plenum to cool a 2100 sq ft home. The OAT is approx. 112 deg F. Two of the ducts coming from the plenum cool those parts of the house ok. The third duct coming out of the top of the plenum is smaller than the two coming out the sides. That third duct from the top cools two small bedrooms and a bath.

The temps at the register are about 60 to 65 degrees in the registers that are cooled by the ducts coming out the sides of the plenum, but the third duct (smaller) coming from the top gives a temp of about 78 degrees in the three registers that it feeds. Can you tell me what the problem could be?

I have looked at the plenum to see the setup for the three cooling areas to the house. I have removed the zip tie from the small duct at the top and pulled back the insulation.

It does feel like there may be some cool air leakage coming from between the insulation and the plastic -wire wrapped "hose" inside the insulation.

I have not removed the duct from the plenum because I did not have the thermal tape to put back on to seal the plastic "hose" to the plenum. - Kyle 7/6/12

Reply:

Kyle as we replied to your separate email (readers please ask questions just once) you are on the right track to check the smaller duct system thoroughly, including an inspection of it's interior for crimps, blockages, leaks, lost connections, a closed damper, etc.

If the problem dates from original installation the size, routing or a similar problem may be at fault.

Question: The AC ducts in my third garage don't blow cool air. How do I diagnose the trouble?

I have a third garage which shows AC duct but does not blow air in. how do I figure out what the problem is? - Ritin 7/16/12

Reply:

Ritin,

This sounds easy enough to me that I must be missing something in your question.

If you have one air handler/blower unit and it is supplying three AC Duct trunks, then if two of them blow air and the third does not you're looking for a closed duct damper, disconnected, crushed, crimped, or collapsed air duct.

If the third garage is a free standing building and has ductwork but no air handler you would not expect to find air coming out of the ductwork - it's there for future addition of an HVAC system.

Question: warm air is coming out of what I think is a return vent

I have central air and a clean filter all the vents are blowing cold air.

The problem is there is a vent above the thermostats and air return that is blowing out hot air when ever the air handler or fan is on. This is keeping the temp too high for the ac to turn off - Tamar B in SC 7/26/12

The vent looks different than the return and cold air vents I think the suction of the return is causing it to draft hot air what could cause this? - Tanner 7/26/12

Reply:

Tanner, if the "vent" has no operable louvers to open and shut it then indeed it could be a return air register. Bur return ducts should be under negative pressure - air moving in not out of the register.

First map out the duct system to see what is installed.

Next look for a misrouted or improperly connected duct - it would be very unusual for air movement through a return duct system to cause one of it's branches to blow out instead.

Any mistake in where a small add on return duct was connected, say right atop and ahead of the cooling coil would however cause the problem you describe.

More common would be a cool air supply duct that is routed through a hot space and perhaps has lost its insulation.

Question: A cat jumped out of the HVAC duct floor vent - the owner says this is normal and is there for ventilation

I am looking at buying a double wide through an estate sale. The "trustee" of the estate is the deceased owners daughter, who know very little to anything about mobile homes (as I).

I am hiring an inspector, how ever he wont be available for 3 weeks and I have an immediate concern.

While standing in the living room I was startled out of my skin to see a cat jump into the living room from a floor vent ( vent cover was pulled off) I looked into the vent and it appears to be completely open - no duct work - just a view of the ground beneath.

The daughter said that that was common and is there for ventilation.

LOL Tell me this isn't so! - Gail 9/7/012 (originally posted at https://InspectAPedia.com/Manufactured_Homes/Mobile_Home_Troubleshooting.php )

Reply: HVAC ducts & jumping cats: let's keep pet doors and HVAC ductwork as separate topics

Gail,

Thank you SO much for the wonderful cat - HVAC system question.

Yes I can tell you "it isn't so" - that is, it is not good practice to simply leave floor vents in a home open to the outdoors such that anything, including a cat (or worse, a raccoon) can hop in for a visit.

I can imagine a few reasons why you might have found the missing duct work and open floor vent, all adding up to some more trouble and work for you.

For example most likely there was a heating or heating and cooling system that used air but that has disintegrated, been damaged, fallen off, or was simply removed and abandoned.

I suppose a less ridiculous explanation that the owner could have invented might have been to explain

"Oh I forgot to tell you, that's just Marion, my mom's cat. Marion comes with the house. The hole in the floor is her pet door."

Sometimes too, when warm or cool air delivery into a home is poor in flow rate or quantity, people try to improve system operation by adding more return air to the system by just cutting an opening that lets outdoor air into the system somewhere.

The problem is this is the most expensive possible way to heat or cool a home since it's a "one way" design - we take un-conditioned air from outside, heat it or cool it, then try moving it into the living area.

Really the most significant implication of the cat in the hat, I mean cat in the vent discovery in your possible future home's heating system is that it's a red flag to watch out for other work done or "problems solved" by the same person on that home - as you may find other amateur workmanship that lengthens the list of repairs and improvements needed to make the home safe and habitable to normal standards.

Keep me posted, and send along photos if you can (use the CONTACT US link at page bottom or top) - especially if ... the cat comes back.

Question: My house is very dusty - it keeps clogging the air filters

The inside of the duct where I replace my filter monthly is very dirty in comparison to the same duct in my mother's house, friends' homes and in my prior home (with metal ductwork) just 3 miles away. My neighbor's duct is as dirty as mine and she says she has to dust daily.

The dust in my house, built in 1998, is excessive, but numerous HVAC companies that have been here that say the flex ductwork is fine and that this enormous amount of dust is "normal".

Both my dog and I are constantly sick with respiratory ailments and other infections, and we are getting worse as our immune systems continue to break down (as shown on blood tests).

In my previous home, which I owned for 17 years and is very similar to my "newer" house, I only had to dust every 2 weeks, not every 2 hours as is the case in this house were I well enough to dust every 2 weeks. I am spending thousands of dollars on vet treatment for my therapy dog and medical treatment for myself, but I'm thinking that I'd save money if I could just get this dust reduced. I have done everything

I can on my own using the EPA's guidelines, but I don't think I can single-handedly replace all the flexible ductwork. By the way, I live in a suburb outside of Phoenix, Arizona, where the summer temperatures can be as high as 126 degrees Fahrenheit.

I imagine the attic can get to at least 150 or higher. Surely the flexible ductwork would deteriorate in these conditions, so why would builders install it?

Does anyone have any recommendations or advice for me? I am willing to pay to have this system properly repaired or replaced but even during these cooler months, no company seems to want to take this on. Is there no one in the U.S. who actually "works" anymore? - Kathy Villa 2/13/12

Reply:

Kathy, yes there are some of who still work. Flexible duct is a popular method in many warmer climates. The integrity of these materials should withstand the temperatures under 200 to 300 deg. F. with no problems.

I only install metal and seal and/or insulation on it. If the cold air return is improperly sized and inadequately sealed, that can cause dust problems as well. Assuming you have AC, it would be interesting to know what your evaporator coil looked like.

Consider the dust sources as well: quality of carpet, dust mites, animal products, body dandruff, etc. If the house is too tight with inadequate make up air in the ventilation, this can contribute to. Good luck - Carl

Kathy: adding to Carl's suggestions, if your home happens to have one of a few brands of flex duct whose vinyl external covering disintegrates in hot environments (see GOODMAN GRAY FLEXDUCT for an example) you'd want the ductwork replaced.

Otherwise, the use of flexduct continues to be economical and widely used;

Before talking about replacing ductwork (read expensive) it makes sense to have an expert inspect the system, map its design and layout, and give you an evaluation. Then you can discuss the duct routing and insulation improvements for the ductwork with your HVAC installer.

Next, in Arizona where you live the cooling season is long and demanding - so your HVAC system may be running more hours than it did in your more-hot home. And of course the house layout, HVAC system design and layout, and even the blower and filter system may be quite different.

Also depending on the type and size of your dog, s/he may be a source of indoor dust as well. Pets who move in and out of doors can bring in a lot of soil dust.

Question: why is it easier for cold air to flow downwards through ductwork than upwards?

please explain, how is it easier for cold air to flow thru a duct downwards than it is upwards. Just exactly how do you A0 calculate this? B0 measure this? - Tom Schlachter 3/20/12

Reply:

Tom, cool air is more dense than warm air (Gas Laws) so a cubic foot of cool air weighs more than a cubic foot of warm air. The exact difference in weight of a cubic foot of air at different temperatures depends on the actual temperatures of the two volumes of air being compared.

In plain English this simply means that cool air, because it is heavier than warm air, will flow downwards in a vertical air duct more easily than it will flow upwards - because in the "down" direction, the weight of the air tends to force it down, assisting any push imparted by the blower fan.

Charles' Law:   V1/T1=V2/T2  says that if we raise the temperature of a cubic foot of air, the air will want to occupy a larger volume.

Because warm air tends to rise and cool air tends to fall in a building, and because in a hot climate the upper floors of a home will be hotter than lower floors (warm air rises), if we're trying to cool the upper floors in a structure the HVAC designer/installer may decide to locate the air conditioning air handler in the building attic rather than the basement.

It's easier for an air handler to blow cool air down than up because gravity gives the cool air a downwards weight.

It's easier for a warm air furnace to blow warm air up than down because warm air, weighing less than cooler air around it, will rise in that environment.

We discuss BOYLE's LAW and CHARLES' LAW in detail with examples of using these equations

at WATER TANK PRESSURE CALCULATIONS.

 

Question: smell indoors coming from nearby portable generator

(May 23, 2014) Dave Wallace said:

I am getting a smell in my rooms that At first I thought was caused by someone running a generator in the neighborhood but it became more often was getting kindoa sick. it smell like a leaky manifold on a car or truck but it's in my house.

Even when I turn all AC and Heater to OFF I get the smell for hours and once in a while it goes away for a few hours. the smell stays even with I have doors and windows open.

I called AC/ Heater tech and he says in 50 years he hadn't heard of my symptoms because the system is off he didn't think it was the heat exchanger since no heat was being used etc
He's coming too look I think tmw 5/23/2014 my bedroom is now smelling as I type this I have to go outside

Reply:

Watch out: in addition to requiring that a portable generator motor not operate where its exhaust enters your building air supply,

Be sure you have a working carbon monoxide and smoke detector installed

Question: why do some of my house's rooms not get heat?

(June 11, 2014) kathialderson said:

all my rooms in house esp living room are cold except furtherst bedroom which dont get very cold at all ! what could be the problem?

Reply:

Look for closed supply registers in the too-cool rooms; Inspect the duct system for crimped, disconnected, or leaky ducts oe a closed register or duct damper.

Start at LEAKY DUCT CONNECTIONS

Question: duct odor problems

(July 12, 2014) AS said:
Hello. i have been having a a strong (to me, anyway - plus i smell it on my clothes when out of the house) tar odor in my house when our AC system runs.

I've had countless HVAC people over, discussed with a former contractor - everyone but one person (more later) pretty much shrugs their shoulders and says, "i don't know." So - here is some background: we live in chicago in a rowhouse with a 'flat' roof. Our AC unit is a rooftop unit (condensor/fan on roof) and our heating system is from steam boilers - completely separate.

the smell only comes around when we run the house fan or AC.

About five years ago (when i noticed the smell but have since had so much going on and it's only been 1 year that i've been working on this problem) we had some work done to the house.

We did some minor reconfiguration and therefore new duct runs in some rooms on the top floor. They also removed 1 of 2 roof vents (the old school kind) to use one as a dryer vent location.

One person who came to fix our AC unit (which is 15yrs old BTW) actually found a section of the return air duct that had an 8inch round hole in it. Yes - the contractor left it open to the 'attic'. i was hopeful that by sealing that up the smell/issue would be resolved.

No such luck. I'm wondering if there are other deficiencies in the duct work. What other advice or options could you suggest to get to the bottom of this? A home inspector? Where to start - i need something more scientific and targeted!

The HVAC people for the most part just advise me to re-do everything and i really can't afford that.

Or even part of it - but i NEED to figure this out. We have a toddler in the house now and i want a happy, healthy home. i would love some suggestions. thank you for your time.

Reply:

AS

It does sound as if there,may be other duct or,return air leaks and contaminants.

Give a call to a large HVAC company, talk with the service manager, and ask for help from a senior, experienced technician.

Keep us posted.

See the diagnostics at ODORS in AIR HANDLERS & DUCT WORK

Question: uneven cooling air distribution, air flow too weak, not enough cool air

(July 23, 2014) Ray Sokolowski said:
Hello...I could really use expert advice.

I've had two different companies come out and "inspect" my concern and they're not offering results.

We moved into a condo (approx.1750 square ft.) three bedrooms, 2 1/2 bath. The register in the master bedroom is blowing at 10% and not cooling the room. I've closed all of the other vents and there was no change to the air flow in the master bedroom. The other vents are blowing pretty forcefully.

The condo was built in 2007 and I am trying to get the building plans. Any suggestions are greatly appreciated.

(Sept 3, 2014) Lisa said:
This sounds lame and amatuerish, but we are having problems with airflow in certain rooms.

All registers are on the floors which I have vacuumed out many times and all air intakes are low on the walls near the floor as well. I have read that airflow often has much to do with intake.

Should the grills of the air intakes be turned to face upward to bring in cleaner air than pointing downward sucking up dust and pet hsir from the carpet that accumulates berween vacuuming??

Reply:

Sure, Ray.

Take a look at the "weak warm air flow" diagnostics article at

AIR FLOW TOO WEAK and also see DUCT & AIR FLOW PROBLEMS

and let me know if questions remain.

Lisa:

see the articles above.

If the weak supply vents share the same return air source as the strong supply registers then the problem is elsewhere - e.g. Crimped or leaky ducts or tight, closed rooms.

If we are heating we want to pick up cooler air closer to the floor. If heating, the opposite.

Question: banging noise from ducts

(Dec 18, 2014) Anonymous said:

Our duct work produces an irritating bang every 20 minutes to half an hour. It does this summer and winter, whether the we are on heating/ coolong or even just the fan running. How can we pinpoint where this banging noise is coming from?
Ron Bradfield

Reply:

Sounds as if you need to make a thorough visual inspection of the HVAC system starting at the air handler and any duct controls, looking also closely at any automatic duct dampers.

Question: weak cool air flow in part of doublewide home

(May 11, 2015) Marty said:
I live in a double wide manufactured home. Why is the air conditioner blowing more forceful air out of the vents on the floor on one side of my home?

And should I call a repairman if my thermostat is set at 74 and the temp in my house gets all the way to 83 with the ac running? The thermostat is in the kitchen.

Reply:

Marty

Thanks for the good question about balancing air flow out of the duct system in your home.

At the ARTICLE INDEX at the end of this article you'll see a series of articles that help diagnose and fix HVAC duct air flow problems, beginning at AIR FLOW IMPROVEMENT, HVAC

Start by checking

- are the supply registers all open fully?

- are there other balancing dampers in the duct system that are partly closed?

- are the lengths of duct run different, and do the weaker sections have more bends or elbows?

- has flexible duct been crimped or squashed?

- are duct sections disonnected or leaky?

Let me know what you find.

There are quite a few reasons why the air supply might vary. A visual inspection of the duct system can often sort this out, and I'd do that before calling for repair.

...

Continue reading at DUCT SYSTEM DESIGN SIZE & DEFECTS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see DUCT SYSTEM & DUCT DEFECT FAQs-2 - more-recent questions & answers about air duct problems

Or see these

HVAC Duct System Articles

• AIR FILTERS for HVAC SYSTEMS
• AIR HANDLER / BLOWER UNITS
• DUCT SYSTEM DESIGN SIZE & DEFECTS - home

Suggested citation for this web page

DUCT SYSTEM & DUCT DEFECT FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC DUCT SYSTEMS

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Citations & References

In addition to any citations in the article above, a full list is available on request.

---

ADVERTISEMENT

---