Defects in HVAC duct systems:
This article 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.
This diagnostic article describes common defects found in HVAC duct work such as incomplete supply or return ducts, poor choice of location or size for supply and return registers, locating ducts in problem areas, leaks into and corrosion of duct work, leaky duct connections, and special notes on product failures by two manufacturers of flexible duct work.
The article includes important safety warnings about dangerous openings in HVAC duct work.
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We explain how to find and correct air duct problems such as missing air conditioning cool air supply or return air registers, undersized air conditioning duct openings, improper cooling duct routing, cooling (or heating) air duct corrosion, leaky air duct connections, defective heating or cooling ductwork materials such as Goodman gray flex-duct, some Owens Corning Flex-duct, and asbestos-containing air conditioning or heating duct work.
Air conditioning duct system defects include a remarkably wide range of errors, from failure to supply cool air or failure to even circulate air in the building, to health hazards such as use of asbestos material in or on duct work, to very dangerous conditions such as drawing heating equipment combustion gases into the building cooling (or heating) air.
The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects. Asbestos in or on HVAC ducts is a possible hazard for which we provide links to a separate document - see "Asbestos HVAC Ducts" at below left.
Below we list some common sources of poor air flow from air conditioning or heating duct systems: check these first before ordering a more costly survey or repair job.
Technical note: The D'Arcy-Weisbach equation for pressure and head loss can be used to calculate the actual pressure loss due to friction in a building piping or air duct system.
The Engineering Toolbox provides the D'Arcy-Weisbach formula:
Watch out: But keep in mind that even this apparently accurate calculation of the effect of piping on air pressure and airflow loss will not include the effects of obstructions in the building return air or supply air duct system such as
Continue reading at AIR FILTERS for HVAC SYSTEMS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: How do I Diagnose Too Little Cool Air Flow Downstairs?
I have a 2 story house the air flows out the registers heavy upstairs but downstairs hardly any flows and from one there seems to be none. i inspected ductwork but find no visible pinching.
Reply: Tips for diagnosing uneven air conditioning air flow between floors in a building
Tips for diagnosing uneven air conditioning air flow:
Stephen hasn't provided much detail about how the air conditioning system in his home is designed and installed except that he doesn't see any duct pinching [DUCT ROUTING & SUPPORT], so pending some feedback we start with some general cool air flow diagnostic tips.
In a two story house at which air conditioning is provided by a single air handler, balancing the delivery of air flow can require extra attention to duct routing, use of balancing dampers in the ductwork, return air register location, and supply air register location and even multiple air filters or air filter condition.
In contrast, if the home has separate air handlers, one for upstairs and a second for downstairs his cool air flow problem would be simpler to explain.
Stephen reports heavy air flow upstairs and poor air flow downstairs.
Let us know what you find. It may help other readers.
Question: poor air flow out of HVAC registers on lower floor of two story house
i have a 2 story house the air flows out the registers heavy upstairs but downstairs hardly any flows and from one there seems to be none. i inspected ductwork but find no visible pinching - Stephen 5/21/011
Stephen: see our Questions and Answers section we've added above. There you will find some suggestions for diagnosing the problem of poor cool air flow downstairs. Keep us posted and send along photos of your system if you can. What we learn may help other readers.
Question: My air conditioner was tested and cleaned but now it's not working. What else can I check?
I've had my unit tested and cleaned and told that all looks good however the house does not cool off and continues to run. What else can I check? - Elise 6/2/2011
Elise, if your air conditioner is not cooling the home and keeps on running (the thermostat is never being satisfied), I'd start with the diagnostic articles listed under LOST COOLING CAPACITY - see the link at the left of this page.
Question: Air conditioner is not cooling - air not coming out of registers
We hadn't turned our air conditioner for a few months and when we turned it on it didn't work. We checked the fuses and pushed the high pressure reset. Now it runs and the air is cold but it doesn't seem to really cool the house the way it used to. I was reading we might need to clean the coils but I also noticed no air comes from one vent. I climbed under the house to check the ducts and there's no leak but a part that dips down seems heavy and squishy like there's liquid or gel inside it. Is this normal? Can that cause the pressure switch to trip? Can it cause the system not to work as efficiently and cool the house? The air feels pretty cold coming out, but it doesn't seem to come out with a lot of force (although it's a big house and I'm not sure how much force it came out with before). Thanks for any suggestions. - Hillary 6/2/2011
Question: The AC is running but not cool - should air coming out of our outdoor condenser unit be warm?
We turned on our a/c but the air blowing out is not cool. We had the unit serviced last year. Should the air blowing out the top of the condensing unit outside be warm? - Eli 6/3/2011
The air blowing out of your condenser unit (the outside compressor condenser that converts low-pressure refrigerant gas back to a liquid refrigerant) should indeed be warm.
Question: Sweating air conditioning ductwork - condensation on or in HVAC ducts
The ductwork in the basement is not insulated and is sweating alot. i have never noticed this in the past what could cause the ductwork to sweat so bad. The house is cool and the unit is cycling. - William 6/21/11
Sweating ductwork: means that you are seeing water on the exterior of the metal ductwork system. Although we often use the word "sweating" ducts or pipes, it's not really sweat - it's not water coming through the metal, it's moisture from humid air surrounding the ducts condensing on the cooler duct surface. You can get this problem inside ductwork too, you just can't see it so easily.
Question: What should be the temperature of air coming out of our HVAC ducts?
Can anyone tell me what the temp of the air coming out the ducts should range? - Steve 6/26/11
Measuring air temperatures at a supply duct is very tough to map to a standard because of the very large variation in circumstances that affect the output temperature where you're measuring: duct lengths, routing, insulation, air velocity, filter condition, blower cleanliness, refrigerant charge, and other variables are all at work. That's why we usually check air temperatures right at the cooling system - at the air handler.
There we look at the temperature drop across the cooling coil itself. If we find that the temperature difference is normal (say 15 degF) we know that the cooling system is working. Then after considering air velocity, any further complaints about the system's cooling capacity need to be examined against the ductwork and similar possible problem areas.
Details about measuring air conditioner operating temperatures are at OPERATING TEMPERATURES
Question: Is it legal for dirt to get imbedded in fiberglass lined heating ducts
The mobile home we live in has hard backed fiberglass insulation for the heat ducting. When trying to clean it is impossible because the dirt and debris imbeds itself into the insulation. Is this legal in Wyoming? - Laurie Sorum 8/2/11
Laurie I can't speak for Wyoming building code details - you'll want to take that question to your local building department. But certainly using solid fiberglass type insulating board for HVAC ducting is very common throughout North America.
Question: Our A/C system doesn't cool and the experts now say the ducts don't match the cooling unit
My property located in Windsor, Ontario, Canada. It uses to have a 2 Ton’s A/C work for this 2 floor old building more than 20 years. 5 years ago this 2 Ton’s A/C was upgraded into a new 3 Ton’s one instead of by a local cooling company. The sales representative told me the bigger A/C has more power capacity with high efficiency, it would be better than never before. But unfortunately Since the new 3 Ton’s one was installed, it never could make cool enough air to properly work for the building, In hot summer days, when you touched the air come out from the registers, it absolutely was not cold. The inside temperature was above 30 C degree or more even the A/C running in 24 hours a day.
I contacted same cooling company many times in every year, they always sent someone to check it, but the issue never could be solved. In the Third year, they gave me the conclusion was my original duct work system doesn't match new 3 Ton’s A/C and the original duct system needs to be improved with extra $2000 cost. I refused to accept this offer. I believe they installed wrong size A/C or any other reason because I never had problem with 2Ton’s A/C work at the same duct system condition. I really don’t know what was real reason caused the problem happened, if I ask them change 2 Ton’s A/C back, is this a reasonable requirement? - Charley 10/12/11
Charley, indeed there could be a duct size or other duct problem, or a fan problem, but to help sort out your question I'd start by measuring the air temperature drop across the cooling coil. If it's not between 15 and 22 degF drop, then the coil is not cooling and the system may not be properly charged. In other words we need to first find out how the existing equipment is working.
Take a look at LOST COOLING CAPACITY.
Question: I have warm air blowing out of the cool air vents
I just bought my first house so i don't know a lot about home maintenance but I've noticed that i have warm air blowing out of the cold air intake vents? Is this normal? If not, what should i for? - Julie 1/13/12
Reply: from Carl
Julie, use a sheet of toilet paper to see if you have positive pressure or negative pressure at your air diffusers. If you have positive pressure at you cold air returns and negative at your diffusers, there are a few problems you may be faced with. The most common is an in-experienced connected your furnace up wrong to the supply and return plenums. Without seeing the unit, some motors are reversible - this would apply only if it is installed right though. Another possibility is they chose the wrong type of flow pattern. eg upflow versus down flow. Good luck. - Carl
Thanks for helping out Carl.
Julie: if you haven't done so be sure that your thermostat and controls are correctly set to cooling. For example if ductwork runs through a hot attic and the A/C is not in cooling mode and the indoor air handler blower fan is set to manual on during hot weather, the fan will run continuously but will just circulate hot air through the home.
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
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
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.
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.
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?
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
Each of those problems as well as other causes for reduced airflow are here at InspectApedia - use the links listed at the "More Reading" links at 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
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?
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
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
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.
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).
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 http://inspectapedia.com/Manufactured_Homes/Mobile_Home_Troubleshooting.php )
Reply: HVAC ducts & jumping cats: let's keep pet doors and HVAC ductwork as separate topics
Thank you SO much for the wonderful cat - HVAC system question.
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."
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
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?
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
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