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.

Heating & Cooling Air Duct FAQs-2

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.

On 2018-07-08 by (mod) - how to fix a problem with poor air flow on lower floors

Eli

Before adding booster fans or return or supply air ducts and registers let's be sure we have properly diagnosed the poor air flow problem.

Where is the air handler?

An attic air handler on a three floor house will have a devil of a time blowing warm air (for heat) down to the lower two floors,

while a basement or crawl space air handler will have a hard time blowing cool air up three floors in a home.

If there is inadequate return air volume then neither air handler is going to work very well..

If the air handler blower is dirty or if ducts are crimped, disconnected, leaky, etc. then air flow will be horrible.

See details at AIR FLOW IMPROVEMENT, HVAC

Then you can also take a look at BALANCING AIR DUCT FLOW

On 2018-07-08 by Eli

Hi! Have a problem in the house — the cold/warm air is coming out only on the 3rd floor, and just barely from 1st and 2nd.

The most reasonable and expected thing to do is, quite probably, balancing.

But, I’m considering other problems, that may have been overlooked by the experts that came.

First of all, I’m concerned that there is only one air return register, on the third floor, just between two bedrooms and near the stairs.

Could this be a bigger factor than the lack of proper in-duct balancing?

Next, could installing air boosters for each floor be a better and more stable option in such case?

Boosters would essentially set the pressure difference between duct and room, and so mirror the pressure between floors in the ductwork, making a sort of auto-balancing, is this correct?

Any other options I missed? Thanks.

On 2017-12-21 by Manfred

Was hoping to raise bathroom ceiling when renovating but just discovered that this is impossible because space between current ceiling and concrete floor of condo above us is occupied by a 12” diameter circular duct carrying A/C to one bedroom register.

Can One simply replace this with a rectangular duct 7” (or even 4”) high but wide enough to allow the same volume of air to pass? MB

On 2017-02-03 by Elizabeth

there is a steady ticking sound coming from the ceiling. It is constant (every 10- 15 seconds) when the heat has not kicked in, and increases in frequency (steady and constant - every second) when the heater kicks on.

On 2016-12-13 by Anonymous

we have a new air conditioner and all registers blow cold air but one air is blowing thru the line to the register but a connection may have came off how do i find where it is without opening up a lot of holes in ceiling

On 2016-08-11 by (mod) -

Price

Aluminum foil tape works well; some experts spray paint first, then cover with foil tape or other material. If the ducts are rusted, in bad shape, it may make sense to replace those sections entirely, removing the old sections intact to avoid making a dusty mess.

On 2016-08-10 by Pricefamily2001@yahoo.com

How to cover an air duct joint that may have asbestos coating/cover

On 2016-07-18 by Anonymous

I had a small electric heat pump A/C installed by this old house 35 years ago in a historic home. The relay switches are no longer available. Is there a company that makes a similar unit @ 8-9 " deep and 3Ft long and 4Ft wide? I would appreciate your answer.

On 2016-04-04 by (mod) - Why would someone disconnect my return air and tape up the hole in the unit where it attached?

Anon:

Sounds odd to me - not something we can know from a one-line question. There must be something we're not seeing here. After all, if your HVAC system had NO return air, you'd have very little air coming out of the supply registers.

On 2016-04-04 by Anonymous

Why would someone disconnect my return air and tape up the hole in the unit where it attached?

On 2016-01-03 by (mod) - fiberglass dust

Brian,

If you are asking about looking for fiberglass dust at a macro level that can be seen by the human eye, please see our article at https://inspectapedia.com/hazmat/Light_Forensic_Guide.php

where you will see that the key is not light wavelength or color spectrum but rather light intensity and aim.

If you are asking about microscopic detection of fiberglass in air or dust samples, there are several key steps such as using the right microscope type and adjustments, using a mounting liquid with the right specific gravity or better, refraction index, and also working at sufficient magnification to spot not just large fiberglass fragments but also the very small ones that might be more harmful - depending on the expousure level.

See the fiberglass detection article series beginning at

https://inspectapedia.com/Fiberglass/Fiberglass_Hazards.php

On 2016-01-03 by Brian

what spectrum of light is best to detect fiberglass insulation indoor air contaminates

On 2015-10-06 by carlos

Air is coming from inside the wall

On 2015-09-27 by Anonymous

Reid, to be well informed and to read about large vs. Small fiberglass fragments search inspectapedia for FIBERGLASS HAZARDS

On 2015-09-27 by Reid Austin

We have been advised to replace our current fiberglass duct work with galvanized steel due to the risk of cancer. We were told that systems over ten years old release fiber glass particles into the air which is a cancer risk.

Replacement of our current system which was installed in 2003 will be very costly. Is this truly a health risk?

On 2015-06-19 by Rick

What to look for when blower fan will not come on

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.

- Stephen

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.

• Is this an A/C retrofit job? Just double check - if this is an A/C retrofit job onto a home that already had warm air heating, and if the A/C air handler was put in the attic, perhaps it is ONLY connected to the first floor ductwork. Rule that condition in or out first.

Stephen reports heavy air flow upstairs and poor air flow downstairs.

• Check for balancing dampers [see BALANCING AIR DUCT FLOW] : since it is easier to push cool air down through ducts from an upper level to a lower level in a building than it is to push cool air upwards from lower to upper floors, the installer may have included airflow balancing dampers - look for these in the visible and accessible ductwork as well as at individual supply and return registers.
• Check ductwork connections. Perhaps there are none or too few delivery ducts to downstairs? In some air conditioning diagnosis jobs we found that an air duct had simply become disconnected entirely.
• Check for return air obstructions: if you are trying to blow cool air into rooms whose doors are shut and if your system uses a centralized air return register, air supply may be blocked.

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

Reply:

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

Reply:

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 end of this article .

If you feel air coming out of the supply registers but it's not cool that will suggest one direction of investigation (lost refrigerant, for example); but if there is poor air flow itself out of the supply registers that's a different direction of investigation (crimped, blocked, disconnected ductwork, dirty filter, dirty squirrel cage blower fan).

It's odd to be told that "all looks good" on your system but then to find it's not working. So why not give a call back to whomever you paid to clean and test the system and ask them to take a more careful look.

Let us know what you're told - it will surely help other readers who are having trouble with their air conditioning system.

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

Reply:

Hillary:

If the air coming out of supply registers is cold and has good force or air flow, but the house still seems not to be cooling down enough, it could be that the system needed to run for a while to not just cool down house air but also the warm building interior surfaces and contents, all of which have thermal mass.

If airflow is weak at all registers I'd be looking for a dirty filter or cooling coil at the air handler.

If AC airflow is weak just at some registers I'd be looking for a duct damper that is shut, a register control that is closed, or ductwork that has become crimped, crushed, or disconnected.

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

Reply:

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.

Eli: The air blowing out of the supply registers in your building should be cool. If your system is not blowing cool air you may have a refrigerant leak that should be found and fixed before the system is re-charged. Take a look at the LOST COOLING CAPACITY A/C diagnostic articles found at the ARTICLE INDEX the bottom of this article for details on how to figure out what's wrong with your cooling system.

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

Reply:

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.

The solutions to this problem are these:

1. Insulate the duct exterior surfaces
2. Check for sources of unusual indoor moisture or leaks and fix those.
3. Add dehumidification to the area where you are seeing this problem - for example using a portable dehumidifier along with a condensate pump that can empty the dehumidifier for you even when it is left on, running, and unattended.

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

Reply:

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 AIR CONDITIONER 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

Reply:

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.

And I agree that mechanical cleaning of fiberglass ductwork is generally impossible to perform without risking damaging the insulation, damaging its inside coating, risking increase of the release of fiberglass particles into the building air, and actually increasing the future accumulation of dust and debris on the disturbed inside surface of the ductwork. Typically the components of house dust that collects in HVAC ducts are dominated by fabric fibers and skin cells. But high levels of dirt in the ductwork can impede airflow, and if moisture condenses in the ducts or there are leaks into the duct system that material can support problematic mold growth.

A better solution to dirty fiberglass ductwork interior is its prevention by installation and regular changing of the system air filter, and assuring that there is not significant bypass air leakage around the air filter.

Once the ductwork has become "dirty" you have these treatment options:

- Install and maintain improved air filters to reduce the movement of debris particles into air distributed into the occupied space

- replace the most-damaged or dirty duct sections (the materials are not costly but labor cost will be involved)

- spray the duct interior with an encapsulant - not an approach I personally like too much due to its cost, questions about long term reliability, and questions about thoroughness of treatment

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

Reply:

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.

...

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 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-2 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

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