Diagnose & cure HVAC duct problems like weak air flow, leaks, noises, condensation:

If not enough cool air is provided by your air conditioner, or if the air temperature is not cool enough, or if you just can't get your A/C unit running, this article helps diagnose and correct the problem with step by step things to check and links to more detailed explanation when you need it.

This article forms part of our series on how to diagnose an air conditioner or heat pump that is not cooling.

Air Conditioning Air Duct Problems and Air Flow Defects - Poor Air Flow

Damaged air conditioning ducts (or heating ducts if you are diagnosing weak heating air flow) such as ducts which have been improperly routed and are crimped, crushed, or have excessive bends can reduce cool air flow in an otherwise properly functioning system.

You've already checked the suggestions made at AIR FLOW TOO WEAK, right?

If not please do so. It may save some trouble.

Quick List of the Most Common HVAC Duct Defects that impair air flow

• Crimped flex-duct (photo at left)
• Disconnected ductwork
• Leaky ductwork - tears, holes
• Undersized or poorly located return air
• Ductwork runs too long
• Ductwork with unnecessary bends or lengths of unneeded but left-in-place flex-duct

Watch out: Don't forget to check the easy stuff first: A client asked us to drive a considerable distance to repair her apartment's central air conditioning system after having had several unsuccessful service calls by local repairmen.

Because access was difficult and nasty, the first fellows who had been called preferred to stand in the apartment and suggest easy but irrelevant remedies like "adding refrigerant" (balderdash!) but no one had managed to get into the rather tight and hard-to-enter attic crawl space where the cooling ducts were routed. For one thing, you needed to bring a ladder, climb to a high scuttle hole, and remove the screws of an access panel.

We had our trusty Little Giant ladder and made the climb, bringing along a flashlight and a screwdriver. In the scuttle opening we found that the main cooling duct had become completely disconnected.

The attic was nice and cool but no cool air was being blown into the living area.

Check the condition of the duct system for blocked ducts, loose leaky connections, closed dampers, crimps and bends, before calling your service technician.

See DUCT SYSTEM DEFECTS for details of various A/C supply and return air duct and register problems and how to detect and correct them.

Cooling Capacity of the Air Conditioning Duct System vs Ductwork

Even if a very high BTUH capacity cooling system is installed, if the duct system is defective the ability of the system to deliver cool air to the occupied space can be severely or even totally lost.

Duct System Efficiency

(in percent) describes the percent of cooled air produced by the A/C equipment which is actually delivered to the occupied space. This number is less than 100% because of air flow restrictions and losses in the duct work.

Duct Delivery Effectiveness

is the percent of cooling capacity which is delivered through the registers into the occupied space.

Registers themselves restrict air flow.

See DUCT SYSTEM & DUCT DEFECTS.

Cooling Capacity of the Whole Air Conditioning System

So the ability of the entire A/C system to cool a building or rooms in it requires that both the cooling equipment and the duct system be in proper working order.

Details about duct system and air handler diagnosis, including duct defects, air filter defects, and air handler problems are provided

at AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself.

Is there no cool air at all coming out of the supply registers?

Or is there air blowing out of the supply registers but it's not cool enough?

Here we explain how to diagnose loss of air conditioning cool air flow or cool air temperature.

Before ordering an expensive air conditioner service call to restore lost cooling capacity, here are a few simple steps to perform.

Some of these can be done by any homeowner, others may require a bit more expertise.

Other Causes of Weak A/C or Heat Airflow Blamed Misdiagnosed as a Duct System Defect

Weak Air Flow due to Dirty Air Filters

If your air conditioning equipment is running but little or no cool air is coming out of the supply registers, check that your air filter(s) have not become blocked with dirt and debris.

Usually the air filter is right at the air handler or blower unit, or your air filter may be installed behind a grille covering a central warm-air return that sends air back to the air handler.

See AIR FILTERS for HVAC SYSTEMS

for details

Ceiling Stains due to Condensation or Ice Leaks

At above left you can see a ceiling air supply register that has leak stains around its opening - further investigation for a condensate leak or roof leak is needed.

Accidental airflow through an idle ceiling duct system in winter can cause accumulation of condensation and ice.

Poor or missing insulation on an attic duct system can also cause excessive in-duct condensation and leaks such as the stain shown here.

Air Flow Too Weak, Too Cold, or Too Warm

Details about diagnosing and fixing too-weak air flow from HVAC systems are

at AIR FLOW TOO WEAK

Air Conditioner Blower fan unit not moving enough air: 

too little air coming out of your air conditioning ducts?

Check the condition of the blower unit:

if it's dirty the blower may be spinning but not moving much air.

See DIRTY A/C BLOWERS for details.

Dirty filters or iced coils 

or crimped or disconnected air ducts can also cause loss of cool air or too little cool air coming out of supply registers.

These items are addressed below in this air conditioner diagnostic guide.

Also see DIRTY COOLING COIL / EVAPORATOR COIL

and  FROST BUILD-UP on AIR CONDITIONER COILS.

Sketch courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Air flow that is too slow 

for any reason (such as a dirty filter or dirty blower fan assembly blades) can cause first, air temperatures that are abnormally low coming out of the air conditioner, and eventually a reduction in air flow as coil ices over.

See A/C COOLING COIL ICING

Air flow that is too fast

 for any reason (improper fan motor, speed, fan belt or pulley size, duct design, duct registers removed, etc) can produce air that is not cool enough and can prevent proper air dehumidification (oversized A/C system).

See A/C NOT DEHUMIDIFYING

Air Conditioner Cooling Coil Icing Problems & Coil Leaks

Several causes of cooling or evaporator coil icing all have the same effect: ice blocks air flow across the coil, resulting in weak air delivery during the cooling season.

For a detailed discussion of air conditioner or other refrigeration (or dehumidifier) cooling coil ice-up diagnosis and cure,

see FROST BUILD-UP on AIR CONDITIONER COILS

Also see REFRIGERANT LEAK DETECTION

and REFRIGERANT LEAK REPAIR.

If your air conditioner or heat pump is a split system or mini split system design (using a wall-mounted cooling or heating unit) our troubleshooting diagnostics are

at SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS.

Get More Cool Air Flow

This discussion is now found at COOL AIR SUPPLY IMPROVEMENT

Get More Warm Air Flow

This topic is discussed at WARM AIR SUPPLY TEMPERATURE & IMPROVEMENT

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2024-01-29 by rndiy - moderator is willing to go a little deeper to understand the issues and troubleshoot solutions

@InspectApedia Publisher, Thanks again. This is the only place I find where the moderator/pro is wiling to go a little deeper to understand the issues and troubleshoot/brainstorm ideas/potential solutions. Much appreciated.

On 2024-01-29 by InspectApedia Publisher

@rndiy,

We have gotten beyond my expertise here but I certainly think that's worth a try.

On 2024-01-29 by rndiy

@InspectApedia Publisher, Thank you. I appreciate your reply. I have no way to eliminate the U-jump over the cross-beam. I could replace that abrupt reducer with a tapered one, but cannot upsize the 8" trunk due to available space.

So, to provide a path to those surplus CFMs, I am thinking of adding a 6" tap on that 12" trunk on the right, upstream to the abrupt 12x8 reducer. Would that help improve the flow and reduce the build up of heat in that trunk?

On 2024-01-29 by InspectApedia Publisher - thorough inspection of the duct routing is what's needed when tracking down an airflow problem

@rndiy,

Thank you so much for taking the time to add that important update to this duct airflow restriction discussion.

What you found is a reminder that a thorough inspection of the duct routing for bends etc. is what's needed when tracking down an airflow problem.

On 2024-01-29 by rndiy

@InspectApedia Publisher, I wanted to provide an update. The 12" trunk on the right has an abrupt reducer 12-to-8" about 8ft from the supply plenum. The 8" trunk goes for another 4ft and forms a "U" to go over a cross beam and tees into a 8" secondary trunk.

I think 4-5 adjustable elbows were used to go over the beam. I used values from the duct sizing tables found on this web site.

Assuming 600CFM are going into the 12" trunk, 325 CFM are taken out before the air reaches the abrupt reducer, leaving 275CFM Using 225 CFM capacity for the 8" trunk leaves 50CFM of airflow without a forward path.

Could this lack of trunk capacity in that supply plus the restrictive "U" transition be causing the heat build up in the 12" supply trunk and in the plenum near the entry to that trunk?

I attached a top view of the duct layout and shaded the portion of the plenum that is hot. Temperature was measured using Klein CL390 thermocouple.

On 2024-01-19 by InspectApedia Publisher

@rndiy,

Thank you that's very helpful additional detail. Sorry that my replies are constrained, I'm in the middle of traveling between countries.

On 2024-01-19 by rndiy

@InspectApedia Publisher, I understand having an expert look at this is the best way to go. I wanted to provide some more information.

I used Klein CL390 temperature probe to measure the temperatures at the points marked in red in the edited diagram. The probe was inserted about 5-6" into the ductwork. I was surprised to see the same temp difference from left to right inside the riser connecting the top of evap coil box and the plenum (measured through the two red circles).

I had an HVAC tech from a reputable company in for maintenance and he did not know why the difference. He looked up the blower exit hole and did not see any obstruction. He suggested I close some takeoff dampers on the 12" trunk.

I did that and it did not make a difference. I found this article about static regain and wondered if the height from blower exit to plenum (45") is enough for 100% static regain and if that is the root cause.
https://hvacrschool.com/duct-takeoffs/

What specific qualifications and experience would someone need to accurately diagnose such a condition?

On 2024-01-19 by InspectApedia Publisher

@rndiy,

The temperature is indeed almost certainly the same at the point of entry to the two different duct sections. However from that point quite a few variables can cause differences in air flow and thus also apparent air temperature and supply registers.

Examples include leaks and variation and insulation and variation in route length and route locations such as through warmer or colder areas.

To answer this question more accurately requires both a detailed and thorough on site inspection of the conditions of your air handler and ductwork and its routing, insulation, etc as well as carefully made objective measurements.

We don't know how your measuring or what you're using to measure or where you're measuring or what are the sources of variation but these are some examples.

On 2024-01-19 by rndiy

@InspectApedia Publisher, the airflow from registers connected to the 10" is about the same as those connected to the 12" trunk. However, the temperature difference at the registers is about the same as at the trunks.

I understand the capacity of a 10" duct is less than the 12", but why would the temperature be that much lower?

The distance from top of the furnace to the center of plenum is 45". if both trunks were the same diameter, would the air temperatures inside the trunks be equal?

On 2024-01-17 by InspectApedia Publisher - difference in duct size may obstruct air flow

@rndiy,

There may be more subtle factors such as the shape and exact location of obstructions to air flow, but I think that the most significant effect is the difference in duct size between 12-in and 10 in.

That is to say, an equal volume and flow rate of air arriving at the t in your ductwork and counters a 10-in opening on the left and a 12-in opening on the right so much more air flow wants to move to the right.

If you're getting inadequate airflow and heat output on the smaller duct line you may need to try installing a duct damper on the larger diameter duct so you can balance the air flow.

On 2024-01-17 by rndiy

I was checking the temperature rise on supply trunks and found that the reading on the left trunk is about 20F less than the right trunk. I attached an illustration of the set up.

Temp is taken about 12" away from the supply plenum. There is no blockage in the supply plenum. As shown, the only difference between the left and the right are the blower outlet is closer to the right side and the whole house humidifier cutout is on the right.

The cutout is blocked as we don't use that humidifier. This is a high efficiency gas furnace Lennox EL296UHV series. The temp rise on the right is right at middle of the range, but the left side is well below the mid point due to this difference in temps.

Would like your insight into what may be causing this big difference between the two trunks. Thank you

On 2024-01-17 by rndiy

@InspectApedia Publisher, thank you. I appreciate your insight. I reviewed the water supply pipe diameter and flow article in your reply and found the pressure loss doubles when I go from a 8" dia. pipe to a 7" dia. pipe.

While the absolute value of the delta is in the thousandths, adding 7-8 such joints brought the total delta to about 0.02" WC. Still not enough to take up the project right away, but I will keep this on my list. Thanks again.

On 2023-12-31 by InspectApedia Publisher

@rndiy,

If that improvement also means you can clean the ducts more successfully over the life of the building it might be worth doing.

But beyond what we've already said, I don't know how to quantify the change impact on airflow.

On 2023-12-25 by rndiy

@InspectApedia Publisher (mod), my thought was to replace that section of the return duct with a longer pipe (10ft) to eliminate joints and crimp the remaining joints the right way.

But, the return duct is in a chase and it would require disconnecting takeoffs as well. I was looking for a way to quantify the impact of reverse crimping (and that more severe crimped edge than the factory crimps which are snug fit) so I can assess the cost & effort vs. the benefit. If they used the factory crimps as is I would not have bothered.

On 2023-12-26 by InspectApedia Publisher - look at the effect on airflow in cfm of changes in duct diameter

@rndiy,

As I think you've found by now, we can look at the effect on airflow in cfm of changes in duct diameter with a general formula.

A1 x V1 = A2 x V2

for a given cfm starting rate the velocity will increase in the smaller duct section

but as I said before we haven't addressed the effect on cfm of turbulence at the reducing connections and my OPINON was that turbulence is probably greater (worse) when the pipes are joined "backwards" as in your illustrating photo.

You might just try some of the free online HVAC duct airflow calculator tools that are around the web. Those will let you plug in a friction (standard that IMO is going to underestimate friction unless the surfaces are nice and clean - as you say yours are).

Also I'm guessing that all of your ducts are round; rectangular ductwork behaves a bit differently in re airflow.

CD Engineering provides "HVAC – How to Size and Design Ducts" https://www.cedengineering.com/userfiles/M06-032%20-%20HVAC%20-%20How%20to%20Size%20and%20Design%20Ducts%20-%20US.pdf

The effect on flow in air ducts (or in water pipes) of sections that are in effect a smaller or later diameter is at

WATER SUPPLY PIPE DIAMETER vs FLOW

On 2023-12-26 by InspectApedia Publisher

@rndiy,

That was a good move cleaning the blower.

Because the curved blades on a squirrel cage fan don't really have much area inside the curve, it doesn't take much dust to make them nearly flat which significantly reduces their ability to push air.

On 2023-12-25 by rndiy

@InspectApedia Publisher, thank you for that suggestion. I use a 4" thick Honeywell MERV 8 pleated filter. So, I was able to access the blower wheel through the media cabinet hole. I cleaned the blower wheel a month ago using my shop-vac and a brush attachment. The fins were dusty but it was not thick or sticky.

On 2023-12-25 by InspectApedia Publisher - Cleaning the duct interior is probably the easiest improvement in air flow.

@rndiy,

Thank you for that update.

Cleaning the duct interior is probably the easiest improvement in air flow.

An interesting technical question is the difference in effect on airflow rate for ducks that are connected as you described it earlier backwards.

What I mean is that the constriction in the diameter of the duct interior is the same regardless of the direction of air flow so what's different is the effect of turbulence or other more subtle technical effects on air flow as it passes over the wrong way facing edge of the crimps.

I don't know the answer to this but it's going to be interesting to look for it.

If you're concerned about improving the air flow in your HVAC system, especially if it's older and as you have found, has or had accumulated dust and debris inside the ductwork, in my experience the single most significant cleaning chore and one that people ignore or simply don't know about is cleaning the squirrel cage blower fan in your air handler.

I've seen a greater than 50% improvement in air flow from that cleaning step. If you want more detail, we have an article about that at this website as well.

On 2023-12-25 by rndiy

@InspectApedia Publisher, Thank you for the reply. Yes, there are three 6" dia. openings/saddle takeoffs that bring return air from the return vents, one U-bend visible at the far end to get this return duct over a cross beam, a 8"-10" tapered increaser to connect this to the primary return trunk,

and finally a 10" collar connecting the primary return trunk to the return plenum. I was able to find ELs for those fittings in the Manual D tables. I cleaned the duct of that dust and debris as well.

On 2023-12-24 by InspectApedia Publisher - Return duct joint crimps pointing wrong way - how to calculate impact?

@rndiy,

I'll need to do more studying myself to give an engineering-line answer, buy my best guess is that you add up the effective length of reduced duct diameter to calculate the total effect on airflow through the duct system.

We've addressed this in our articles on the effect on flow in gpm (or in your case flow if air in lpm) of adding a section of larger (or smaller ) diameter water pipe into an existing piping system.

Keep in mind that there are quite a few other important factors that touch your air flow rate: debris I see in the piping also reduces the effective duct diameter AND increases friction losses. There seem to be some bends that also reduce air flow rate.

An interesting amateur alternative is to get actual objective data - using a velocimeter to measure air output at the blower and air output in lpm at the supply register at the end of the duct run.

On 2023-12-24 by rndiy

Return duct joint crimps pointing wrong way - how to calculate impact?

Hello. I was cleaning an 8" return duct and found the crimps were pointing upstream / against the airflow. My understanding was crimps should always be in the direction of the airflow. The crimp edges were bent further inward reducing the opening by about 1/2" all around (1st picture)

There are 6-7 joints like that on that duct run of 26 ft physical duct. The return duct is 8" for about 18ft, it increases to 10" dia. to the return plenum. There is another 10" return joining the plenum as well. The furnace is gas.

I cleaned up the duct but kept thinking what would the effect of the wrong way crimping be on the effective length of that joint, and if those joints make it too long an effective length.

ACCA Manual D have tables of various fittings and their equivalent lengths. So I thought of using those tables to calculate the EL on these wrong way joints. I drew a diagram of what I think the air will experience as it flows through that joint (2nd picture).

Two of ACCA tables seemed relevant; the tapered increaser and the abrupt increaseer (3rd picture). It wasn't clear what values to pick from those two ACCA tables. So I thought may be an EL of 10ft per joint? There are 7 joints like that. So, about 70ft added to the physical length of 26ft making the EL of that duct 96ft? (that's before adding the EL for the elbows etc.).

- Would like to know if calculating the EL is a reasonable way to estimate the effect of that joint?
- And, what values to select from the two ACCA tables based on the data provided in the 2rd diagram?

At this time, I am only trying to find a way to quantify the impact of these 6-7 wrong way crimps. Thank you.

On 2023-04-17 by InspectApedia Publisher

@Alexander Riccio,

Keep in mind you need enough light, a sharp lens, and plenty of borescope length to inspect the whole assembly without having to drill hundreds of holes and to be able to inspect longer inaccessible duct sections.

Better to rent a proper tool or hire a technician. I've done this task. A 30 inch dim borescope can't do what's needed.

On 2023-04-17 by Alexander Riccio

Pfft, you think that's an oddly crimped duct?

Check out what we found after demolition for the family kitchen renovation:

On 2023-04-17 by Alexander Riccio

@Doug, not gonna lie, if you just wanna see the inside of some ducts in your own home, get a cheap borescope camera on Amazon. Personally, I tell people to just get the USB camera ones, they plug into any computer and don't require you to install some sketchy software on your phone.

On 2022-06-01 by InspectApedia-911 (mod) - plumbing or duct inspection camera can examine otherwise inaccessible duct sections

@Doug,

Yes a plumbing or duct inspection camera can examine otherwise inaccessible duct sections.

Be sure you've read the Recommended Articles near the end of the page above - that may help diagnose your lack of air flow at your supply registers.

Below: the FLX-108REC from Shenzhen Flexi Electrical Co., LTD, an inspection camera sold by HVAC suppliers and other vendors and online, providing a 100 ft. cable and video screen on which is displayed the view of the cable-end camera.

Described as a "...pipe inspection camera supports video & audio record, picture snap, distance count, LED lights adjust, battery recharge and waterproof IP68(can work underwater at least 33ft/10m depth), it can inspect the pipeline dia from 1inch(25mm) - 8inch(200mm)."

On 2022-06-01 by Doug

I have some vents that air is not flowing through. They're open. Is there any type of probe (like a plumbing snake) made to inspect duct work to avoid taking down drywall?

On 2022-03-24 by Inspectapedia Com Moderator

@Mike,

Radiant heat through a floor uses electrical cables or hot water tubing from a boiler. There will be no cold air returns to the heating system.

The radiant tubing or cables warm the floor that in turn radiates heat into the occupied space above it.

On 2020-08-26 by Mike

So I’m in the process of finishing my basement and have a question. I have radiant floor heat throughout the basement and I was unsure about cold air returns. Do I need to add some as there is none? And if so should they be up high or down low. The upstairs is heated and cooled by forced air

On 2020-08-20 by danjoefriedman (mod) - running on high the air is not as cold as when its running on low

Barry

The system may be working normally; faster air movement across the cooling coil means the air won't be as cooled as when the air movement is more slow.

On 2020-08-17 by Barry

I have a question, I have a 2 stage ac unit, when its running on high the air is not as cold to when its running on low. This problem just started any thoughts?

On 2019-11-14 by danjoefriedman (mod) - duct system design size and defects

Charlie

I repeat your question and give a more-detailed reply now found at

DUCT SYSTEM DESIGN SIZE & DEFECTS - home

Please take a look and ask further questions as you need.

On 2019-11-13 by Charlie

Hi My name is Charlie I have a question? . Im installing a 3ton with Furnace unit 1200 CFM locating the unit in the middle of the basement

doing a extended trunk off each side from what i have looked up I can install 16x8 duct @ 700 CFM with two trunks would give me 1400 CFM. is this correct.

On 2019-07-17 by (mod) - white debris coming out of the air ducts

While a lab analysis might help point to a source, I'd start in the air handler, looking for a plastic component that's getting shredded.

On 2019-07-17 by Wil Dupuy

I have small bits of plastic coming out of my vents in all rooms. The particles are of different sizes some big as a grain of sand?

On 2019-06-29 by (mod) -

Perhaps you could answer description or attached some photos since I'm not quite sure what we're discussing

On 2019-06-28 by Ginger

Wondering what this boxed feature could be that is part of the duct. it is on the intake line about 10 feet before the furnace. Really can't image why it has a "wash" switch in the duct system.

On 2017-08-17 by Tim

We just had our A/C & Furnace replaced with a new unit that was very expensive and since the installation we are experiencing a high pitch whistling noise from the Return Air Plenum, the installers used the same plenum 12"x20" and bent a bunch of sheet metal to adapt to a larger return air grille 17"h x 30" w.

The company has sent their warranty tech out twice to no avail, they keep trying to make the return air plenum bigger by adding bent sheet metal. Everything I have read says the return air plenum needs to be much larger. I am looking for answers and getting very frustrated. Answers anyone?

3.5 ton Lennox A/C and Lennox variable speed Furnace (fan).

On 2017-07-24 by (mod) - get more air flow out of my ducts

Kent,

Did you try the diagnostics at

AIR FLOW IMPROVEMENT, HVAC ?

On 2017-07-23 by Kent kessler

Having trouble with air flow for a/c. I would like some insight. I can go into great detail but am checking to see if there is anyone here first

. It may take a little time for me to give all the info needed for someone to recognize the problem. So if your ready for a problem and would like to help me solve this please respond
Thanks kent

On 2016-11-08 by (mod) - R410a will have a temperature of 4.4 C (40 F) at 118.9 psi.

MD

There is no simple single number that would be a right answer to your question. The temperature of refrigerant leaving the compressor is a high pressure, high temperature gas that as it flows through the condenser cools to a liquid.

At the evaporator the temperature drops depending on the refrigerant metering rate into the evaporator coil and varies depending on the specific refrigerant (R410 vs. R22, etc), the pressure in the system as well as where along the evaporator (cooling coil) you take measurements.

Example: R410a will have a temperature of 4.4 C (40 F) at 118.9 psi.

If you use the search box just above and search InspectApedia.com for REFRIGERANT TEMPERATURE & PRESSURE you'll find both an explanatory article answering your question (there is no single right answer) as well as a PDF file with charts showing refrigerant temperature and pressure ranges.

On 2016-11-07 by md akram - What is temperature of refrigerant in evaporator

What is temperature of refrigerant in evaporator

On 2015-11-09 2 by (mod) - is there more than one air filter?

I can't tell from just your e-text, Randy.

Indeed some HVAC systems have multiple filters and sometimes at multiple locations.

Searching InspectApedia for "Find the Air Filter" recalled this helpful article

AIR FILTER LOCATION on finding all the air filters in a heating or air conditioning system.

On 2015-11-09 by randy Shafer

My central air unit is in the attic do I have more than one filter

On 2015-07-03 by kishan chauhan

duct ac all-ready gas charge presser 70/290 suction return not cooling

Question: My Air Conditioner won't turn off - what to do?

Carla said: My outside air unit will not turn off on its own. I have to manually turn the breaker off to turn it off. Does anyone know why or how I can fix it?

Reply:

Carla if your A/C won't turn off it could be that the thermostat is set to a temperature that the system cannot reach - due to lost cooling or due to a setting below the capability of the system. If your A/C won't turn off even if you set the thermostat to a temperature that is above the current room temp, then the thermostat or an A/C control board or switch is bad and needs replacement - in that case you need a service call from a trained HVAC Technician.

See OPERATING TEMPERATURES HVAC and

also CONTROLS & SWITCHES, A/C - HEAT PUMP

Question: My Air Conditioner (or heat pump) keeps cycling on and off too rapidly - what might be wrong? short A/C on-cycle problems

Joe said: short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!

Matthew said: short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old

Five causes of a home air conditioner compressor short cycling on and off too rapidly - Short Cycling Air Conditioner Diagnosis & Repair

(mod) said to Joe and Matthew (re questions just above):

1. Loss of air conditioner refrigerant -
   
   a refrigerant leak in the system. You may be able to repair the short cycling problem temporarily by recharging the air conditioning system but the proper repair is to find and fix the leak. Otherwise you will have to keep repeating the costly service call to just add refrigerant.
   
   See REFRIGERANT LEAK DETECTION.
2. A/C Coil Icing -
   
   the evaporator coil (cooling coil) is iced over (such as due to improper refrigerant charge or dirt or a reduced air flow due to a dirty filter) - take a look in the air handler to see if the coil is blocked by ice or dirt.
   
   See FROST BUILD-UP on AIR CONDITIONER COILS
3. Oversized air conditioner -
   
   if the short cycling has always been a problem since the day the system was installed, there is a good chance that the unit is too big (too many BTUS) for the space being cooled.
   
   A more subtle version of this same problem is that you've done something like closing doors or adding a partition that had the effect of reducing the size of the space being cooled. Sometimes we can mitigate this problem by running the blower fan at a lower speed or by opening interior doors to increase the size of the space being cooled or even by moving the thermostat.
   
   See AIR CONDITIONER BTU CHART
   
   and OPERATING TEMPERATURES HVAC
4. A/C control problem -
   
   it's less likely, but a damaged control board or switch could also be causing rapid equipment on-[off cycling.
5. Compressor damage or compressor start-troubles
   
   I've seen these other causes of air conditioner short cycling: if someone manually turns the air conditioner thermostat up and down or on and off too frequently, a hard-starting compressor may find that it has been shut down with high internal head pressure inside the compressor.
   
   Normally that head pressure bleeds off over time, making it easier for the compressor to re-start (against low head pressure) the next time it turns on.
   
   See HARD STARTING COMPRESSOR MOTORS

But if someone is turning the system on and off quickly, the compressor may have a hard time re-starting against the pressure on its outlet side. A starter capacitor addition or replacement might fix the problem. If your A/C compressor is showing this symptom but works OK if you leave it shut off for 30 minutes or longer, that may be the trouble.

We have also see or a damaged compressor internal refrigerant valve causing high head pressures;

In sum, you need a service call from a professional to correctly diagnose and repair the problem. Ask the service tech what she/he found and let us know - what you find will help other readers.

If your air conditioner or heat pump has the opposite problem, staying on too long,

see LONG-ON CYCLING AC COMPRESSOR.

Question: Why is my A/C not producing enough cool air?

A/C not producing enough cool air - I had my air conditioning system gassed up last week - $210. ! The unit is not producing enough cold air. The unit is set on 72 and does great at nights but during the day where the temp. outside is reaching mid 90"s it's getting up to 80 in the house. Is my duct work screwed up? How do you repair trailer metal ducting? - Amanda

90 degrees outside and 83 inside with thermometer at 76. Cools off to 76 when sun starts to go down and house then gets cold. New capacitor and condensor just put in. Help. - Ginny 5/17/12

Reply:

Amanda: if your system is not cooling there could be any of a number of problems - see the article above as a place to start. If your basic complaint is that the A/C temperature at the supply registers is cool enough but the volume of air flow is too weak, we'd start by:

• Install a new clean air filter
• Check that the air supply registers are open
• Check that ductwork has not become disconnected or crimped or crushed
• See DUCT SYSTEM DESIGN SIZE & DEFECTS where we list quite a few steps in diagnosing poor air flow out of an HVAC duct system

Weak air conditioner air flow - is there a relationship between refrigerant charge level and weak cool air flow rate?

Les said: Weak air conditioner air flow: Our A/C was serviced two months ago and the repairman said it had a leak. $400 later it was recharged with coolant and now the ac is doing the same thing. Very little pressure coming out of vents and no cold air coming out. Does anyone know what I can do for the weekend? It is stifling!

See DUCT & AIR FLOW PROBLEMS

(mod) said:

Les: A leak that was fixed by a re-charge is not as good a repair as a leak that was fixed by finding and fixing the leak - you'll just have to keep adding refrigerant.

But weak air flow out of the vents would not be due to a refrigerant leak; more likely a clogged filter or crushed or disconnected ductwork, or a blower fan problem.

Weak air Flow traced to Clogged Air Filters: Can Clogged Air Filters Affect the Room Thermostat?

(May 13, 2011) Jim said: If air conditioner filters are clogged will it cause the thermostat to shut off?

Jim: clogged A/C filters won't cause a room thermostat to shut off. The thermostat responds to room temperature.

However clogged A/C filters that reduce air flow, cause coil frosting, or otherwise reduce or stop the flow of cool air into the room where the thermostat is located would mean that the thermostat would remain "unsatisfied" and should mean that the thermostat says "on" - continuing to call for cooling.

See AIR FILTERS for HVAC SYSTEMS

Air Filters & Cooling System Amperage Draw: Could Removing the Air Filters Cause the Electric Motor in the Air Handler to Draw Higher Amps?

Question: got one for you. i put an ammeter on my air handler and it read 8.25 amps, I removed the filters and it went up to 9.75 lmao at the situation the amperage should have gone down. what gives here

Reply: causes of variations in electric motor efficiency and current draw measured in amps

Lost: this amps variation is beyond my expertise, but in general reducing the load on an electric motor will show up as lower amps or current draw, not higher amps. Here are two interesting explanations of amps or current variations on an electric motor that I found when researching the question:

1. Voltage variations and current draw at electric motors:

If your supply voltage is varying from your power company that can show up as higher amps draw on the motor (though it's a suspicious coincidence to see it exactly when you removed the filters and supposedly reduced the load on the motor).

Quoting from motorsanddrives [dot] com: "The effect of low voltage on electric motors is pretty widely known and ... The amount of power the motor draws is roughly related to the voltage times current (amps). Thus, when voltage gets low, the current must get higher to provide the same ... To summarize the situation, low voltage can cause high currents"

2. Load variations and electric motor efficiency:

A second possible source of seeing higher amps or current draw on your blower motor when you pulled out the air filters and thus reduced the load on the blower motor might be illuminated by this U.S. DOE pamphlet "Determining Electric Motor Load and Efficiency"

Quoting: "Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range of 5 to 10 hp; peak efficiency is at 7.5 hp. A motor’s efficiency tends to decrease dramatically below about 50% load."

Question: what causes warm suction lines in an air conditioner or heat pump system? Warm A/C suction line (should be cold) and zone control dampers that stopped working

JMONTE said: warm A/C suction line question: After my condenser is turned on for about 4 minutes the suction line starts to get warm to the touch. can you tell me what the problem may be

Reply:

JMONTE: If the HVAC suction line gets warm, you may be out of refrigerant, or the system may be running in heating mode if it's a heat pump.

See OPERATING TEMPERATURES HVAC.

Becky, If a motorized HVAC zone control damper is not opening or closing, most likely the motor has failed, or the thermostat that operates that zone control is off or set in an incorrect position.

See ZONE DAMPER CONTROLS

Becky said: motorized air conditioning zone dampers not working

My house has "zone" control with dampeners to close off the upstairs over the garage room unless that thermostat is on - the room is not cooling. I have located the damper under the house. What are some causes for the damper not opening and how to repair them? condensation, motor to damper? silicon glued properly?

...

Continue reading at SUPPLY DUCT AIR LEAKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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Recommended Articles

• DUCT SYSTEM DESIGN SIZE & DEFECTS - home
• AIR FLOW IMPROVEMENT, HVAC
• AIR FLOW TOO WEAK

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