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: this article explains how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperature, or an air conditioner that won't start.
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Duct problems: 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.
Watch out: Don't forget to check the easy stuff first: A client asked us 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.
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.
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.
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 A/C Filter Problems for details
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.
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
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.
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
see REFRIGERANT LEAK DETECTION
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.
This discussion is now found at COOL AIR SUPPLY IMPROVEMENT
This topic is discussed at WARM AIR SUPPLY IMPROVEMENT
Continue reading at SUPPLY DUCT AIR LEAKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see AIR FLOW IMPROVEMENT, HVAC
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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?
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
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
DanJoeFriedman (mod) said to Joe and Matthew (re questions just above):
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.
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
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:
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 A/C Air Duct Problems
DanJoeFriedman (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.
(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 "un-satisfied" and should mean that the thermostat says "on" - continuing to call for cooling. See AIR FILTERS for HVAC SYSTEMS
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
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."
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
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?
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