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Air conditioner air flow too weak:
How to diagnose weak or too-warm air flow from an air conditioning or heat pump system.
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.
How to Diagnose Air Conditioner Output Cool Air Flow Too Weak, Too Cold, or Too Warm
WEAK AIR FLOW: is the air conditioner (or when heating, the heating system) blower fan unit not moving enough air, so that there is too little air coming out of your air supply registers? Here is a list of things to check, in the best order.
2. Check for a dirty or clogged air filter: replace the filter.
Watch out: some HVAC systems have more than one air filter in more than one location. Be sure you've found all of them. For example a filter may be located at a central return air inlet grille and another may be located at the air handler itself.
Clogged Air Conditioner filters can lead to lost cooling capacity first, because the clogged filter reduces the air flow
through the system, meaning that you'll feel less air flow at the supply registers than was previously present.
3. Check for a dirty or broken blower fan: if the squirrel cage fan in the blower unit is dirty the blower may be spinning but not moving much air. If the blower is a belt-driven unit check that the belt is intact and that the blower spins.
4. For Cooling Systems: Check for an iced cooling coil inside the air handler; a refrigerant leak can cause frosting while later the when more refrigerant has been lost the result is delivery of inadequately-cooled air.
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.
5. Check the ductwork for any cause of blockage, partial blockage (crimps, excessive bends), leaks, disconnects. Blocked, crimped or disconnected air ducts can also cause loss of cool air or too little cool air coming out of supply registers.
Reader Comment: faulty HVAC Duct Fire Damper Blocks Airflow
2016/01/22 Peter said:
No air coming from vents in your commercial building? We had a 7 ton unit that was supposed to heat and cool two big rooms and some bathrooms. Nothing was heating up those rooms. The 7 ton Rudd was new, everything was checked, even a bigger gas pipe was put to it. While the ducts at coming directly from the 7 ton were shaking with air flow, no air flow was coming from the ducts in the rooms, which were about 50 feet away. At first we thought it was the ductwork design. It literally had about 15 turns in it to get from the roof to the building. We thought it might be leaks. Perhaps insulating the roof ductwork.
Because nothing seemed to fix the problem, and we had tenants, we spent a fortune putting in at first, temporary radiant heat, and then permanent electric radiant heat. After months of trying to figure this out, my HVAC guy finally discovered the problem. Commercial ducts have Fire dampers in them. Fire dampers are passive fire protection products used in heating, ventilation, and air conditioning (HVAC) ducts to prevent the spread of fire inside the ductwork through fire-resistance rated walls and floors. Fire/smoke dampers are similar to fire dampers in fire resistance rating, and also prevent the spread of smoke inside the ducts.
When a rise in temperature occurs, the fire damper closes, usually activated by a thermal element which melts at temperatures higher than ambient but low enough to indicate the presence of a fire, allowing springs to close the damper blades. So, apparently with us, a fire damper in our ductwork decided to close. We easily spent thousands and thousands in emergency heating only to find out it was a defective $100 fire damper.
Thank you for the important report on the big hidden cost of a defective fire damper on heating costs. While I've made the general point about looking for duct obstructions I'd not considered fire dampers, devices designed to "fail" in the closed position for fire safety. I'll add your remarks to the article as you will surely thus help other readers.
Other HVAC Air Delivery Problems: no air flow, air flow too warm, air too humid
NO AIR: Is there no cool air at all coming out of the supply registers at all?
Check all of the heating and cooling controls to be sure that they are set properly and working normally. The thermostat is set to COOL, FAN to AUTO or ON, HEAT to OFF, and the set temperature on the thermostat is set below room temperature (if you want cooling).
SNAFUs in the individual controls and relays and switches for the air handler, blower, compressor/condenser unit and refrigerant metering equipment will, if not working properly, lead to these same complaints.
Check the following causes of complete loss of air flow
Are there closed supply or return registers, or a closed ductwork damper.
Is the thermostat not calling for cooling or heating as appropriate
Has the system lost electrical power
Is there a blower unit that is not operating because a blower door is open (door switch shuts off the system)
is there a condensate pan switch that has shut down the blower assembly, disconnected ductwork
Is there a completely frost-blocked cooling coil
AIR TOO WARM: Or is there air blowing out of the supply registers but it's not cool enough?
Typical causes of too-warm air flow out of an HVAC system in cooling mode are
Refrigerant leaks & lost refrigerant
Air flow moving too fast across the cooling coil
Backup heat is stuck on (discussed below)
Insulation missing from the duct system
Ducts routed through a hot, un-conditioned space
AIR TOO HUMID: if the air conditioning system is not dehumidifying, and presuming no one has left windows or doors open to humid outdoor air, air flow may be too rapid or the system may be over-sized.
See DEHUMIDIFICATION PROBLEMS
HVAC Control SNAFUs That Can Lead to Too-Warm Air Conditioning or Weak Air Flow
As you read below, a reader (Pete) reminded us that even when the A/C system seems to be working properly a too-warm air output problem in a heat pump system could be due to emergency or backup-heat being left in the "ON" position.
Question: why is my heat pump blowing warm air?
4/30/2014 Pete said:
I have an old unit that will not blow cool/cold air. I have had a tech out twice and I am trying to see if there are any things that I can check or ask my technician to check. The tech had the unit working properly, after his last visit, until about 2 weeks ago. At that time we had a cold-snap and I had to turn my heat back on. When I turned it back to AC it has not worked properly since.
The outside unit (condenser) is running as is the air handler. Plenty of air is blowing out of the registers but it is only about 70-71 degrees. I do not know anything about fixing ACs but I am pretty handy and I have been researching on the web.
Some of the viable options I found are a stuck or bad Reversing Valve, coolant line could be blocked or contaminated or the Thermostatic Expansion Valve could be bad. Does that sound right? Any thoughts? Also, I noticed that the high pressure line (small copper tubing, I think) coming out of the outside condenser is cold to the touch. Should that not be warm/hot? I really would like to get one more season out of the unit. PS: all filters are clean.
With the benefit of hind-sight - your comments below, that cold refrigerant line was an important clue.
Reader follow-up: (2 hours later) Problem solved: emergency heat stuck on
Found problem. AC was working but the emergency heat strips were stuck on. So the cold air was getting heated. Replaced and is working. Hooray!
Fix Existing Air Conditioning Problems Before Trying to Improve its Cooling Capacity
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.
Questions & answers or comments about air conditioner system diagnosis & repair
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Timothy Hemm, Yucala, CA, contributed photographs of electrical wiring and equipment installed in California buildings. Mr. Hemm can be contacted at TimHemm@yahoo.com
A/C - HEAT PUMP CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
Thanks to reader and research scientist Cyril Roberts, Barbados, for technical discussion and investigation of air conditioning system dehumidification problems (April 2009).
Thanks to readers Beth & Dennis for asking about how to improve an inadequate air conditioning system supplying cool air through crawl space ducts and floor registers. (May 2010).
Thanks to reader William Smith for discussing cooling coil leaks and lost cooling capacity diagnosis - June 2010
Thanks to reader Jacob Behrends, FL for discussing how a clogged condensate drain line can overflow condensate into a condensate pan that in turn may contain a safety switch that shuts down the whole air conditioning system. August 2010.
Determining Electric Motor Load and Efficiency, U.S. Department of Energy, web search 08/01/2011, original source: http://www.p2pays.org/ref/40/39569.pdf [copy on file at InspectAPedia.com]
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
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