This article discusses the diagnosis and repair of cooling coil or evaporator coil problems that occur in the air conditioning or heat pump air handler unit such as frost or icing, dirt, blockage, refrigerant leaks, or improper sizing. Our photo at page top shows the cooling coil in the attic air handler component of a central air conditioning system.
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The liquid air conditioning refrigerant entering the cooling coil through the metering device (a capillary tube or THERMOSTATIC EXPANSION VALVE) is increasingly changed to gas form as it "boils" or evaporates as the liquid refrigerant flows through the cooling or "evaporator" coil, so that at the end of the cooling coil the refrigerant is totally in gaseous form.
[Click to enlarge any image]
This state change (liquid to gas refrigerant) absorbs energy, cooling the tubing and fins of the cooling coil and thus indirectly, cooling and dehumidifying indoor air that is blown across the coil.
A cooling coil which is blocked by debris or ice and frost, or which is damaged can obstruct air flow and reduce air conditioning system output. The air conditioning system evaporator coil and problems include ice and frost build-up, dirt or debris blocking air flow through the coil, and damaged or leaky cooling coils.
We also discuss how cooling coils may be cleaned in-place and what to watch out for during that procedure. Cooling coils which are part of an air conditioning retrofit installation onto an existing warm air heating system can also present special problems of sizing and air flow, discussed further at ADDING A/C: RETROFIT SIZING. Sketch of heat transfer at the inside coil, also called the cooling coil or evaporator coil, courtesy of Carson Dunlop Associates.
If ice, dirt, or damage block air flow across or through the cooling coil (evaporator coil) in an air conditioner, the cool air output will be substantially reduced or may even stop entirely.
Below we describe how the cooling coil works, what goes wrong with this component, and how its problems are diagnosed by simple visual inspection (inside of the air handler) or by some simple temperature measurements.
How To Inspect, Test, & Diagnose Cooling Coil (Evaporator Coil) Air Conditioner or Heat Pump Problems
First, make a visual inspection of the cooling coil. Most air handlers provide an access panel or cover that can be removed to give at least partial view of the cooling coil surfaces.
Turn off electrical power to the system to be safe from electrical shock.
On opening an access cover or panel on the air handler you can recognize the cooling coil from our photos and sketches shown here and elsewhere on this website. You may need to use a flashlight and mirror to see the coil surfaces.
Remember to inspect the cooling coil from the incoming-air side - the side of the coil facing the blower fan assembly. That's because any dirt or debris entering the coil will come principally from this direction. If you inspect the wrong side of the coil it may look perfectly clean even though it is totally blocked by debris on its other surface.
DIRTY COOLING COIL has photos of just how blocked a cooling coil can become in an air conditioner or heat pump.
Here are some common defects to look for at the evaporator coil (cooling coil) in an air conditioner or heat pump:
Temperature measurements at the cooling coil: see OPERATING TEMPERATURES for a discussion of where and how air temperature measurements are made to diagnose cooling coil or other air conditioner operating problems.
Below we introduce some of the more common air conditioner or heat pump cooling coil or evaporator coil defects and repairs.
Air flow requirements across the air conditioning evaporator coil: if airflow is weak for any reason (dirty coil, duct system defects, blower fan defects, dirty blower squirrel cage fan), the air conditioning system will not operate properly. Some experts write that there should be between 350 and 400 cubic feet of air per minute (CFM) moving across the evaporator (cooling) coil for each ton of air conditioner capacity.
One ton of cooling or heating capacity = 12,000 BTUH so if your AC unit or heat pump is a 24,000 BTUH unit it is a "two ton" unit and needs to see 700 to 800 CFM of air across the evaporator coil.
Some home inspectors and air conditioning service technicians carry a small airflow meter that can actually measure this number with fair accuracy. (The same tool is nice for comparing air flow and balancing air flow at various building supply ducts and registers.
How Air Conditioning & Heat Pump Evaporator Coils (Cooling coils) are Cleaned
Evaporator coil cleaning often requires cutting refrigerant lines, removal of the coil and other components for cleaning, and reinstallation, pulling a vacuum on the refrigerant lines, and recharge with refrigerant. Such service and repair may involve significant expense, although there are some "in place" cleaning methods using foams and sprays that are a simpler procedure. See DIRTY COIL CLEANING PROCEDURES for details of this topic.
FROST BUILD-UP - Frost Build-up on the Evaporator Coil in an Air Conditioner
The ice or frost formed on a cooling coil in an air conditioner air handler unit is usually caused by an improper refrigerant charge, possibly by inadequate air flow across the cooling coil, or by a thermostatic expansion valve (TEV) or other air conditioner or heat pump control defect.
Ice blocks air flow through the coil, thus reducing air conditioner output; if the ice formation is extreme nearly all of the airflow across the coil is blocked and the air conditioner system runs but does not produce cool air flowing into the occupied space.
Frost and ice can also form on refrigerant tubing at other locations, and frost and ice can form inside air conditioning duct work itself, leading to troublesome leaks into the building.
Details of what causes frost on air conditioning equipment, what problems that creates, and how to diagnose and repair icing or frost on cooling coils or other air conditioner parts are provided at FROST BUILD-UP on AIR CONDITIONER COILS. This article explains locations and causes of condensate, frost or ice formation in air conditioning systems, air handlers, compressor/condensers, refrigerant lines, and in air ducts.
Note that frost formation at some cooling coils (not air conditioners or dehumidifiers) may be normal. We discuss frosting and non-frosting cooling coil types and coil defrosting methods further at Frosting vs. Non-Frosting Types of Evaporator Coils
BLOCKED COOLING COIL - Air Conditioner Evaporator Coil Blocked by Debris or Dirt
Ice is not the only (nor even the most common) cause of blocked air flow in an air conditioner. This photograph shows how easily debris can stick to and clog the inlet side of the cooling coil in an air conditioning system. This evaporator coil was nearly totally blocked with dust and debris. How does this happen?
There was no air filter installed in the system. Ordinary house dust is comprised largely of fabric fibers and skin cells.
These and other debris in building dust such as soot and organic particles like pollen and mold spores all join to form a gray mat on the fins of the cooling coil in an air handler.
Debris sticks particularly quickly to this surface because of the combination of close spacing of the cooling fins (about 1/16" apart) and the fact that condensate forming on the coil keeps the surface damp.
Details about the detection and cleaning of dirt and debris which block an air conditioner cooling coil are at DIRTY COOLING COIL.
Temperature Measurements at the Cooling or Evaporator Coil
Reader Question: what should the air temperature be when leaving the cooling coil of an air conditioning system?
What is normally the air temperature leaving the evaporator?
I never looked into it but I think it should be as close to 32F (freezing) as possible (-- with out reaching freezing - Not to cause ice accumulation on the evaporator fins).
I want to put a thermometer by the evaporator fins and see how good the air cooling is, its probably an good indirect way to see if there is sufficient refrigerant in the system or if there is air or other gases mixed in with it -- making the cooling inefficient. - E.K.
Reply: look at the air temperature drop across the cooling coil rather than looking for an absolute or specific air temperature
Air temperature leaving the evaporator: there may be some standards that I don't know (probably are) but the way I look at it, because of variation in refrigerants, air speed across the evaporator, and temperature of the incoming air aimed at the evaporator, we look more at the temperature difference across the evaporator to see if it's doing its job.
Figure that 15-20 degF would be a good temperature drop across the coil for a typical air conditioning system. Other experts add that the temperature difference across an evaporator (cooling coil) may be as little as 14 degF or as much as 22 degF.
To a beginner HVAC service tech [DF] it was striking to see how dramatic and critical was the effect of airflow across the evaporator coil on coil behavior and coil frosting or icing.
During an actual case of refrigeration system diagnosis  (the coil was icing over and the system was not cooling) I sought to adjust the TEV to bring the frost line to the end of the cooling coil where it belonged. But one learns immediately that only if the blower fan was sending air across the cooling coil could one expect the system to behave as designed.
Without that airflow, at just about any TEV setting of refrigerant flow rate into the evaporator coil the coil would ice up quickly.
Temperature measurements at the cooling coil: see OPERATING TEMPERATURES for a detailed discussion of where and how air temperature measurements are made to diagnose cooling coil or other air conditioner operating problems.
Types of Evaporators or Evaporator Coils or Cooling Coils: Dry vs Flooded Evaporator Coil Designs
Dry Evaporator Coils: in a dry evaporator coil design, all of the refrigerant entering the evaporator coil enters as a vapor (or gas).
In a dry type evaporator coil (or cooling coil) the refrigerant oil travels constantly in the system along with the refrigerant, and some oil is discharged into the condenser. That is, only liquid refrigerant can actually carry oil.
In the evaporator the refrigerant is vaporized and the oil travels through, but the vapor is less capable of actually carrying the oil through the coil.
For the oil to pass through we need refrigerant gas velocity and turbulence in the evaporator coil, so we do not want much pressure drop across the evaporator coil.
Therefore dry evaporator type coils are usually short - to avoid much pressure drop.
Otherwise we get oil traps at the bends in the evaporator coil.
These are the more common type of evaporator coil or cooling coil in refrigeration systems. So, for example, for a small air conditioner that has to be packed into a small space, to keep the evaporator length short the manifold system may used to run several short evaporator loops in parallel - to avoid long individual tubing runs that might cause an ensuing refrigerant pressure drop and oil traps in the system.
[An oil trap will clog or prevent refrigerant flow through the evaporator and thus will prevent the system from working. A symptom might be loss of cooling and high refrigerant pressures on the high side]
Flooded Evaporator Coils: in a flooded evaporator coil design, the evaporator is constantly full of refrigerant, whether the cooling system is "on" or "off". See our cooling coil sketches shown here.
Our sketch (left) shows the basic layout of a commercial refrigeration system. Here we detail the difference between frosting and non-frosting evaporator coils and we explain how frosting-type systems must be defrosted to keep working.
Non-Frosting Evaporator or Cooling Coils - No Defrosting Needed
Dehumidifiers are examples of non-frosting type cooling coil designs. These devices are basically little "air conditioners" or cooling systems in their design (though their warm air output is exhausted directly into the same space). The dehumidifier system is a refrigeration system designed such that the coil will never form ice or frost.
Room air conditioners (portable or window or through wall units) are also examples of non-frosting type cooling coil designs.
These "frost-proof" or non-frosting systems (in normal operation) ar more difficult to charge: you must use a precisely measured charge or a temperature-sensing device and matching gauge with the temperature-sensing device - you find where the liquid ends in the evaporator line - where there is no further change in temperature in the evaporator coil tubing, there is no more liquid refrigerant present.
If you see ice or frost on these cooling coils it's an abnormal condition that needs to be diagnosed and repaired. See our diagnostic advice at FROST BUILD-UP on AIR CONDITIONER COILS
Defrosting Methods for Cooling Coils (Evaporator Coils) in Refrigeration Systems
Frosting Evaporator or Cooling Coils Require a Defrost Cycle
Examples of frosting evaporator coils or cooling coils include refrigerators (or freezers). When more than 1/4 of the surface is ice or frost that condition acts as an insulator that reduces the efficiency of the appliance, so the appliance will have to defrost itself - either automatically or manually.
There are two defrosting methods commonly used in frosting-evaporator coil designs:
Defrost by electrical resistance heating (common on refrigerators, including frost-resistance for door faces and jambs using extra resistance heating elements in those areas too);
Defrost by hot gas: a solenoid in the compressor discharge line shuts [sketch above left] down vapor from the condenser and deposits high pressure/high temperature refrigerant gas directly into the evaporator coil, bypassing the refrigerant metering valve (TEV or cap tube).
The problem with dumping high temperature refrigerant vapor into the cold (iced, needs defrosting) evaporator is that it causes it to begin to condense - back pressure of the gas goes up and head pressure at the condenser goes down - now liquid refrigerant can back up to the compressor (where it would cause damage).
To avoid compressor damage from liquid refrigerant during this defrost cycle we add heat at the end of the evaporator coil (cooling coil) to insure that refrigerant reenters the compressor as a vapor, never as a liquid. Typically, setting a maximum of 20 minutes of defrost cycle adds protection against warming up food in the refrigerator or freezer where this design is used.
The refrigeration compressor continues to run during the defrost cycle in the hot gas method case, but the compressor will not keep running during the defrost cycle in the electrical resistance defrost cycle method.
When is cooling coil frosting abnormal?
Note that on dehumidifiers and air conditioners or heat pumps frost or ice formation on the cooling coil is not normal and is an indication of the need for repairs. See our diagnostic advice at FROST BUILD-UP on AIR CONDITIONER COILS
How cooling coils are changed-out or replaced
When an evaporator coil or cooling coil needs replacement (perhaps because the old one is damaged or leaky):
If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.
See How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building.
Page top photo of an iced-up air conditioning evaporator coil are courtesy Guy Benfante.
Continue reading at DIRTY COOLING COIL or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: why is the coil raining water droplets?
(May 27, 2011) DEVIN said:
What can cause an evaporator coil to rain droplets of water , the water is not running down the coil but falling off. Very little but enough to make a puddle a day. The coil has been cleaned several months ago. And has a proper trap , and seem to have plenty of air flow
It is normal for moisture to collect on the evaporator coil during the cooling cycle, as moisture condenses out of warm moisture-laden air onto the evaporator coil (cooling coil) surface.
If that condensate is not being properly drained, however, the drain system needs repair lest you have a leak into the building.
Question: air coming from duct is not blowing enough cool air
(June 1, 2011) Y Riggans said:
Air coming from duct is not blowing cool enough air. Temp inside with central air on was still at 85 after 2 hrs and temp outside was extremely hot today 6-1-11. Worked up until today!
If air is coming out of supply registers at a good rate but the air is never cool then your system is not cooling. Common causes are lost refrigerant or a refrigerant metering device (thermostatic expansion valve or cap tube) that is not working. Check out the LOST COOLING CAPACITY diagnostic article linked to at page left.
Question: Fan on AC unit stopped working
(June 15, 2011) Rod said:
Fan on A/C unit stopped working, replaced capcitors, fan is still not coming on I can hear the condenser coming on any ideas?
Question: Replacement A-Coil (cooling coil) was too small?
(July 18, 2011) David said:
If our a-coil was replaced by one not large enough would that cause our cooling not to be good enough when the it is very hot outside? Ours was replaced because it was leaking and now when it is very hot outside it wont cool the house more than into the 80's. Year before that we had a new outside condenser unit put in.
David: if the new A-coil in your indoor air handler is significantly smaller than the one that was taken out, it may be that it's not providing enough cooling surface for the volume and rate of air being blown across it. Take a look inside the air handler to see if there seems to be a larger (than before) space through which air moving through the air handler is actually bypassing the cooling coil and ask your HVAC tech about that observation. Let us know.
The a-coil is a little smaller but supposed to be ok for our 3 ton system. The tech said no leaks but added 2lbs of freon (10 months since last added). Said our compressor valves were bad because of noise he heard. Payne compressor only 3 years old. Replaced compressor but hasn't been hot enough to see if it made a difference.
David, an HVAC tech that added two pounds of refrigerant after one year of system use and told you that there are no refrigerant leaks leaves me mystified. Unless your system was short-charged in the first place, having to add refrigerant always means that we've lost refrigerant from a leak. Take a look at the article linked at page left titled REFRIGERANT CHARGING PROCEDURE
Question: Air handler stops and stdarts too often
(July 22, 2011) Bill said:
My air conditioner goes off and my air handler stops and seconds later comes back on. This will continue for several minutes before it shuts down. Any ideas of problem?
Bill an air handler that stops and and starts very frequently - seconds apart - sounds like a thermostat or control board problem.
Question: intermittent delivery of cool air
(July 23, 2011) Marc said:
Our central air runs when we turn it on, but sometimes it blows cool air and sometimes it doesn't. Sometimes the air pressure is strong and sometimes it not. Sometimes the air will blow for 30 min and sometimes 3 hours. On a good day it doesn't seem to cool the house past 78 degrees. The last 2 days in the upper 90's the ac would run, but there was little to no air pressure coming out and if there was a little air pressure coming out, it was not cool air.
Our indoor house temperature read 85 degrees. Today it is blowing hard and the air is pretty cool, but seems to still take a while to cool the house down and doesn't get below 78 degrees (It is about 90 degrees out today outside). I think the air coming out should be cooler as I have been in other houses with central air that are cooler. We have had this problem the past 3 summers.
We had a guys take a look and he said it could be a small leak somewhere and he just recharged the system. I don't want to call out the guy again if I don't have to. Could the problem be a leak still or something else because it works semi properly inconsistently. Any help would be greatly appreciated. Thanks.
I'd look for a control problem or an automatic duct damper problem since you note that when the system runs it's cooling capacity is satisfactory
Question: Condensate doesn't drain when blower is running and overflows drain when blower stops
(Aug 3, 2011) Mark said:
I have a horizontal fan/coil unit that wont let the condensate drain while the blower is operating and when the blower shuts off , the water comes out faster than the drain can handle it and overflows the pan
Mark it sounds as if there are two problems:
1. you may be getting condensate blown down the ductwork during system operation
2. your condensate drain is clogged or undersized and the pan may also be undersized.
I'd ask an HVAC tech to look at and correct both problems, though first you might see if you can clear your condensate drain yourself.
Question: Lennox heat pump compressor locked up
(Aug 18, 2011) David S said:
In December of 2010 my Lennox heat pump compressor “locked up” and my service tech Larry Luker gave me two options
1) replace the faulty compressor
2) replace the entire outside unit and air handler unit evaporator coil. Because the existing unit was 10 years old I opted to replace the unit and evaporator coil.
The home is a three level townhouse with HVAC zone control and I never had any real problems with heating or cooling before this event. Once the unit was replaced the heating cycle worked fine and we had a mild spring and I rarely used the cooling cycle.
I got married in April and in mid May we rented the unit to a nice couple. Just after they moved in the weather turned hot, very hot and the AC started to act up. The unit would cut off and not restart for extended periods of time and we would call our service tech and he would come over and say that the thermostats were’t in sync or there was a stuck bulb in one of the thermostats or the unit was low on freon, the unit was froze up because the thermostats were’t all set the same, or there was a problem with the zone board etc etc.
All in all he made about seven or eight trips and in desperation we had the zone board and all the thermostats replaced, the freon topped in hope to eliminate the real issue. The unit would eventually fired back up and run for 5 or 6 days and then cut off once again. Finally the compressor just refused to start but when it did attempt to start the circuit breaker that feeds the outside unit would buzz (not trip just buzz).
The tech put a hard start capacitor on the unit to get the compressor to start but when it did start it sounded like it had a bunch of bolts rattling around in side. Our tech call Lennox locally and they said that all they could do is give him a replacement compressor (they only had one left so it sounds like this may be a bad run or design).
I called consumer affairs and pleaded for a new unit but the person on the phone told me that all Lennox would do is “furnish a replacement compressor”. I ask to speak to a supervisor and held for one hour and got cut off, I called back twice and left two messages and never got a call back.
When we took the door off the air handler to get the nomenclature off of the new evaporator coil we found found that it is substantially smaller that the original coil and I thinking that this could be the reason the unit keeps freezing up.
Our tech put in the new compressor and the unit ran for a week and now it is down again. This unit has cost us well over $10,000.00 now and we still don’t have a reliable HVAC system and will probably loose our tenants and they will potentially take legal action against us.
Question: intermittent start up problems at air handler
(June 3, 2012) joe g said:
After I turn my a/c unit on sometimes my evaporator unit will turn off then on again maybe once or twice or sometimes never?? While the compressor would be running with out going off .
Look for a control problem, wiring short, or failing start capacitor
Question: inadequate cool air supply
(June 14, 2012) Dan said:
My air conditioner blower motor is running but there is inadequate air flow coming through the vents, this just started today and was running fine prior
June 17, 2012) Anonymous said:
check ur filters Dan and the inside coil/evaporator and see if it's dirty
Thanks Anon, we agree completely. When airflow in an air conditioning system is inadequate the place to start is with a dirty air filter.
Other sources of reduced airflow include:
a dirty blower fan assembly
Question: HVAC unit not working; tech added refrigerant
(July 11, 2012) Sweaty Jane in GA said:
After one year, our HVAC multi-zone unit (Bryant) is not running at all. Last year our tech had to put in more freon and then in December 2011, he had to add more, because the heat was not working. It worked fine for 3 months, then out again in April. Now, he says we have a freon leak, and need new evaporator coils, after a supposed spring tune-up and more freon. Will the new coils fix the leaking freon or could it be elsewhere?
Reply: regular addition of refrigerant is poor practice - find and fix the leak
Indeed the freon leak needs to be repaired - otherwise your system is both contaminating the environment and creating a regular delivery route for the A/C guy. I can't say if the leak is in the coils or elsewhere - so before replacing the coils, ask to be shown where the leaks are occurring.
Question: do air filters restrict airflow at the cooling coi8?
(Aug 7, 2012) Judy said:
Do the air filters for the evaporator coils restrict the air flow around the coils on my manufactured home Nordyne unit as the handy man who came to repair the outside unit told me?
Do air filters block air from flowing over the evaporator coils?
Well maybe: if the air filters are dirty they restrict air flow. If the air filters are reasonably clean then the amount of air flow restriction is within the design limits of the equipment. Certainly DO NOT operate the system with no air filters installed. Doing so will allow dust and crud to accumulate inside the equipment, leading to more serious blocked air flow and an expensive cleaning bill.
Question: frost on the TEV
(Oct 18, 2012) Robert firstname.lastname@example.org said:
I have frost build up on the TEV and condenser inlet tubing of my geospring heat pump water heater. I had an error message of "clean filter" even though filter was clean followed by "heat pump failure" I noticed temp at inlet was 15f which is right at bottom of normal range. I think this temp goes below 15f triggering error message. Would this be a sign of leak in the sealed system? How about faulty TEV? I am DIY'r with no HVAC experience but time on my hands. How can I test TEV or if a leak in system? The frost does not extend into the condenser coils itself, just on the inlet tubing and TEV.
A dirty filter can slow air flow at and thus frost up the coil;
But frosting at the TEV may indicate low refrigerant or a dirt-clogged or failing TEV.
Question: AC unit freezing up - added freon, now it's freezing up again
(Oct 22, 2012) Tracy said:
One month ago we had an ac serviceman come out because our unit was freezing up. He cleaned out the system and put in more freon. Well, it is doing the same thing again. What needs done now? We can't afford to keep having service calls.
Frosting can be caused by low refrigerant; but adding refrigerant is only a temporary fix since without finding the leak the refrigerant just leaks out again.
(Mar 4, 2013) sharon said:
H how much to repair a heat pump coil.
Question: Anon claims bad capacitor causes freeze-up if there is not enough freon
Mar 8, 2013) Anonymous said:
capacitors normally cause there freezing if there is enough freon.
Anon - the capacitor is an electrical device that starts an electric motor or keeps it running - that device has nothing to do with the freon charge.
Question: what are the signs of a freeze-up at the heat pump?
(Jan 31, 2014) Anonymous said:
what are the symptoms of a heat pump Freezing up.
If by freeze-up you mean literally, that would not be likely to occur unless the unit heater or base heater failed or lost power. The unit would not work, and quite possibly the compressor motor would be destroyed if someone tried to run the unit. You might hear horrible clanging, or perhaps just a hum with no compressor operation.
About the prior remark, I'll be darned if I can understand the relationship someone suggested between the capacitor and freezing because of not enough freon. This looks odd to me. A capacitor is an electrical component that assists in starting an electric motor. It knows nothing about freon levels.
Question: condensate leaks out of the air handler or blows into the duct work
(Apr 14, 2014) MIKE said:
My issue has to do with condensation dripping off the coils onto the blower housing, then out any crack it can find. The issue has nothing to do with the coils freezing, or the condensation drains being plugged, a lot of the condensation is not draining off the coils fast enough to make it to the drains, it drips onto the blower housing
This is a common problem in HVAC systems with high air movement rates and high indoor humidity. Usually the HVAC tech adjusts the system's operating rate to dehumidify the building adequately and, provided there is not an unusual indoor moisture source (wet crawl space, leaks) the problem is not chronic. If your unit has an adjustable fan speed, run the system at a slower speed for longer intervals to solve this problem. If an HVAC system is over-sized (too much cooling capacity) the condensate blow-off problem is often worse because the system cools down the building too fast - before it has also removed enough moisture.
Question: why do we always connect the refrigerant inlet at the bottom of the coil?
(Apr 21, 2014) anil kumar said:
please give me the clarification abt cooling coil connections.Why we connect inlet all ways at bottom of the coil.
Reply: we don't
Anil it's not clear to me that the the inlet is always at the bottom of a cooling or evaporator coil. For example some refrigerator designs insert refrigerant at an evaporator coil top - allowing it to flow downwards through the coil as it changes from liquid to gas (absorbing heat and thus cooling the coil).
You'll see cooling coils in a variety of positions depending on the system design.
If you want to pass on to me where you read that "the coil inlet always connects at the coil bottom" I'll research the question further but on the face of it I think the premise of your question is incorrect.
Question: is it OK to re-use the drip pan overflow safety switch?
(June 16, 2014) Phillip said:
Can the auto shut down water detectors be safely reused after they have been soaked in water and have shut down the AC system one time, or do they have to be replaced once they've done their job?
That's a good question. The switch is expected to shut down the system before it has itself been flooded. If that's what happens the switch does not need to be replaced. Testing it would be easy - just set the switch in a pan of water or pour some water into the drip tray.
But if a switch was actually flooded and if its design and wiring connections were not intended for that submersion it would be unreliable in the future - risking internal corrosion for example - and I'd replace it. It's an inexpensive part.
If you give us a switch part number and brand we can research the specific details.
Some condensate switches such as the Condensate Cop are sealed units that operate magnetically - and should not be damaged by flooding.
See these articles on HVAC condensate trays, pans, and switches
Question: worried about wet cooling coil after cleaning off mold
(June 30, 2014) Diane said:
Just had AC cleaned it was full of mold. The cooling coil is wet all the time. the mold will grow back. My health is suffering. Why does th coil stay wet?
It's normal for the cooling coil to be wet because moisture from air passing over the coil condenses on the cool surfaces.
Question: leaky condensate pan "should be inspected"
(July 6, 2014) Rick said:
I am purchasing a home that has a 4 ton York AC system. My home inspector indicated that the condensate pan should be inspected since there is evidence of a leak (rust stain) on the side of the air handler. The seller had an AC tech certify the system but he did not inspect the pan. I called a HVAC company and was told it typically takes an hour or two to open the unit and that they normally replace the coils if they are spending that much time. That sounds like BS. Is it difficult to do this inspection? How much does it cost to replace a condensate pan? Can the pan be replaced without replacing the coils or does that depend on the unit?
Reply: goofy inspection report - the inspector was afraid to call for "repair" but that's what's needed
In my OPINION telling you to have an obvious defect "inspected" is a CYA move - if the condensate pan is leaking it needs repair or replacement, and a check of what got wet and what damage may have ensued is needed too.
"Certification" such as you describe is of no value.
The ability to replace the condensate pan without having to cut and repair refrigerant piping and other components depends on where and how the unit was installed. In some cases, yes the replacement is trivial, in others not at all. Sorry I can't see your system so can't say which.
But certainly replacing a condensate pan alone with no other system wear or damage is no reason to replace a cooling coil unless the coil is itself corroded or damaged.
Question: lost cooling at Carrier 38TRA024311 and inside I have a Air Handler Model FF1B - cleaning the coil helped
7/12/14 Peter said:
I have a 14 year old unit for my apartment. Outside I have a Carrier 38TRA024311 and inside I have a Air Handler Model FF1B. I noticed the apartment was not cooling. When I looked into the Air Handler the evaporator coil was completed frosted. I also noticed the blower motor not working. I shut off power over night. I thought at first that the coil was frosting over because there was no air moving. But I was able to get the blower going by manually spinning the squirrel cage; I will replace the capacitor?
In any event - the blower running did NOT fix the frost build up. I tested it this morning and in just a few minutes the copper tubing was totally white and soon followed by the complete frosting of the evap coils. I shut it down - I know otherwise I can damage the outdoor compressor. The coils don't look that dirty - obviously I will clean - but what do you think? I read low refrigerant can cause this. The evap unit looks surprisingly good but I know there are expansion valves etc that I can't see. Hot here in Florida! Thanks.
DanJoe - part of the reason I am even tackling this is because the a/c on my old Subaru (also 14 - don't like to throwaway things) stopped working and the dealer wanted $920 to replace what he said was a bad compressor. But I could see the compressor was spinning. Anyway. I bought some gauges and a Robinaire vacuum pump and a new expansion valve.
Replaced the expansion valve, vacuumed system and reloaded with R134 and now my Subaru a/c is now working fine. In my reading, they said that the expansion valve frequently caused ice build up. The valve was a pain to change behind dash.
My guess is that it may have been the valve because after I vacuumed the lines and turned the pump off - it held a vacuum - no leaks - so maybe it was the refrigerant level? Anyway I mention that because I wonder if it is possible for a layman like myself to even change the expansion valve on my air handler system? The more I look at my coils - I realize I do need to clean them. Any products that you suggest? I plan on ordering (Carrier Products P291-0503 RUN CAPAC 5 MFD 370V OVAL) capacitor from partsmart.us - only $4. Thanks in advance for your patience.
DanJoe - I went out any bought some foaming cleaner for my evaporator coils. They are dirtier than I first thought. First pass - I still get frosting on coils and piping but not as much. Running a/c now to try and cool down the apartment. I ordered a 'Ge 5x370 Run Capacitor 5uf 370 Vac' capacitor from Amazon and will replace this week. (Right now I am manually starting blower cage.) I am beginning to wonder if my freon is low and that is the cause of the icing. I will call a tech in this week to check but I have two questions. Question 1. I have a set of manifold gauges that I used to fix my 14 y.o. Subaru's a/c this spring. Can I use them on my balcony unit to see psi charge? - what are normal readings? Question 2 - will the new capacitor improve my blower or simply eliminate my need to spin the squirrel cage. Thanks. Can't believe how great this site is!!!
Peter, in addition to replacing a fan motor start capacitor it sounds as if there is a low refrigerant problem or a refrigerant metering device problem frosting the coil.
Cleaning the coil was a good diagnostic move.
Do not hook up the gauges as you describe. Without also using a gas canister you may contaminate the system or lose more refrigerant. And pressure readings alone don't fully diagnose the trouble.
About a DIY expansion valve replacement, if cutting and soldering refrigerant lines is required there is risk of contaminating the system.
Thanks for the reply. Dan Joe - first of all this is such an awesome site! The frosting was because my coils were dirty. Two cans of 'a/c safe foaming cleaner' from Home depot did the trick for $12. I will now stay on top of maintenance. Any thoughts on the capacitor? Does it just help to start the blower motor? I will get it this week. Right now I have Fan Speed set to 'ON'.
Your description of a fan that will run if given a starting spin is a good diagnostic of a failed motor start capacitor. (Sometimes the motor itself is failing and ultimately needs replacement). I'd replace the capacitor or add one - see
Keep me posted
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