Effects on A/C or heat pump system operation due to cooling coil ice & frost formation.

This air conditioning repair article series discusses evaporator coil icing: the problems of ice and frost formation in air conditioning system air handler units, blower units, or AHU's, duct work, or other air conditioning system components.

A freezing or frosted A/C coil blocks air flow and leads to loss of cooling.

The air conditioning system evaporator coil and problems of frost build-up on the air conditioning coil are explained and diagnosed here Our page top photograph of a thoroughly ice-blocked air conditioner evaporator coil was contributed by a reader who described: "I cleaned the coils & installed a new filter - obviously I have a low refrigerant problem. This is an 11 year old furnace/air handler with no history of other problems, but low on Freon."

Effects of Coil Frost & Ice in the HVAC System

What Happens to the Ability of the Cooling System to Cool the Air When an Evaporator Coil Ices Up?

When the cooling coil has a nice thick ice build-up on its surface there will be no cool air produced by the air conditioning system at all. The fan runs, outside compressor/condenser run, but little or no air moves through the duct system. The page top photograph shows icing on the cooling coil and refrigerant lines exiting the coil inside air handler close to the evaporator coil even.

You might see ice formation on the suction line just outside of your air handler even though you cannot see the evaporator coil itself - on most residential air conditioning systems, the surfaces of the cooling coil are not readily accessible by the homeowner. But if you don't see ice on the suction line, ice could still be present on and blocking air flow through the cooling coil.

The cooling coil, or evaporator coil is visible if the air handler is opened on some air conditioner units. At other installations the cooling coil is completely covered and can't be seen at its location (say on a retrofit installation atop an existing hot air furnace) unless an inspection opening has been made (by cutting the steel and installing an access panel cover), or unless there is an opening that was made previously to install a humidifier in the same plenum chamber.

When an air conditioning system with a frost-blocked coil is turned off and allowed to warm up the ice on the coil melts and spills into the internal condensate collector tray in the air handler. Then when the air conditioner is re-started it may for a while produce cool air before becoming ice blocked again. If an air conditioning system behaves in this way coil icing is a possible explanation.

Frost build-up indicates an air flow or refrigerant problem. A blocked coil (by dirt) or a blower fan which has lost its ability to move air (such as a dirty squirrel cage fan) will reduce air movement across the coil and lead to frost build up there. We suspect this is the more common cause of this defect. We discuss the problem of dirt on the cooling coil slowing air flow and leading to ice-build-up

at DIRTY COOLING COIL / EVAPORATOR COIL.

[Photograph of ice formation at the suction line of an air conditioning compressor/condenser unit (and some odd insulation there) courtesy of Mark Cramer a past president of ASHI and a Florida home inspector.]

Reader Questions: What causes ice to form on the cooling coil in an air conditioner or heat pump?

the unit get frozen,cleaned, freon ok,could it be the compressor/ - Anon 6/6/11

how often is the compressor the problem with icing - Anon 6/6/11 -

My friends condensor coil is iced up and his house is warm. The outside temp is 89. What can be the cause and if we run a hose on the coil will it help the problem by melting the ice ? - Jack Christian 7/8/11

I have a small haier air conditioner window until, you can open it right up in the front and see all the exposed ice. What would most likely be the problem and how could I find it to fix it? - Brianna Shollenberger 7/8/11

The article has cleared some doubts which I had since a long time. As I understand now, likely causes of ice formation are unclean filter/coil, incorrect amount of refrigerant, or thermostat not working properly. Hope this helps to solve problem I have been having with our bedroom AC. Ice starts forming if I set the temperature to 24 degrees centigrade, or less. Without cooling, the room temperature is around 30 degrees. - Ramesh Chopra 7/17/2011

Reply: the most common causes of ice formation in the air conditioner are:

Ice formation inside the equipment on the coils or refrigerant metering devices is not a normal operating condition, but it's a common defect often traced to

• Any problem that has stopped or reduced air flow through the system, such as a dirty air filter or crimped or disconnected air ducts. Collapsed or crimped air ducts, collapsed duct interior insulation, a closed duct damper, closed supply registers, or a dirty blower fan in the air handler unit, even a mis-wired thermostat or bad system control board can also cause reduced or halted air flow. Find and fix the air blockage.
• Low or lost refrigerant - can cause evaporator coil icing because of abnormally low pressures on the low side of the cooling system. Often this condition deteriorates to no more coil icing and no cooling whatsoever as the refrigerant leak continues and eventually there is just little or no refrigerant gas left in the system. Find and fix the leak and recharge the system.
• A refrigerant metering device problem such as a dirty or mi s-adjusted thermal expansion valve or a partly-blocked capillary tube. Dirt in the system, compressor oil circulating into a capillary tube, and ice on the TEV can cause these problems as well.
• Please review the more extensive description of causes & cures for ice formation in the article above

If a cooling coil or other components inside the air handler or air conditioner is freezing up the cause needs to be found and fixed; Iced coil means less or no air flow across the coil - it stops working. That's why the indoor temperature creeps up and why air flow is further reduced in the system.

A small amount of frost found on the refrigerant lines right at the uninsulated line at the entry to the cooling coil and sometimes at the compressor is common and may not be serious.

Brianna in your small A/C unit icing is most likely one of two things:
a dirty air filter is slowing airflow across the evaporator coil OR the level of refrigerant in the unit has dropped a bit too low.

Ramseh: yes; I'd add that a bad thermostatic expansion valve could also cause coil icing. The first cause I suspect is a dirty air filter, the second cause I suspect is low refrigerant.

Reader Question: why does a low refrigerant or "freon" charge cause cooling coil frost or ice formation?

(May 25, 2014) Anonymous said:
sir i use 19 mr 27 (1.5)orient split ac but in night elder pipe show ice and coolong not good

(July 1, 2014) Jonathan said:
Why does too low a charge of refrigerant in the A/C system can, for a while, lead to too-low temperatures in the coil which will then cause frost or ice build-up on the coil? Please excuse me for lack of knowledge. I'm a novice at HVAC.

Reply:

Jonathan it's a good question and I've wondered if I needed to offer more explanation - thanks.

The problem is this: if the total refrigerant charge is low but not exhausted, then insufficient refrigerant is avaliable on the liquid side of the refrigerant metering device to adequately feed the cooling coil with liquid refrigerant. IN that condition the vaporization temperature of the refrigerant inside the cooling coil is around the same as the freezing temperature of water - which makes condensate freeze on the coil surface.

If on the other hand there were adequate refrigerant, then there is more liquid refrigerant inside the cooling coil and heat is being removed (the coil is being cooled and heat in air blowing across the coil transfers into the coil) at a higher temperature - above the freezing point of water.

The result is abnormally low pressure on the low side of the refrigeration system (inside the coil) which causes very rapid refrigerant evaporation of the small amount that *is* present, resulting in abnormally low coil temperature.

Later when still more refrigerant has been lost, there is insufficient refrigerant to cool the coil at all - so at that point there is no more frosting or icing but rather the cooling coil simply remains warm.

You can observe the key data by looking at a chart of the properties of a specific refrigerant to see its behaviour at different pressures and temperatures.

At refrigeration school we learn that ONE way to determine if we've got the right amount of refrigerant in the system is to see where the frost line is on the system. If the frost line stops in the top third of the coil then we don't have enough refrigerant - we are running out of liquid refrigerant inside the coil too far from the end of the coil.

If the frost continues well past the end of the evaporator coil and down the rest of the suction line we may have too much refrigerant in the system - we're continuing to see a state change (liquid to gas) in the refrigerant well past the evaporator coil.

(July 2, 2014) Anonymous said:
Your great explanation gratified my curiosity! I do really appreciate it.
Many other articles in this site are also helpful to me. Thanks.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: ice build up on one side of the cooling coil

I have a lot of ice build up on one side should I just turn it all off and let it melt? I think it's a dirty filter and will try first after ice is gone Just concerned about where all the water will go. When ice drains. And a little concerned about why all the build up is all on one side. Love to hear ideas or suggestions. - Brandon Tully 5/25/12

Reply:

Brandon, you can turn off the A/C to let the iced coil melt and clear, but it will ice up again if you don't find and fix the problem. Typical causes are dirty filter, dirty coil fins, or improper refrigerant charge. Icing may be asymmetric depending on the level of refrigerant in the system, OR on some systems that inject refrigerant via cap tubes into more than one place in the cooling coil, if one tube section is blocked and the other is not, you can get one sided icing on the evaporator coil.

Question: had the AC coil replaced with a used one - would this cause ice up on the compressor/condenser

had a coil changed out, on side of box stated that this coil was filled with new 410 refridgerant. the unit outside was replaced with a used one and the installer said he used r22 , would this caused a coil to ice up and back up to outside unit? - Cathi 6/7/11

Reply:

Cathi: certainly mixing two different refrigerants will not permit proper air conditioner operation.

Question: can I use a blow drier to get ice off of my cooling coils?

Can I blow dry the ice that is on my ac coils ? Paris 6/10/11

Reply:

Yes Paris, or you can just leave the system turned off to allow the ice to melt; however this is more of a diagnostic step than a repair since unless the problem that caused ice to form is fixed it will just recur.

Question: I've been told closing off some rooms in the house can cause coil icing

My wife insists on closing two of the five rooms upstairs off, and sealing the vents. I've been told this can cause the coils to freeze. Is that correct, and if so, will opening the doors and removing the vent covers thaw it out or will I need to do more? - BDM

Reply:

BDM:

I don't think that closing off two of five rooms would be enough A/C air blockage to cause coil freeze-up, though indeed if most of the air flow were blocked that coil icing problem would be likely.

Try closing off the rooms as you wish - just shut the registers using the adjustable lever. You shouldn't need to "seal" the vents. A little leakage out of them won't make much difference in system operation.

Then if you notice a reduced air flow at your other room supply registers you'll want to check inside the air handler (turn off power first for safety) to see if the coil is icing. If it is icing up and never did before, and assuming that nothing else (like a dirty air filter) is blocking air flow, leave the system off for an hour or so to let the ice melt and go back to opening the vents in question.

Question: owner does her own diagnostics on poor air conditioner performance

Been waiting all day for the HVAC repairman and it is 100* out, so thought I'd do some research of my own & be knowledgeable when he gets here. First noticed this morning, house was warmer so I checked the air vents on both 1st & 2nd floors of my house & there was GREATLY reduced airflow, almost nothing coming out. But I heard the A/C unit running on the outside of the house ...sounded a little louder than normal.

I went and checked it & the metal piping leaving the A/C had a lot of ice build-up very near where the piping leaves the AC unit (this piping is covered w/a foam insulter(I am guessing)) & it is the piping that runs from the AC to the entry on the side wall of my house. The only thing we have done differently in the past few days is adjust the lever on the furnace unit inside the basement so more airflow goes upstairs to our bedrooms & less airflow downstairs & we shut the main floor vents to help force more air to the warmer area upstairs. I did have the thermostat on 65* for several hours but that was on almost 2 days ago, but when we got the upstairs cooled down, I turned it back to 70-72*. Any help would be great. Oh, also, the outside unit/fan was running fine, but had to turn it off to thaw it out, and haven't turned it back on because I don't want it to freeze again before AC guy comes to take a look. THANK YOU! - Kristen 7/26/11

Reply:

You're doing some good diagnostics on coil frost or icing on your air conditioner, Kristen. The basics to check before calling the HVAC repair technician is to assure that both air handler indoors and compressor/condenser outside are running, that the air filter is clean and in place, and that no one has adjusted a supply or return air duct damper nor damaged ductwork so as to block air that should be flowing across the evaporator coil.

Provided the air routing adjustments you made didn't simply cut off air flow inside the air handler I wouldn't expect that to have caused coil icing. Let us know what your tech says - it may help other readers.

Reader follow-up:

Thanks for your comments Dan. The AC repairman came and ran a test on our AC and it needed Freon, so far so good & it's been running for 6 hours trying to get back to a decent temperature in the house. It was up to 82* & now back down to 75*. Hoping to get it back to 70*. I went outside (now after dark) & checked the pipe with a flashlight. No ice, but pipe is VERY cold since it has been running so long. Is it okay for the the AC to run this long continuously since we are trying to get the house to cool down?? Or do I need to adjust the thermostat to 75* and give the AC a break from running. I'm not sure if these things are built for this/or meant to run this long continuously. We have about 2200 sq ft (I think) above ground 2-story, & another 800 square feet of finished basement, but mainly trying to get the upstairs sleeping quarters cooled down. I would hate for the AC man to have to make another trip out since he is actually about an hour from our house. Any thoughts ..I'm afraid the pipe could freeze again since it was so cold to the touch (even though the Freon level is back to normal). Thanks again.

Adding comments to my earlier problem w/freezing pipe on outdoor home AC unit. The pipe did freeze again, & we figured out why on our own. Since it is okay now, we believe it was a combination of the low freon in the beginning, and our furnace fan was pulling so much air to cool the house that the filter was actually almost stuck in the slot (like it was pulling a lot of suction), not to mention when needed to change our filter. So our diagnose for the freezing pipe as of now is this: a combination of low Freon, dirty furnace filter that was making it work overtime & running to long w/out lowering the indoor temperature enough, & we also had closed all the main floor vents, so we opened a few back up to make sure air was coming in and out properly. Hope this helps someone. - Kristen 7/27/12

Reply:

Good going Kristen on diagnosing those cooling system problem sources. You need a service tech to fix a refrigerant leak and charge a system, but a dirty filter or collapsing air filter are tasks most homeowners can tackle.

When your home has been quite hot for days, it could take the air conditioning system quite a while, (hours not days) to get the temperature back down, as the thermal mass of the home affects the cooling load. For example drywall walls and ceilings that are warm have to be cooled off too. - DF

Question: the hvac tech added refrigerant - what should I do to avoid repeat service calls?

My AC output was low for some time. On opening the cover I noticed heavy icing on cooling ducts. The young technician who came instantly said that gas had leaked and it requires a refill. Should there be other reasons to avoid repeat technician visits? - Kulbir Labana 7/30/11

Reply:

Kulbir: yes, if the refrigerant gas has leaked out of the system then just putting more refrigerant in is only a temporary fix. You will want a service tech to find and fix the gas leak or other causes of icing that are described in the article above. .

Question: AC Heat Pump Keeps Freezing Up Outside

AC heat pump unit is freezing up inside, I guess the coils, and the outside unit had ice on one pipe. the unit is about 10 years old and i think it has only been serviced once during that time. I am recently divorced and broke. Will the heater work this winter, I can do without ac till spring, or do I need to repair or replace it to have heat? - Lee 9/7/11

Reply:

Lee you need a service tech to diagnose and correct the problem or just as you'll have no cooling now, you may have no heat in winter - which will, unfortunately, be costly itself.

You don't have to use the A/C in hot weather if you can tolerate the temperatures, though depending on where you live there could be mold risks if the indoor humidity is too high.

In winter, without your heat pump you'll be running on backup heat that is likely to cost more over the whole winter than the cost of repairing and using the heat pump.

Question: how do I close off return air ducts when not used in winter

We have an air-handler installed in our attic, laying on its side, for air-conditioning. There are the circle vents of supply and return air, in each room. The article says to close them both in the winter, but only the supply ones have a little handle where I can close them off. I hear this is the common way to do this, so how is it possible to close the return air ones?

2nd question: When we bought this house, there were several places where the ceiling had leaks, and we thought it was from a bad roof, but now find that it is where the drain tube from the condensation, froze and cracked in the winter. How can we stop this, it is pretty much on the floor in the attic, and no way to get it higher as it goes to the outside wall. - Anon 10/23/11

Reply:

Anon, at WATER & ICE IN DUCT WORK we describe the causes of ice formation in HVAC ducts routed through a cold attic or ceiling. Indeed by natural convection warm moist air can rise up into ceiling or high-wall HVAC supply or return air ducts when the HVAC system is not operating.

And yes it's easy to close off a supply duct as most have operable louvers or register closures. To close off high or ceiling mounted return ducts we recommend a magnetic cover sold for that purpose; some homeowners just cover their ceiling return openings with clear plastic wrap or aluminum foil - more ugly but functional. Just remember to remove it before turning the system back on.

Question: ice on an ammonia based chiller

why ice on ammonia chiller or evaporator in an arena? - Ali Khan 1/30/12

Reply:

Ali ice can form on any HVACR equipment when conditions produce temperatures below freezing. The root causes are the same as those listed in the article above, regardless of the refrigerant gas or chemical used.

Question: no air flow at some of the HVAC ducts, good air flow at others?

I understand that low refrigerant and low air flow can cause freezing I get that. But I have Carrier split heat system where the air handler is in the attic standing upright. On top of the air handler are three air supply "taps". Two on front and one in the back. The two taps on the front go to the back of the house. The one tap in the back goes to the front with spiders taps of off that to registers. The back tap off the air handler has no air flow what so ever. None at the register it feed.

I went to the first spider leg of this tap and took the duct work off and turned on the unit. No air flow and no blockage. Why would my unit have two taps with air flow and one with none and ice up every time I turn it on? Also I had the unit charged last night, which didn't take much. By 6 am this morning the coil was frozen. Any thoughts would be appreciated. - Anon 5/16/12

Reply:

If the same air handler is feeding all of the duct systems and two have good air flow while the third does not, you want to look for a closed duct damper, missing or leaky duct connections, a crimped, constricted air duct, or an air duct where internal insulation has collapsed. There is a blockage or a closed duct damper or something to find.

Question: can I use water to get ice off of the refrigeration equipment?

Can I pour water on iced pipes - Terrence 5/26/12

Reply:

yes, it wont hurt the piping, but it won't fix anything, and indoors could be quite a mess. And of course don't wet any electrical components.

Question: frost on the evaporator coil, slugged compressor motor

had frost on evaporator coil and suction line. cleaned dirty coil and melted ice by turning furnace on. Turned A/C back on but only got 75F at nearest register. Did I ruin the compressor? Is there a protection device for compressor? - Greg Nelms 4/17/12

Reply:

Greg, you could be running out of refrigerant. Running the compressor briefly = may be ok, but don't keep running it since compressor motors rely on the presence of refrigerant for cooling and lubrication.

Reader follow-up:

Dan,The day the coil iced up was an unusually warm day in NJ. Wife turned A/C on in the afternoon, I did not get home till 9 PM and found the iced condition. The evaporator coil was excessively dirty {even though I change the 90 day filter every 45 days}. Temperatures here are back to normal and have not tried the A/C since that day. Afraid I slugged the compressor. - Greg Nelms 5/1/12

Reply:

Greg,

Unfortunately if the compressor was slugged, you can't un-do it. You can have the system cleaned and inspected. If the compressor is damaged it'll be noisy, or won't work at all.

Question: is frosting caused by an AC compressor with poor pumping?

evaporator fan ok and evaporator coil clean , I suspect the cause of the frosting of the suction pipe is because the AC compressor has poor pumping. - Simon Yengi Wani 5/6/12

Reply:

I don't think so, Simon. If the air conditioner compressor were not pumping or could not develop a vacuum the result would be no frost and no cooling.

Question: air leaks out of the drip line and cables at my air conditioner, freezing coils half way up

I was in my crawlspace under the house and noticed a lot of air coming from where the drip line and other cables were coming from. My coils have been freezing but only about halfway up. Could this be a leak from the blower to the duct? Could this be causing my coils to freeze? - Alan 6/23/12

Reply:

Alan, I'm unclear what that air leak is - perhaps a leak out of the blower compartment? Out of a supply duct? If so it ought to be sealed, but I don't think that'd be a cause of frost formation; Refrigerant lines should be insulated. Coil frosting is discussed in the article above.

Question: added refrigerant, but four days later ice was back - back to the original issue

Two weeks a go, we placed a service call for additional freon as we thought that was why air wasn't reaching thermostat setting. Service guy suspected a possible freon leak, but added 3 gallons of freon. Four days later, we were back to original issue. I checked lines running into outside unit (where fan is located) to see that there was significant ice build up. We've turned off A/C for now. I looked for filter, but couldn't find where one would be. Based on above sounds like dirty filter, faulty TEV or refrigerant leak are all potential causes. Anything else I should think of before calling a different service guy? - Brian 7/8/12

Reply:

Brian, at AIR FILTERS for HVAC SYSTEMS (article links listed at the ARTICLE INDEX the bottom of this article ) you will find a description of various places to look for the air filter on an air conditioning system - or search InspectApedia for "find the air conditioner filter"

"back to the original issue" sure suggests the problem was not cured, or your refrigerant leak is a big one; I don't like adding refrigerant to a system without finding and fixing the leak - HVAC equipment does not normally consume refrigerant. Sometimes a tech will just add refrigerant because it's cheap and quick rather than tracking down a hard to find leak. But 3 gallons is a huge addition - a big leak - that ought to be found and repaired.

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Continue reading at REPAIRS for FROST / ICE on COIL or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
• COOLING COIL or EVAPORATOR COIL
  • COOLING / EVAPORATOR COIL REPAIR
  • DAMAGED COOLING COIL
  • DIRTY COOLING COIL / EVAPORATOR COIL
  • DIRTY HVAC COIL CLEANING PROCEDURES
  • FROST BUILD-UP on AIR CONDITIONER COILS
  • MANUALS & PARTS GUIDES - HVAC - home
  • RETURN AIR, INCREASE
• DIAGNOSTIC GUIDE A/C or HEAT PUMP
• DIAGNOSTIC DETAILS A/C or HEAT PUMP
• FROST BUILD-UP on AIR CONDITIONER COILS
  • FROST / ICE IMPACT on A/C
  • HEAT PUMP OUTDOOR UNIT DEFROST CYCLE
  • REPAIRS for FROST / ICE on COIL
  • REFRIGERANT LINE FROST
• REFRIGERANT LEAK REPAIRP
• AIR CONDITIONING & HEAT PUMP SYSTEMS
• WATER & ICE IN DUCT WORK
• WET CORRODED DUCT WORK

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FROST / ICE IMPACT on A/C at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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