Thread: What causes this? See photo.

Guys, take a look at this photo. After I started the unit, within a few minutes I got this frost build up on condenser suction line. Outside temperature was about 72 degrees, humidicy aboit 40%. The unit seemed to have a decent differential. The condenser unit was fairly new but the evaporator in the attic looked very old 20 years+ maybe. Any idea on what causes this frost on the outside suction line? What do you make of this or what causes it, and could it represent a problem?

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Originally Posted by Gene South

What do you make of this or what causes it,

could it represent a problem?

.
Low on Refrigerant.
.
Yes defer to HVAC.
* after The A Coil Freezes it will block air flow ( No Cool AC out of the supply ducts. )
.

Originally Posted by Gene South

Guys, take a look at this photo. After I started the unit, within a few minutes I got this frost build up on condenser suction line. Outside temperature was about 72 degrees, humidicy aboit 40%. The unit seemed to have a decent differential. The condenser unit was fairly new but the evaporator in the attic looked very old 20 years+ maybe. Any idea on what causes this frost on the outside suction line? What do you make of this or what causes it, and could it represent a problem?

Low refrigerant or a blockage in the line/coil would be the most likely culprit.

Gas is not cold. Evaporating liquid (refrigerant) is cold. The only way you can see frost on the gas line at the condenser is to have liquid in the line. The most probable cause in this case is a severely mismatched evaporator to condenser. The problem might be correctable to a degree by changing the size of the orifice at the evaporator and adjusting the charge. It will never be 100% correct and will not be efficient.

I would suspect the evaporator coils are dirty.

rick

The only thing frost on a line tells you is that the line is below 32 degrees. If the indoor temp was very low, you might just be below 32 on the suction line due to low load. I would be looking further if I suspected the filter or coil was blocked. Might rise to the need to have a tech look at it but I was not there and need more information before forming an opinion.

Did you happen to take a look at the EVAP coil?

Dirty, clean? You indicated that it was older ... could be related, but as JL noted ... I wasn't there and need more information.

As Rick, Nolan and Jim said and maybe others it is more likely to be a dirty evaporator coil. What I see in almost every instance every time the coil is blocked up It will ice the evaporator coil up and also the suction line gets frosted up.

There could be contributing causes but that is more than likely the case.

It seems all the seasoned inspectors have there own guess, some probably right on, but still guessing. Refer client to an licenced HVAC man and you shouldn't get bit later.

My thoughts would lead to me to saying dirty evap coil or low refrigerant on the icing problem that Gene has come across.

[quote The condenser unit was fairly new but the evaporator in the attic looked very old 20 years+ maybe./quote]

Genes statement about the age of condensor versus evaporator prompts me to ask a question that I have been wanting to ask for some time. Most HVAC people that I probe for answers from time to time tell me that if a Condensor Unit is replaced that the Evaporator should be replaced also. This is certainly understandable to me and makes perfect sense to maintain the seer rating of the unit, and now compatability with old freons versus the newer blends.

I would like to hear what my peers think so I can share the knowledge of all of you with my clients that get into this same predicament from time to time.

Originally Posted by Ricky Wells

My thoughts would lead to me to saying dirty evap coil or low refrigerant on the icing problem that Gene has come across.

[quote The condenser unit was fairly new but the evaporator in the attic looked very old 20 years+ maybe./quote]

Genes statement about the age of condenser versus evaporator prompts me to ask a question that I have been wanting to ask for some time. Most HVAC people that I probe for answers from time to time tell me that if a Condenser Unit is replaced that the Evaporator should be replaced also. This is certainly understandable to me and makes perfect sense to maintain the seer rating of the unit, and now compatibility with old freons versus the newer blends.

I would like to hear what my peers think so I can share the knowledge of all of you with my clients that get into this same predicament from time to time.

As Brian said, we are only guessing from a distance. Recommending a HVAC contractor is the best (only) way to go.

Having said that, my reasoning that the mis-match was the most likely cause was: (1) OP said the temp differential was good. Can't have blocked coil with no airflow and have a good temp differential. (2) Low refrigerant causes the liquid refrigerant to boil at below 40 degrees and can cause the evaporator to frost up, but this takes time as there is not enough liquid to fill the evaporator coil. The indoor coil has to be almost fully blocked by frost before the line starts to frost at the condenser. OP said there was air flow and it happened within a few minutes.

This makes me think the evaporator was flooded due to mis-match that could not handle the low ambient temperature. On a hot day the problem would not be apparent as the refrigerant would boil off before it got to the condenser.

That's my best guess.

Thanks to all you guys for all good feedback as usual. I left out that the attic temperature that day was about 75 degrees. I did flag this item and referred to licensed HVAC tech.

Thanks again.

Gene

There are only 2 reasons a suction line freezes, lack of air across the evaporator coil or a lack of refrigerant thru the coil for whatever reason ( bad TXV or clogged metering device, low charge, blocked filter drier)

Originally Posted by David Bell

There are only 2 reasons a suction line freezes, lack of air across the evaporator coil or a lack of refrigerant thru the coil for whatever reason ( bad TXV or clogged metering device, low charge, blocked filter drier)

How can it freeze without refrigerant? Remember, the old guys used to charge systems by watching for frost at the suction line. When it began to frost, it was full! Not a recommended procedure, but did work to an extent.

I didn't say no refrigerant, I said low refrigerant. Frosting of the suction line shows that the refrigerant has not gained enough heat from the evaporator, a low charge will vaporize before entering the evap. coil where it should become vapor. The low pressure and tempurature will begin to freeze the coil and further restrict airflow until the coil becomes a block of ice. In extreme conditions I have seen the ice build up to the compressor suction port.

Originally Posted by David Bell

I didn't say no refrigerant, I said low refrigerant. Frosting of the suction line shows that the refrigerant has not gained enough heat from the evaporator, a low charge will vaporize before entering the evap. coil where it should become vapor. The low pressure and tempurature will begin to freeze the coil and further restrict airflow until the coil becomes a block of ice. In extreme conditions I have seen the ice build up to the compressor suction port.

Vapor is not cold! Liquid is cold. The liquid flashes when it passes through the orifice and 20 to 30% of the refrigerant becomes gas (boils off), leaving 70 to 80% liquid, which continues to boil off as it travels through the evaporator coil. If for some reason the liquid does not completely boil off before it reaches the condenser, liquid reaches the compressor. One reason liquid can reach the condenser is a blocked evaporator coil, which can happen due to frost. One way for frost to develop on the coils is low pressure due to low charge. Low pressure makes the liquid boil off below the nominal 40 degrees used in air conditioning. But this takes time as the coil frosts only where liquid is touching the coil, and there is not much liquid because it is under charged.

Super heated vapor is colder than sub cooled liquid. Pressure/ temp. relative.

Originally Posted by David Bell

Super heated vapor is colder than sub cooled liquid. Pressure/ temp. relative.

You might want to research that!

R-22 boils at -41 F at atmospheric pressure, as you raise the pressure you raise the boiling point and temp. A typical R-22 system on a given day may have a SUB COOLED liquid pressure of let's just use 200 PSIG leaving the condenser. At that pressure the temp is going to be about 102 degrees F. The SUPER HEATED vapor pressure at the suction line should be about 75 PSIG with a temp of 44 degrees F. Now if you have a low charge in that system you could see a suction pressure of 30 PSIG giving you a temp. of 8 degrees F. Any moisture contacting the coil or suction line at that temp will freeze.
With low pressure conditions you will see bubbles in the liquid line sight glass because the refrigerant is boiling off prematurely. If you have liquid coming back to the compressor, you have an overcharged system and risk blowing the compressor.

Originally Posted by David Bell

R-22 boils at -41 F at atmospheric pressure, as you raise the pressure you raise the boiling point and temp. A typical R-22 system on a given day may have a SUB COOLED liquid pressure of let's just use 200 PSIG leaving the condenser. At that pressure the temp is going to be about 102 degrees F. The SUPER HEATED vapor pressure at the suction line should be about 75 PSIG with a temp of 44 degrees F. Now if you have a low charge in that system you could see a suction pressure of 30 PSIG giving you a temp. of 8 degrees F. Any moisture contacting the coil or suction line at that temp will freeze.
With low pressure conditions you will see bubbles in the liquid line sight glass because the refrigerant is boiling off prematurely. If you have liquid coming back to the compressor, you have an overcharged system and risk blowing the compressor.

But SUB COOLING is measured at the HOT GAS line. Not the the LIQUID line. More research!

Originally Posted by Vern Heiler

But SUB COOLING is measured at the HOT GAS line. Not the the LIQUID line. More research!

Sub cooling is the amount of heat removed at the condenser turning the refrigerant back to a liquid.

The liquid refrigerant leaving the condenser is sub cooled!

Originally Posted by David Bell

The liquid refrigerant leaving the condenser is sub cooled!

You are correct. My bad, measured at the liquid line. However, the fact that sub-cooling is measured (there is a drop), is an indication of a fully charged system. Which begs the question, what does "Super heated vapor is colder than sub cooled liquid. Pressure/ temp. relative." have to do with anything?

If the low side pressure has dropped to 55 psig. (30 deg) it is low on refrigerant and only a small amount of liquid is touching the evaporator coil. It will freeze moisture that touches the coil, but only at a small area until that area has blocked air flow. Then the liquid can then move on to unblocked air flow area and continue to frost the coil. Which is why I said, low refrigerant charge takes time to block the entire coil and show up at the condensor as frost on the low pressure line. OP said it was only a few minutes.

Originally Posted by Vern Heiler

You are correct. My bad, measured at the liquid line. However, the fact that sub-cooling is measured (there is a drop), is an indication of a fully charged system. Which begs the question, what does "Super heated vapor is colder than sub cooled liquid. Pressure/ temp. relative." have to do with anything?

If the low side pressure has dropped to 55 psig. (30 deg) it is low on refrigerant and only a small amount of liquid is touching the evaporator coil. It will freeze moisture that touches the coil, but only at a small area until that area has blocked air flow. Then the liquid can then move on to unblocked air flow area and continue to frost the coil. Which is why I said, low refrigerant charge takes time to block the entire coil and show up at the condensor as frost on the low pressure line. OP said it was only a few minutes.

The problem with low pressures is the refrigerant is turning to vapor before it gets to the evaporator, as the pressure drops further in the coil the refrigerant temp drops also. The frosting is occuring everywhere after the metering device to the compressor. This process does not take long and can be compounded by low airflow across the evap coil. The less airflow, the less heat that the refrigerant can gain.

Vapor is not cold! Liquid is cold.
By the way. My statement was in response to this.

Originally Posted by David Bell

The problem with low pressures is the refrigerant is turning to vapor before it gets to the evaporator, as the pressure drops further in the coil the refrigerant temp drops also. The frosting is occuring everywhere after the metering device to the compressor. This process does not take long and can be compounded by low airflow across the evap coil. The less airflow, the less heat that the refrigerant can gain.

I disagree. The only place frost will appear is where there is liquid refrigerant touching the coil. We have all seen the single band of frost on evaporator coils and on outdoor coils when heat pumps are in heating mode. The frost begins at the begining of the circuit. Most systems have multiple circuits so there will be 2 or 3 stripes of frost, but seperated by the part of the coil that does not have liquid in it. No liquid no frost.

Originally Posted by Vern Heiler

I disagree. The only place frost will appear is where there is liquid refrigerant touching the coil. We have all seen the single band of frost on evaporator coils and on outdoor coils when heat pumps are in heating mode. The frost begins at the begining of the circuit. Most systems have multiple circuits so there will be 2 or 3 stripes of frost, but seperated by the part of the coil that does not have liquid in it. No liquid no frost.

Get yourself a pressure temp chart and look at the numbers. Low pressure vapor is what freezes a coil, the lower the pressure,the lower the boiling point the lower the temp of the refrigerant

David Bell, you nailed it! low refrigerent, not enough air across the evaporator, both cause a low coil temperature and then freezes the suction line and evaporator. Could be a plugged filter, dirty evaporator, plugged metering device, low charge. On a fully charged system just turn off the furnace fan and see what happeneds with no air flow. It will freeze up and your suction pressure will be low and look like a low charge. I like to charge by head pressure / ODA temp and super heat.

Originally Posted by Dan Hagman

David Bell, you nailed it! low refrigerent, not enough air across the evaporator, both cause a low coil temperature and then freezes the suction line and evaporator. Could be a plugged filter, dirty evaporator, plugged metering device, low charge. On a fully charged system just turn off the furnace fan and see what happeneds with no air flow. It will freeze up and your suction pressure will be low and look like a low charge. I like to charge by head pressure / ODA temp and super heat.

And when you disconnect your gauge lines, it is the liquid refrigerant that hurts your fingers from the high pressure test point, not the vapor from the low pressure test point. Think about it. Vapor refrigerant is not what cools the coil. If it was there would be no need in fully charging a system. When water boils evaporation cools the liquid, the steam does not become cooler.

By the way, I have not said low charge, blocked air flow, or other problems could not cause frost. I said that with the given information, those were not the most likely causes.

Originally Posted by Vern Heiler

And when you disconnect your gauge lines, it is the liquid refrigerant that hurts your fingers from the high pressure test point, not the vapor from the low pressure test point. Think about it. Vapor refrigerant is not what cools the coil. If it was there would be no need in fully charging a system. When water boils evaporation cools the liquid, the steam does not become cooler.

By the way, I have not said low charge, blocked air flow, or other problems could not cause frost. I said that with the given information, those were not the most likely causes.

Vern, this is the last post I will make on this subject. The refrigerant that hurts your fingers on the high side instantly vaporizes( remember r-22 boiling point at -41 degrees atmospheric) it is the high pressure and the amount of vaporizing refrigerant that can give you frostbite.the high pressure line is warm until you open it up to the atmosphere and it vaporizes. Your theory of what a closed system does is suspect. I hope you study up on the states of refrigerant in each part of the system and begin to understand basic refrigeration.

When the indoor unit and the outdoor unit are not a matched set, the HI should be skeptical. When the system does not function as intended, the skepticism should be raised.

If you are going to give possible reasons for the system not functioning it is OK to suggest that it might be correctable by adding refrigerant, but I would lean on the possibility that it will be a much more expensive fix.

Originally Posted by David Bell

Vern, this is the last post I will make on this subject. The refrigerant that hurts your fingers on the high side instantly vaporizes( remember r-22 boiling point at -41 degrees atmospheric) it is the high pressure and the amount of vaporizing refrigerant that can give you frostbite.the high pressure line is warm until you open it up to the atmosphere and it vaporizes. Your theory of what a closed system does is suspect. I hope you study up on the states of refrigerant in each part of the system and begin to understand basic refrigeration.

David I think I have it...


For any who are watching, I will hold my finger in the vapor above the liquid if you will hold your finger in the liquid.

David is correct I service hundreds of AC units a year!! Its usally dirty coil, dirty air filter or low on Freon.

Originally Posted by Vern Heiler

David I think I have it...


For any who are watching, I will hold my finger in the vapor above the liquid if you will hold your finger in the liquid.

Vern, you are missing the science here and I understand David Bell's frustration and decision to discontinue posting on the matter.

When you hold your hand on the liquid line it is warm, when you hold your hand on the suction (vapor) side it is cold. Very, very simple.



Holding your finger in the liquid side, assuming you could withhold the pressure, will be safe. It is the expanding vapor that causes the cold.

I agree with Bill Brooks. I'm an inspector an also an A/C Tech.
99% of the time it's going to a dirty coil, dirty filter, or low on refrigerant.
If low refrigerant, start looking for a leak.

Originally Posted by Vern Heiler

David I think I have it...


For any who are watching, I will hold my finger in the vapor above the liquid if you will hold your finger in the liquid.

Vern,

You don't have it. You can not compare the behavior of a refrigerant suddenly released to atmospheric pressure to its behavior within a closed mechanical system where it is intentionally being taken through a continuous loop of carefully controlled phase transitions.

Ultimately you seem to be stuck on just a small portion of the overall equation here. I am not saying you are wrong because you are actually right up to a certain point. The problem here is that your limited focus seems to be preventing you from grasping the larger concepts involved in what actually goes on within a closed refrigerant loop and the ultimate behavior of the refrigerant within a given close system under different circumstances.

I have read this thread through and David is right on track with his evaluation and offered descriptions. If they appear "wrong" to you then I would suggest that you understanding of the refrigerant cycle remains incomplete.

Originally Posted by Rod Butler

Vern, you are missing the science here and I understand David Bell's frustration and decision to discontinue posting on the matter.

When you hold your hand on the liquid line it is warm, when you hold your hand on the suction (vapor) side it is cold. Very, very simple.



Holding your finger in the liquid side, assuming you could withhold the pressure, will be safe. It is the expanding vapor that causes the cold.

Rod I respectfully contend that you do not understand the enthalpy of liquid refrigerant. Keep in mind that water is a refrigerant, as is methane, amonia, and many liquids. I am sure you are familuar with the "water boils at 212 deg. at sea level", well R-22 (for example) boils at a much lower temperature of -41.4 deg. at the same pressure. The liquid boils to maintain the temperature/pressure relationship.

I will still challenge anyone to hold there finger in liquid R-22, R-12, R-134 etc., while I hold myfinger in the vapor above the liquid. (Yes this can be done, and yes I know it is agaist the law!)

While low refrigerant can cause the the low pressure line to frost, it will first frost the evaporator coil solid, due to the liquid being below 32 deg., causing liquid refrigerant to flow back to the compressor. The same is true if the filter or evaporator coil is blocked. This is due to the fact that the refrigerant did not evaporate in the evaporator and is evaporating in the low pressure line. The liquid is what is cold not the vapor.

While low refrigerant can cause the the low pressure line to frost, it will first frost the evaporator coil solid, due to the liquid being below 32 deg., causing liquid refrigerant to flow back to the compressor.


The compressor is a vapor pump,,not a liquid pump. When liquid returns to the compressor it is called slugging and can severelydamage the compressor in a short time. Slugging can be caused by an overcharged system, a lack of proper metering device, or a malfunctioning metering device. It is defined by very similar high pressures in both the liquid and suction lines. Again your refrigerant circuit theory is suspect. Please educate yourself further.

David, the use of the word toward rather than to the compressor would have been more correct. My point was that if the liquid refrigerant does not boil off in the evaporator, due to frost, it will still be liquid when it gets to the service valve in the low pressure line and that is the only way you will have frost. In sever cases it can get all the way to the compressor and damage it.

Your contention that it is the high pressure vapor that is cold is what is so basicly wrong. If I can use water as an example, as all of us understand that water will boil at the same temp, no matter how much heat is given, as long as the pressure is held the same. When the temp of the water reaches its boiling point any additional heat is carried of in vapor, keeping the liquid cool. The same thing happens in the refrigerant system. The liquid boils at the pressure determined by the low side pressure. Additional heat picked up at the evaporator coil and the low pressure line, is carried off as vapor and is refered to as heat laden vapor.

Your statement, "Slugging can be caused by an overcharged system, a lack of proper metering device, or a malfunctioning metering device. It is defined by very similar high pressures in both the liquid and suction lines." "Is defined by..." is nonsense.

Originally Posted by David Bell

R-22 boils at -41 F at atmospheric pressure, as you raise the pressure you raise the boiling point and temp. A typical R-22 system on a given day may have a SUB COOLED liquid pressure of let's just use 200 PSIG leaving the condenser. At that pressure the temp is going to be about 102 degrees F. The SUPER HEATED vapor pressure at the suction line should be about 75 PSIG with a temp of 44 degrees F. Now if you have a low charge in that system you could see a suction pressure of 30 PSIG giving you a temp. of 8 degrees F. Any moisture contacting the coil or suction line at that temp will freeze.
With low pressure conditions you will see bubbles in the liquid line sight glass because the refrigerant is boiling off prematurely. If you have liquid coming back to the compressor, you have an overcharged system and risk blowing the compressor.

I was just looking back over the thread and found another jewel of yours:"SUPER HEATED vapor pressure at the suction line..." showing you have no idea of what is going on! Super heated vapor is at the discharge of the compressor, the HOT GAS line! Measuring super heat is done at the end of the low pressure line, near the service valve. It is a measure of the number of degrees picked up in the low pressure line from the time the liquid has completely evaporated. The lower the super heat, the closer the liquid refrigerant is to the compressor. Why would we want as low as possible super heat? Because it is an indication of how much liquid is in the evaporator, which is what cools the coil. Not the vapor!

Vern, I would love to be able to see things from your point of view but, with your head firmly entrenched, there is no room in your butt.

Sub cooled liquid is on the discharge line after the condenser.

Originally Posted by David Bell

Vern, I would love to be able to see things from your point of view but, with your head firmly entrenched, there is no room in your butt.

David, my view and knowledge of the refrigerant circuit, conforms to the federal governments mandated competency test, administered and proctored by the state of North Carolina. I have shared with you and all that have read this thread my scores on all of the classes of refrigeration. Maybe you would like to share your scores? After which I will be interested in your detailed explanations of how vapor cools the coil.

Originally Posted by David Bell

Sub cooled liquid is on the discharge line after the condenser.

And???

Vern
Watch this
21footxpress's Channel - YouTube

Originally Posted by Vern Heiler

Recommending a HVAC contractor is the best (only) way to go.
.

Originally Posted by Vern Heiler

And???

.
That's the way I feel as well.
*see attachment

Originally Posted by Rick Cantrell

Vern
Watch this
21footxpress's Channel - YouTube

Thanks Rick, I would like everyone interested to pay close attention to 7:02 thru 7:40, (it's the liquid) the point of this disagreement! (See post #27) I do concede that any gas above its boiling point is technically "super heated gas" though most references and concerns are at the output of the compressor where getting rid of the super heat is the major objective.

When I see "frost build up on condenser suction line" I haveattributed it to a filter that is completely blocked with debris (dust particles).

Jerry's answer is the best: "I agree with Bill Brooks. I'm an inspector an also an A/C Tech.
99% of the time it's going to a dirty coil, dirty filter, or low on refrigerant.
If low refrigerant, start looking for a leak."

The inspector can easily verify the dirty filter. The dirty air filter will cause low air flow over the evaporator coils, which means that not enough heat will be removed from them, thus causing lower refrigerant temperatures inside the unit, which creates the frost on the suction line. Other things can cause low air flow, such as blocked supply or return air vents. If the unit was properly sized and the tubes well braized, low air flow is the most likely culprit.

Dirty filters can also cause dirty coils. This may be easy to verify. If the unit has an easily removed panel, the inspector can check the coils in about three minutes. If not, recommend the hvac contractor.

If you cannot verify low air flow, recommend the hvac contractor.

How about lack of insulation on the condenser suction line, perhaps being in contact with the ambient air is causing it to frost, in addition to the other poster's comments.

RE: lack of insulation on the condenser suction line

The suction line contains hot gas (from the evaporator). The purpose of the insulation on the suction line is to keep the gas line hot - to prevent that hot gas from condensing into a liquid before it gets to the compressor in the condensing unit. Compressors condense gas very well. Liquids are NOT compressible. If liquid gets to the compressor inlet, it causes what is called slugging - very bad for compressors.

While missing insulation on the suction line may contribute to the hot gas cooling lower than it should (since the gas is hotter than the ambient air), it will not contribute to frosting. In air conditioning season, the ambient air is obviously much warmer than freezing and would melt any frost on the line, not contribute to its creation.

Originally Posted by Garrett Merrick

RE: lack of insulation on the condenser suction line

The suction line contains hot gas (from the evaporator). The purpose of the insulation on the suction line is to keep the gas line hot - to prevent that hot gas from condensing into a liquid before it gets to the compressor in the condensing unit. Compressors condense gas very well. Liquids are NOT compressible. If liquid gets to the compressor inlet, it causes what is called slugging - very bad for compressors.

While missing insulation on the suction line may contribute to the hot gas cooling lower than it should (since the gas is hotter than the ambient air), it will not contribute to frosting. In air conditioning season, the ambient air is obviously much warmer than freezing and would melt any frost on the line, not contribute to its creation.

Insulation is on the low pressure (gas) line for two primary reasons; #1 is to prevent condensation and consequent moisture damage to insulation, ceilings, wood, etc. #2 is to reduce super heat, picked up in the line from the evaporator to the compressor, which will improve efficiency. Insulation on the low pressure line actually increases the chance of liquid getting back to the compressor. Just one more reason not to over charge a system!