Carrier 52BQA312301AA window AC/heat pump evaporator coils icing over

[stainlesssteel]

I'm living in a studio apartment with a Carrier "window" style A/C/heat pump, built 1985. The air is cold but not as cold as I'd expect, and the evaporator fins ice over after about 4 hours of running. At that point, there is essentially no cooling at all, as air can't flow past the clogged fins to be cooled. I would think this is a sign of not enough air circulation through the fins. There is some suction through them, as verified by a sheet of paper or the lint filter "sticking" to the fins against gravity.

The apartment maintenance guy says it's not enough Freon, and noted the cooled air felt not as cold as it should, though I pointed out it had been running only about 3 minutes. He said he'd come back and "add some Freon"; he hasn't done this yet. He seems knowledgeable enough, having worked here six years, although he's not necessarily a specialist in HVAC.

If anything, i would think that ice forming on the evaporator coils means there is more than sufficient refrigerant. My guesses:

* Somehow there is insufficient airflow over the evaporator fins...maybe the internal fan is operating too slowly? I don't have a frame of reference

* Thermostat keeping the compressor on too long. I DO have it set to COLDEST, but even then it's not cold enough.

I also manage a 9 unit apartment building (not the one I'm living in) and have experience repairing a refrigerator...noting when a top freezer's freezer compartment evaporator coil was NOT displaying a "frost line", it was a strong indication there was not enough refrigerant in the system.

Is there a way that insufficient refrigerant can cause icing of evaporator coils?

[RegUS_PatOff]

this is low Freon:

[stainlesssteel]

Yes, RegUS_PatOff, this is exactly the picture I was referencing...no significant frost line, just a tiny % of the evaporator displaying frost. Now imagine the entire evaporator covered with frost (I'll post a picture). Do you know of a scenario where low freon could cause that, as this maintenance guy is claiming? Or what else would?

[RegUS_PatOff]

[user=16268]stainlesssteel[/user] wrote:

.. Now imagine the entire evaporator covered with frost...  Do you know of a scenario where low freon could cause that...

No.

Is the Fan Motor blowing good ?

Are there any Fan Shrouds missing ?

Is The Fan Motor blowing in the right direction ?

[Pegi]

You are low on freon, no....you are almost out of freon, probably only at about 15 PSI instead of the 70 aprox PSI it should be running on the low side.  You of course have a freon leak, he can either try to find it and repair the leak, evacuate the system and recharge it, or he can evacuate the system and recharge it and see how long the freon lasts, you might have a big leak or a very small one that will take years to leak back out, no way of knowing unless he sees oil on the freon lines.  Suggest he puts the system into a vaccuum and see if it holds at -30 for a day or two, if so probably too small of a leak to find, if it looses vaccuum then need to find to leak, cause the freon will leak right back out.  Low freon=freon leak.....the window units will start freezing up the whole evaperator at about 35 low side running psi, at the frost line in the photo you would be down to about 10-15 low side running psi.  We worked on window units 30 years, Type 1 certified, you are low on freon.  He needs to evacuate the system and recharge however, not just add freon, need to remove the air and moisture that has been drawn into the sealed system because of the leak and drawing in of the outside air into the system.

[RegUS_PatOff]

I think when he said that picture is the one he was referencing.. 

I think that was his thought, if it were low on Freon (which he didn't think it was)

"Now imagine the entire evaporator covered with frost (I'll post a picture). "

 

[stainlesssteel]

RegUS_PatOff - i haven't taken the air conditioner apart to check what you're describing about the fan/shroud, but it seems to be blowing in the correct direction.

Pegi, thanks for your thorough reply. However it sounds like you were assuming RegUS_PatOff's "Low Freon" photo of a refrigerator evaporator with almost no frost, rather the actual photo of my air conditioner evaporator, heavily frosted, below--what it looks like after running about 3 hours: (I woulda posted it originally but I couldn't find my digital camera USB cable)

To reiterate: I think the fact most of the evaporator surface can get frosted is a sign of sufficient refrigerant. Is there a way that insufficient refrigerant can cause frosting of evaporator coils? If so, how?

I have a better guess what the problem is: bad temperature sensor: At first I guessed maybe the temperature sensor was not positioned where it would ever get cold, as in the photo, so that the compressor ran indefinitely, causing the evaporator fins to get colder than necessary to cool the air....so cold that frost started forming on the evaporator fins, until the frost blocked airflow through the fins and stopped the supply of cooled air.

However I tried moving the sensor close to the evaporator, where it would get cold....and the evaporator still frosted over. Maybe the sensor is broken....

But a bad temperature sensor wouldn't explain the not-that-cold air initially...

high resolution version of photo at

http://r7s8t9.info/fixitnow/DSCN6997carrierfrosted.jpg

[Pegi]

You are not understanding the differences in a sealed system in a refrigerator and a window unit cooling unit, or a central a/c unit for that matter.  They are not identical systems.  On a refrigerator the coil has to frost up to provide proper cooling, hence the reason to have to either manually defrost the evaporator or have a self defrosting system in a refrigerator.  They have different freons and run at different pressure's...

What you are showing me is a window unit a/c that is at about 35-45 psi low side running pressure, it should be around 70-85 aprox. low side running pressure.  At about 15 psi you would get a frost pattern like was posted before on that refrigerator evaperator coil....not sure why you are questioning all of this when the maintance man already told you what is wrong with the unit....you need to have the unit evacuated, drawn into a vaccuum to see if it will hold, if not find and repair the freon leak, if so then recharge the unit and see  how long it will last...have the maintance man put his gauges on the low side of the unit and see what the running pressurers are, he will have to add a tap if it has not been tapped into before since these are sealed systems like refrigerators, sealed from the factory.  Be sure he uses the freon type that is posted on the model/serial tag, cannot mix different freons but yours should use R-22 according to the date you posted for the unit. 

So I suggest having the maintance man see what the running pressure is on the low side of the unit, not the high side....then go by what the gagues tell him...betting around 35-45 give or take 10....  that is the freezing point of R-22 hence the frost on the evaperator coil...

[Pegi]

Also see what the temperature is of the air that is coming out of the air vents at the top, if it is cooling properly the air will read somewhere between 52-58 degrees, if it is higher than that the unit is low on freon and works more like a swamp cooler. Be sure you do not get the thermstat feeler or sensor, whichever our unit uses against the evaperator coil, if you do after the unit is fixed it will read the temp of the coil and shut the compressor off too soon since it would be reading the temp of the coil instead of the return air temp from the room..  Central units work this same way as window units, refrigerators are a whole different type of system and freon..

[stainlesssteel]

Pegi,

Thanks for another thorough post! Yes, of course A/C and refrigerators are not identical; in posing my questions I was assuming only similarity.

"not sure why you are questioning all of this when the maintance man already told you what is wrong..."

hahaha....you've never had a apartment maintenance guy guess wrong??!!

I took some output air temperature vs. time data--see graph--with a digital thermometer and stopwatch. Much better than the vague "should be/seems/pretty cold" estimates. And I made sure the temperature sensor (red tube in photo) wasn't stuck right in the evaporator.

* OAT gets down to near freezing 33.6 deg F. How could an A/C roughly 50% low on refrigerant, as is your diagnosis from the picture, still get air that cold?

* An alternate explanation of the frost forming: the compressor is never cycling off. During four hours of measurements i never noticed a sound resembling a compressor shutting off. A bad thermostat tube could cause this, right?

* I noted the frost started forming right around the time the minimum OAT of 33.6 F is reached, about 85 minutes after starting the A/C. THe OAT then slowly rises, as more and more frost blocks airflow over the evaporator fins. About 7-8 hours later (not shown on graph) the OAT is insufficiently cold--the original problem.

So typical air conditioner output air temperature is 52-58 deg F? Seems pretty warm. IN any case, the graph shows this AC gets much colder than that.

Also, in one of my previous posts, you responded with an awesome online service manual for a side by side Whirlpool refrigerator....Do you have a similar source for the Carrier service manual?

Higher resolution graph at

http://r7s8t9.info/fixitnow/carrieracgraph.gif

[stainlesssteel]

The thermometer I used and orientation, to take the data in the graph:

[Pegi]

The compressor will never shut off with the evaperator clogged with frost, no air flow across the thermostat, and the air would not be cold enough to let it shut off.  Not sure about your temp readings however....I do not have any other ideas for you, I just post what I know....good luck with your unit. 

[dkpd1581]

To answer your original questions:

1. YES - low airflow across the evaporator coil can cause the frosting you see.  Typically PTAC units never have the evaporator  coil chemically cleaned.  If that unit has in fact been in service since 1985 (and at best the cheapo foam filter gets rinsed off every now and again) the interior of the coil may be impacted where it is not visually obvious.  I would suggest a good cleaning of the evaporator coil.

2. YES - the return air thermistor located at the inlet face of the evaporator could be defective.  Depending on the type used (solid state, gas charged, liquid charged) will dictate the best course of action for troubleshooting it.  Try adjusting the temperature to see if raising the "Cold" setting will in fact cycle off the compressor.

Low R22 definitely will cause the frosting as the pressure/temperature relationship reaches the 32F mark thereby freezing the moisture on the coil.

Operation of an otherwise good unit in low outdoor ambient air conditions will give the same results unless head pressure controls are in place and properly adjusted - a remote possibility in a PTAC application.

A system restriction of some sort that is not overtly apparent (stopped up strainer, filter drier, stopped fixed meter, defective TXV or powerhead, obstructed check valve) or even non-condensables in the refrigerant (taking up space otherwise needed for R22) can give you similar symptoms.  Moisture from improper practices can freeze either the fixed or TXV orafice and create a restriction that dissappears mysteriously only to return at a later time.

Airflow obstructed by soiled/impacted blower wheel, faulty motor slowing down over time are other issues that will show similar frosting

At best we are all guessing unless we know the solid fundamentals:

Metering device type

Suction and Discharge PSIG

Super Heat and Subcool

Temperature drop across Evaporator

Temperature rise across Consensor

Ambient outdoor temp

Amp draws of compressor and blowers (evaporator and condensor)

Yes I agree that the maintenance guy can guess wrong or be wrong after analysis.  In fact, I make a very good living from the incorrect guess or best intentioned "diagnosis" of units every day.  Ignorance and ignorant people more often than not pay the bills and make the mortgage in my household.  Many maintenance guys run for the common cure "Add more freon Man!"  Have a bad contactor - add freon; bad blower motor - add freon; low tire pressure on your car - add freon (you get the idea).  If you can give accurate answers to the preceeding 7 items, we can give you better answers in return.  Best of luck.