Cold Liquid Line & Bottom of Coil


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Hi, I noticed that our AC unit was not cooling well and determined that the copper liquid line going into the air handler, which is in a closet in the upstairs hallway, is very cold when the AC is running. I removed the front panel of the unit, and the smaller liquid lines feeding the coil get frosty when the unit is running. The bottom few inches of the coil also becomes cold, but the top 75% of the A frame coil is not cold at all. On the outside unit, the air leaving the top of the unit is also ambient temperature, it is not warm like it is normally. Replaced the capacitor on the outside unit just in case, but that did not help. Any suggestions on what could be the possible issue would be greatly appreciated. (The unit is a 3.5 ton and 12 years old.) Thanks in advance for any suggestions.
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Any time you have frost an internal refrigerant restriction is indicated. There is a line restriction between the condensing unit and the coil distributing tubing. It could be a liquid line filter/drier. I would start at the distributor tubing and work back toward the condenser.
Remember frosting=restriction, causing the liquid refrigerant to flash into gas at the restriction.
 
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Thanks Cajuray, when you indicate " I would start at the distributor tubing and work back toward the condenser. ", do you mean look for frost along the line? Also, if the unit was just very low on refrigerant, could that have the same symptoms? Thanks again!
 
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It could be low on refrigerant as well. Generally the entire coil will freeze up with refrigerant charge low. If you know how to check pressures and temperatures, you will find very high superheat at the suction line. If you don't have gauges and temp. tester, you are better off getting a service tech to solve the problem, it is not a do it yourself job. Also if it is low on charge do you know how to properly "top off"?
 
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As I'm learning about how to add refrigerant, I'm trying to determine what the Subcooling value should be on my unit. It is not listed on the tag on the outside unit. I found information in the product manual that indicates:
"Design Subcooling 7 +/- 2° F @ the liquid service valve: ARI 95° F test conditions"
Do I need to be concerned with the "ARI 95° F test conditions" or can I consider a subcooling value of 5-9°F acceptable on my system?

Thanks again!
 
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If it is an older system superheat (With TXV) should be 15-20 degrees fahrenheit, and subcooling 20-25 degrees fahrenheit.
The proper, factory approved method is to weigh the charge into a system that has been vacuumed and leak tested. If that cannot be done use the superheat/subcooling method. The newer systems have much lower subcooling requirements some as low as 8 degrees.
It is very important to check with manufacturer to correctly charge a system as there are variations in specifications.
 
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Thanks Cajuray, the "Design Subcooling 7 +/- 2° F @ the liquid service valve: ARI 95° F test conditions" came directly from the manufacturers service manual. My question is regarding the 95°F test conditions. Does that mean that it would have to be 95°F outside for me to expect 7° +/- 2° subcooling readings? So, if its 80°F outside, I should be looking to achieve different subcooling numbers?
 
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Also, do you use a vaporizer for r410A or do you just open and close the valve in short bursts?
 
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I checked the system pressures/temps this morning. System Recap: 3.5 Ton HeatPump, 12 Year Old Unit, R410A with TXV, Mfg. Target SubCool 7°F +/- 2°F @ 95°F Outside Temp
Today's Outside Temperature: 80°F, Ran System 15 Min prior to recording measurements, no sweating on Suction Line.
Gauge Low Pressure Side: 82 PSI ~22°F, Suction Line 81°F - so SuperHeat of 59°F
Gauge High Pressure Side: 215PSI ~72°F, Liquid Line 80°F - so I guess that is a negative Subcooling: - 8°F

Is it possible that the system has a slow leak? I know we had it leak tested a few years back because it had lost some refrigerant; however, the leak was not found and it was only a small amount of refrigerant and so the thinking was that we recharge and see what happens. Its been a few years and we have 3 other AC units and this one is in the master bedroom. I think there has been a tiny leak for a long time and the unit has been very slowly leaking R410A this entire time. I don't think it was noticeable because we have other units, and we have the thermostat to raise the temperature during the daytime, and the temperature is lower at night so the unit does not have to be working perfectly in order for our bedroom temp to be fine. I think it may have just lost enough refrigerant to go over the tipping point to what was acceptable to where we are now. I would not even have noticed except that I noticed the unit running more often at night and then I felt the liquid line and it was cold, not warm.

or is that not a possibility?

Thanks again for any information you can provide! :)
 
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> and the smaller liquid lines feeding the coil get frosty when the unit is running

this can only happen if gas is expanding into the coil: otherwise there would be no cooling on any line whatsover

> On the outside unit, the air leaving the top of the unit is also ambient temperature, it is not warm like it is normally.

this isn't possible: it's impossible for the HIGH PRESSURE side to be cool, the coil to have no cooling

it's physically and scientifically impossible.

You also said the bottom 1/3 is cool but the top 2/3 is room temperature - which contradicts "no cooling"

Perhaps you mean "poor cooling"
 
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Thanks Cajuray, the "Design Subcooling 7 +/- 2° F @ the liquid service valve: ARI 95° F test conditions" came directly from the manufacturers service manual. My question is regarding the 95°F test conditions. Does that mean that it would have to be 95°F outside for me to expect 7° +/- 2° subcooling readings? So, if its 80°F outside, I should be looking to achieve different subcooling numbers?
there are a few Youtube videos on how to diagnose an evap freeze

there are countless Youtube videos on how to measure subcooling outdoors

there are countless Youtube video on how to add r410a refrigerant

However: you didn't mention finding and fixing a leak or other problem, or pressure testing with nitrogen, or vacuuming (these are needed depending if air got in due to low freon - or if low freon is infact the problem)

However; you didn't mention if you'd isolated the problem yet - so adding R410A is CERTAINLY NO, NOT, DO NOT
 
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Your observations are unusually good however: knowing the temperature at bottom and top of coil.

Coolness means gas or liquid did infact expand in the tubes (liquied, if your freon level is high enough). (good cooling requires the latent heat of liquid - but never mind that)

The design is that before the liquid exits the evaporator it becomes a gas (loosing/absorbing all latent heat of) due to the heating of the airflow (which should be a full designed airflow - ie no air vents closed)

If we are warm 1/4 way through this means only a fraction of the tube is in use. If there was more fluid entering the tubes would be cold throughout and cold on exit (the fat pipe back to the outdoor unit would be, say 40-50 deg or less).

GO ON YOUTUBE

FIND "HVAC SCHOOL"

watch every class they have

subcooling (for txv) actually diagnoses your CONDENSER not your EVAP. superheat diiagnoses the EVAP (but TXV controls SH). however: NEITHER SUBCOOL NOR SUPERHEATING DATA MEAN ANYTHING if you have (1) PSI pressures under or over limits (2) restrictions (3) improperly insulated or cross lines changing line temperature.

your unit looks new - unless the installer "dumped welding ash and metal" into the pipe i kinda doubt you have a newly developed restriction (your hvac has been working a while right? a restriction would come up pretty quick unless you had a filter/drier fall apart. some manufactures have stopped installed them btw)

Let's NOT assume a (rather new indoor installed) filter drier has failed.

The best bet? go outdoors with some Spray-9, spray your tubes in the most likely places a leak would be - while the coil is not hot from running of course.

 
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My big warning here is: if your unit failed you DO NOT have time to learn in, buy equipment, all in time - learning and fixing hvac takes more time and money than you think.

Here's my advice: if you think your going to learn hvac and fix this by replacing parts or installing R410a: GET A MINI SPLIT to keep your home safe from humidity/mold because it will take at least weeks.
 
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So. SC and SH. You know you have low gas/liquid flow into the evap from very good observations.

Anything can cause that, low freon due to outdoor leak is by far the most likely.

other possibles: restriced line between condenser and evap. txv not opening. air in freon (you should probably hear a noise at your txv if there is many of air bubbles) "non-condensibles". not enough liquid made available and stored in condenser (ie, a bad / weak / damaged pump might not compress gas to fluid correctly even if freon is high enough)
 
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CORE COSTS:

your cheapest bet if you have a leak: install a new outdoor unit. the skills need are less than that of repair by far. but this will take you a while! so you'll still need a (temporary) mini-split while you figure it all out

alternative: diagnose, test, buy allot more equipment, wait week(s) for parts and tools

whatever you do: DON'T RUSH IT. don't take steps before you are sure of them
 

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