only buttom half of coil cold


  #1  
Old 06-20-04, 04:27 PM
freakius
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only buttom half of coil cold

hi everyone. i have an a/c unit in my attic. it started to blow barely hot air 2 years ago. had a guy check it out. he said i was low on freon, found a broken shrader (sp?) pin, replaced and recharge. it worked fine for a little while, then started happening again. since it was toward the end of the summer, i waited till last year. another company found a leak (showed me too) in the line. also said the condensor is almost bad because it's been running with the leak for so long. finally this year i had my condensor changed out. it worked well for a few days, then the air started to get warmer and warmer. i had the company out for warranty. the guy checked other things, and said the coil is too big (2.5 ton) for the condensor (2 ton). after he left, i opened up the pannels in the furnace and checked coil. i found out only the buttom half of the coil is cold. the top coil is not cold at all. any idea why? is it because the coil is too big and the condensor cannot fill the coil?

thx (hot in atlanta)b
 
  #2  
Old 06-20-04, 07:28 PM
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There are a couple possible problems for the symptoms you describe. You could be low on refrigerant, or there could be a problem with your expansion valve. In either case there is insufficient refrigerant entering the evaporator coil. That's why it's getting warm near the top. There should be about a 10 degree (max) difference in temperature between the top of the coil and the bottom. If you see more than that you need additional refrigerant entering from the expansion valve. There should be a sensor line from the expansion valve that should be in good contact with the line exiting the evaporator. It will most likely be a very small copper line that ends in a bulb, and the other end is connected to the expansion valve. If that bulb has become disconnected from the tail coil of the evaporator I could see how you could have the problem you describe. Of course, if there's insufficient refrigerant in the system then the expansion valve could be wide open but little refrigerant is available to get through. On other possibility comes to mind while I was typing out an answer, you could have a plugged filter/drier, I suppose. That would prevent the liquid refrigerant from arriving at the expansion valve in sufficient quantities to completely cool the evaporator coil.

I could see that if the evaporator coil were too big for the condensing coil how you could have problems, but you would most likely only see it under worst case conditions. If it was very hot outside and your A/C was working at max capacity then you could have a problem with insufficient liquid returning from the condensing unit. Assuming that you are operating at somewhat less than max I'd say the winner is "low on freon" with the other two bets as place and show.
 
  #3  
Old 06-20-04, 09:13 PM
freakius
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thank you for the detailed reply. i thought about the possibility of low freon. however, the guy who came back to check on it says it's okay. the two gauges read 60 and slightly below 240. is that normal? in fact, the amount of freon has always been a sore point for me. i have had 3 different companies come to look at it, they each gave a different diagnosis. the new condensor unit has a "line dryer" on it (the liquid line). could that be "choking off" somehow? the temnperature difference is big between the buttom half and the top half of the coil.

before they changed out the condensor, the guy said i'm 5 pound too much in freon. he said it's because the last guy put extra in to compensate for the leak. when the guy came out to check out on the new condensor unit, he said it was still a pound too much and let some out. before he let it out, the condensor was blowing out very hot air. i thought it was working well in that sense. now with some freon out, it's not blowing as hot air (no pun intended the new guy (same company) said it's better this way because it was too high of pressure and overworking the condensor. they said that's how the condensor was going bad before.

so, what SHOULD be the correct reading for freon level?

thx
 
  #4  
Old 06-21-04, 07:47 AM
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You can't say exactly what your pressures should be because it depends on the load and the outside air temperature and greatly upon what kind of refrigerant is installed. I'm going to assume that you are using R22 because that's the most common one used in A/C units. Your 60 psig low side pressure would be some low if you were trying to cool a hot house, but the 240 high side could be about right on a hot day. With a reading of 60 you could easily have a restriction in the liquid line or a bad thermostatic expansion valve (TXV). When the freon picks up heat from the evaporator coil it turns from a liquid to a gas and the more heat that is absorbed the higher the suction pressure will be. If you had a well working evaporator coil and just started the unit in a hot house I wouldn't be surprised to see 70 or 75 psig on the low side with R22. As the house cools off the low side pressure will slowly fall towards the 65 psig area and stay there when things are nice & cool inside. The main thing that concerns me is the big temperature difference between the inlet of the evaporator coil and the outlet. As I've stated in a previous post, you shouldn't see more than about a 10 degree difference in temperature there. Anything more than 10 degrees means that the coil is starved for freon. The main controller of the amount of freon entering into the evaporator coil is the TXV. It senses the difference in temperature between the liquid side and the suction side of the coil and opens & closes the valve to let more or less freon enter into the coil to keep the temperature difference between about 5 and 10 degrees. The technical term for that difference in temperature is the superheat. You don't really get any benefit from the superheat other than it makes darn sure that the freon is a vapor before it enters into the suction side of the compressor. Getting liquid into the compressor would be bad and could cause the distruction of the compressor. The fact that you are only getting cold on about half of the evaporator coil means that only a small amount of liquid freon is getting past the TXV into the coil and all of that refrigerant is being boiled off and converted to a vapor well before the end of the coil and you are loosing the cooling benefit of the other half because no liquid gets that far. All of the refrigerant should be converted completely to gas only at the very end. Most of the coil should operate at the evaporating temperature cooresponding to the operating pressure with a small 5 to 10 degree temperature rise at the very end.

Again, take a close look at your TXV and sensor. Make sure that it's in good order. Be sure that the TXV sensor bulb is firmly fixed somewhere on the outlet side of the evaporator and isn't dangling free somewhere. Put your hands on any filter/drier in the liquid line. See if you can feel a temperature difference between the inlet and the outlet of that. If you can, you have a plugged filter that will have to be replaced. From the pressures you gave I would say you either have a plugged liquid line or you have issues with the TXV. A plug in the liquid line would mean that the TXV could be wide open but there still is insufficient freon flow available to produce the desired cooling effect.

At least that's my guesses based upon the information provided.

I just thought of another possibility......If they A/C guys didn't know what they were doing you could have a lot of air (and/or moisture) in the refrigerant lines. That would give you weird pressure readings and little cooling. You have to use a vacuum pump to get everything out of the lines and you usually measure how much freon you are putting into the system with a scale. If you have long lines, however, using a scale can be tricky unless you know exacly how long they are and someone has left you that information along with a chart telling you how much (in pounds) the manufacturer says it's unit should hold.
 

Last edited by jughead; 06-21-04 at 08:16 AM.
  #5  
Old 06-21-04, 08:18 AM
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Wink

Im with jughead here. Also this time of the year the air off the outdoor unit should be HOT if all is right. Also with the TXV are an orifice there. Can it be they got noncondensables in the system now and they have contaminated it. Did they vacuum the system when they put the new unit in??????????


ED
 
  #6  
Old 07-02-04, 06:48 AM
freakius
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There may very well have been contaminants in the line. In fact, that's what one of the other guy guessed before, because of the leak I had. They put in a line dryer at the end of the liquid line toward the condenser outside. They said that would get the contaminate out. Is that true? I don't know if they vacuumed the line or not. I'll check. I'm also going to check the TXV. Thx a lot everyone.
 
  #7  
Old 07-02-04, 07:38 AM
freakius
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Okay I just checked. I assume control flow adapter is what you are talking about, right? It's screwed on tight on the lines (4) leading into the evaporater coil. It's also iced up. It's an A-frame evaporater coil. There are half-circle lines feeding into the coil. Only the buttom 3 half-circles are cold. The top ones are about room temperature. I have called the company to come back out to add some freon. Also, I noticed one of the ducts is torn at the connection into the distributor (right word?). That doesn't seem to be what's messing up the top half though, because they are not cold at all. Any other suggestions? Thx again Jughead and Ed.
 
  #8  
Old 07-02-04, 08:31 AM
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When you had the condensor changed did you also get a new evaporator? If it worked good the 1st few days and gradually got warmer it sounds like a leak and if you didn't change the evap there's a very good chance that's where it is. Any good tech don't add and remove refrigerant they take superheat or subcooling and charge it by that to get it exactly right.
 
  #9  
Old 07-02-04, 08:58 AM
freakius
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Actually, found the leak last year in the liquid line leaving the condensor. It's very close to the condensor. Bad welding job. They didn't think there's a leak in the evaporater. They are coming out today. Should I ask them to charge it by doing super and sub temps? Thx.
 
  #10  
Old 07-02-04, 09:30 AM
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If it's a TXV "expansion valve" they should use superheat if it's a fixed orifice they should use sub-cooling.
 
  #11  
Old 07-02-04, 10:23 AM
freakius
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There is a little label that says flow control adapter. Seems more like a fixed orifice to me. What does it mean to "use subcooling"? Thx Mattison.
 
  #12  
Old 07-02-04, 12:18 PM
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I type to fast when I'm in a hurry like I am now. You charge by subcooling with a txv since they maintain a constant superheat and you charge a fixed orifice with super heat sorry for having it backward.
 
  #13  
Old 08-06-04, 08:56 AM
gandl2123
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A/C Piggy Back......Thanks

Wondered if I might jump on our band wagon and get some of this great help you have gotten. Hope your problems have been solved. Now for mine....

I just hooked up my A/C system (99 Mobile Home) 2.5T Ducane Condesor Unit and Un-Cased 2.5T Evaporator Coil. Filled with freon and wasn't blowing cold enough so we took the door off and noticed right away that the skinny lines going from the (expansion valve/piston)??? to each part of the evaporator coil were white with ice (frozen up). Turned the sytem off and it was so cool to see the ice melt rom the expansion valve down....Ha. The tech said that the problem was with the evaporator coil as many of the fins (aluminum) on the coil had been bent over during a recent cleaning with a garden hose.....ME. HeHe!

Is this the problem...does it really matter how much air is flowing thru the coil that it would make it freezeup there?

Thx
Mr. Tripp (Please respond to site and gtripp@aug.edu would be even better. Thx Again!
PS I'm shopping for an uncased 2.5T evaporator coil but don't want to start buying parts to fix if that is not the problem. The tech said the system was full of freon and the compressor sounded perfect....
 
 

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