Central AC Output Suddenly Varies: Cold-Warm-Cold-Warm


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Old 03-18-11, 06:51 AM
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Central AC Output Suddenly Varies: Cold-Warm-Cold-Warm

Our AC has been fine then suddenly last night I noticed the the temp output in the house felt warmer than is "should have" been. I began monitoring the temperature output every minute using a standard digital thermometer with probe pushed into the vent which happened to be furthest from the evaporator:
AC Output Temperature Observations

Time(PM) Temp F
Getting colder
8:58 65.3
9:01 59.2
9:03 56.3
9:06 55.8
9:07 55.0
9:08 55.8
9:09 53.8
9:10 55.2
9:12 53.2
9:13 54.5
9:14 52.7
9:14 53.1
9:18 52.5
9:33 52.2
9:34 52.3
9:35 51.1
9:38 51.3
9:39 52.3
9:42 I backflushed condensation drain pipe just in case something was stuck in it. But prior to backflushing it, it was dripping water.
9:43 52.5
9:43 52.3
9:43 52.2
9:43 51.8
9:43 51.4
9:44 51.1
9:44 50.9
9:45 51.1
9:45 51.3
9:45 51.6
9:45 52.0
9:45 52.3
9:46 52.5
9:46 52.3
9:46 52.0

You can see that just about every minute the temp is getting either cooler or warmer, opposite of what it did 1 minute ago. I have never seen my AC perform this way. I know this is not right.

I believe the temperature "swing" was a bit more dramatic than those above temp readings show because it takes time for the digital thermometer to "catch up" to the actual temperature.

I also noticed that there was some frost on the thick cold return refrigerant tube at the entrance to the condensor. Both the compressor, condensor fan, and evap fans are running steadily. That is, they are not cycling on and off.

I changed my partially dirty air intake filter but that had no effect on the temperature swing. I also turned off the condensor for about 30 minutes in case there was any ice on the evap then it could melt off, but after running the AC again, the temperature swing continued.

The condensor unit (Ruud 3 ton) was installed in Apr 2000 so it is about 11 years old. I have done some basic repairs to this unit myself including replacing the contactor, putting a new terminal on one of the compressor supply wires after it fell off and shorted, installed a new circuit breaker which was getting "weak" and causing brownouts to the condensor unit, and also washing the condensor fins with a hose.

The evap unit was replaced some time prior to the condensor unit so it is probably 12-14 years old or so. I do have to admit that the evap coil was last cleaned as long ago as approximately Mar 2004. I will be going into the attic this weekend to see how dirty the evap coil is, clean it up a bit, and to also flush out the condensation line.

The last time the refrigeration circuit was serviced was in Sep 2004 when it sprung a leak at a solderpoint near the condensor. The first tech who worked on it used the compressor to vacuum the system, a practice that I understand is not correct. The second and last tech who worked on the system right after that was needed because refrigerant leaked out through a schrader valve and loose cap. I am not sure if the circuit was vacuumed that time but I am guessing no.

What in the world would cause a temperature swing as I observed? Low refrigerant charge due to leak in the refrigerant circuit? I also read that a faulty expansion valve might cause this condition. Further I read that moisture in the line might be causing the expansion valve to freeze shut - thaw open - freeze shut - thaw open causing the temp swing. I am thinking about getting some gauges and seeing what the pressures are doing. I am quite mechanically inclined, but in no way shape or form will I be discharging/charging the refrigerant circuit myself. I am mainly interested in the cause and detailed explanation so that I do not get ripped off when I do call a tech out (I have been ripped off before).

Thank you for any direction. But whatever I do I cannot replace my AC units right now due to the high expense.
 

Last edited by PepZ555; 03-18-11 at 07:25 AM.
  #2  
Old 03-18-11, 09:33 AM
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I think those temperature numbers are OK, 1 degree swing is fine, (you are not in a controlled research lab.) your return air (feed back to your AC unit) temp may vary accordingly too. What is the outside temperature at the time? what is the set temp on your T-stat?. but I do worry about the frost on the pipe, that needs to be checked out.
 
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Old 03-18-11, 10:03 AM
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Outside temp at the time was approx. 65-70F. Inside temp was about 70-75.

I've had that digital thermometer on the vent for nearly a decade now and have never ever seen a swing. The temp might inch up or down as the day progresses, but never up and down every minute. Something is definitely not stable.
 
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Old 03-18-11, 04:24 PM
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Well, I see two problems here. first is the frost that I mentioned before. second, why doesn't your system cycle on/off ? with outside temp only 65-70F, most time you don't even need A/C, another word, A/C won't even tun itself on (unless you have other heat source in your house). but it continue run from 8:58 to 9:46 with register incoming air temp around 52 (a perfect 18F split), that does not make sense. I suggest you wait for a few weeks, when outside temp get to at least 75 or 80F and then do another test.(you may not have any problem)
 
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Old 03-19-11, 06:03 AM
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Here in FL, you might run the AC not so much for the temperature differential, but just to get rid of some of the humidity.

Last night I opened the side of the evap unit. I could only access the "clean" side of the coils. The intake side of the coils is not easily accessible so I couldn't take a look there. But the clean side was indeed clean - hardly a blemish of dirt on it. I hope the other side isn't totally plugged with dirt... I did take a garden hose to the coils and rinse them down, and also I flushed out the condensation drain.

Then I ran the unit and observed frost develop on the cold side of the expansion valve tubes and also some of the coil "U"s on the side of the evap. Then I would hear a bubbling sound and the frost would go away. After a short time the frost would come back, then I would hear the bubbling sound and the frost went away again. This repeated over and over.

Outside temperatures were warmer last night. Perhaps 75F.

Inside, vent temp continued to vary.

I am no expert here but I think it is low on refrigerant...?
 
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Old 03-19-11, 02:08 PM
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I did some more investigating today.

I purchased a dual pressure guage and hooked it up. I noticed after the AC was on for a while the pressures started going up and down (both high and low sides) at the same frequency that the temperature variations ocurred inside (as described above).

The low side would vary from 24-35psi while the high side would vary from 225-235psi, all at 81F outside temp. I determined that as the pressures went higher, the temp coming from the vent was cooler. Likewise, as the pressures went lower, the temp coming from the vent was warmer.

Then I decided to clean out the condensor coils with a garden hose so I thourougly did that. When I turned the AC back on it operated properly from 2:23pm, the vent temp inside steadily dropped until it leveled off at 52.2F at 2:38pm. It remained 52.2F through 2:45pm, and the pressures remained stable. When I checked the temp again at 3:02pm, the problem had returned; the temp was climbing and dropping, and so was the pressure readings.

Earlier, when the AC was stable, I checked the highside pressure reading which was about 230 or so and compared that to the charge chart that I have for my unit. At the ambient temp of 81-82F, 230 is just about right on the money.

Futher troubleshooting, I removed the evap unit's return tube that was held on with a strap, because I highly suspected there to be all kinds of dirt on the evap coils. I was surprised to see fairly clean coils. There was just a little black stuff in some areas. So I used the garden hose to get all of that off and reattached the return duct. This too had no effect on the temp/pressure variations.

The problem continues and I am clueless now. Could the compressor be leaking between high and low sides internally? This is really bugging me.
 

Last edited by PepZ555; 03-19-11 at 04:24 PM.
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Old 03-19-11, 07:21 PM
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Now I see your problem, The low side pressure is too low. the most common cause is lack of freon. every unit is different, but in general, the low side pressure should be around 70, not 20-30. large pipe should feel cold like a glass of ice water, and sweat outside. YOu can easily get a tech to put some freon into the system, but the problem is you have a leak somewhere that needs to be fixed first. good luck .
 
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Old 03-21-11, 08:27 AM
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OK thank you very much for your help. I really appreciate it.
 
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Old 03-25-11, 10:42 PM
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I finally had a tech come out today, the same one who came out and serviced our AC unit last which was in Sep 2004. He was an old neighbor of ours and I trust him. He said the R22 level was low, and he put in approx. 3lbs. He brought the low side up to where it should be which brought the high side up a bit higher than it should be, and he advised there is likely a bit of a restriction in the air handler coil, but it is not too too bad.

He charged me $150. Previously, I almost had another guy come out and he would have charged $180 for a refrigerant fill. And I know that many other companies will charge quite a bit more than that even. What I don't get is how some of these guys charge so much per pound of R22. With the right licensing & documentation, you can get a 30lb tank of R22 for $7.53 per pound, shipped. How are some of these guys charging $30-$55 per pound? Basically this tech earned $125 for the hour he was there. That is some heck of a profit.

I am in the wrong business; I use to think fixing computers was a good money maker. Not so when compared to the AC business...

Thank you for your help and suggestions.
 
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Old 03-26-11, 08:34 AM
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Originally Posted by PepZ555 View Post
What I don't get is how some of these guys charge so much per pound of R22. With the right licensing & documentation, you can get a 30lb tank of R22 for $7.53 per pound, shipped. How are some of these guys charging $30-$55 per pound? Basically this tech earned $125 for the hour he was there. That is some heck of a profit.

I am in the wrong business; I use to think fixing computers was a good money maker. Not so when compared to the AC business...

Thank you for your help and suggestions.
Do you know what it takes to run a business?

Anyone can buy a part, just like anyone can purchase the products to build an airplane. However, can just anyone assemble the parts correctly to ensure the end user is safe? Why is it that so many people disregard the professional training that goes into becoming a top notch trades person. All they see is the price of the part. What about the cost of obtaining a trade certification, the continuing education, the liability insurance, fuel, vehicle maintenance, vehicle payments, insurances, office expenses, advertising, phone systems, banking fees, bad dept, capital depreciation, accounting and legal, workers comp, holiday pay, vacation pay, training pay, pension plans, benefits package, software and hardware costs, equipment costs, storage costs, cost of handling stock, warehousing costs, lights,utility costs, non billable time costs, office staff, answering services, bookkeeper costs, rent or lease costs, taxes, etc? Are we as trades people to absorb all these costs? Who is to absorb these costs? I guess we're supposed to sell the part at the same price as we purchase it for and then charge only for our time at the job, not for the non billable time, and some how try to cover all those other costs plus some how support our companies, our employees, and our families. Is it no wonder why there is a shortage of skilled trades people. When is someone going to start educating the public on the costs associated with running a business?
 
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Old 03-26-11, 01:44 PM
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$150 is on the low side. most people charge more than that. By the way, did he say anything about the leak ? leaked 3lb in one year (assume your AC or freon level was OK last year) is pretty big. You need to check the low side pressure in about 2 weeks and again in 4 weeks to see if the pressure is held up.
 

Last edited by clocert; 03-26-11 at 04:05 PM.
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Old 03-26-11, 08:01 PM
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150 is a fair price. good price actually. Sounds to me as if you have a little bit of a blockage in the line somewhere, either that or some air trapped in the lines. nothing to worry about really with only 10psi swing on the high side.
 
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Old 03-26-11, 10:12 PM
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You missed the point. the problem is you lost 3 lb of freon in one year. there has to be a leak some where. that is why I suggested you check low side pressure to make sure your freon level is OK. (don't worry about the high side, like you said, 10 psi swing is not an issue, but that has nothing to do with your freon leak)
 
 

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