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air flow

Here we go:

At an 80-degree ambient, the high side pressure for a standard efficieny system should be about 245 psig. No problem there.

The low side pressure could indeed be low, since at 50 psig we are getting a 26-degree evaporator coil. But, then again, it may be correct, especially if the system is operating when there is no-load on the coil (early in the morning, etc.)

Here's a neat trick. For a standard efficiency system, the evaporator saturation temperature should be 35 degrees lower than the dry bulb temperature of the return air. For example, if the return air temperature is 80 degrees, the evaporator saturation temperaure should be 50-degrees. This is referred to as the delta-t OF the evaporator coil, not to be confused with the delta-t ACROSS The coil, which is the difference between the return and suply air temperatures.

I can't stress the following enough. BEFORE you make conclusions about the system, it is imperitive that you take a complete set of readings AND evaluate them before altering the system.

From the information you provided in your post, the following are possible causes that would result in the coil freezing:

Blocked or restricted return air system

Blocked or restricted supply air system

Insufficient blower speed

Underfeeding metering device

Liquid line restriction

Noncondensables (HIGHLY UNLIKELY)

Guys, in order for us to provide the best possible help, please provide as much information about the system as possible. By that I mean a complete set of tempertures (pressures are not important) including saturation t emperatures, coil outlet temperatures, ambient conditions, humidity, return air temperaures, supply air temperatures, etc.

One of the major traps that AC technicians fall into is that they troubleshoot the system BEFORE they should. Unfortunately, the mentality goes something like this:

Low operating pressures = undercharge

High operating pressures = Overcharge/noncondensables

AC systems operate under a very predictable set of laws. They are called the laws of physics. The really cool thing about this is that, unless you are some omnipotent being or entity, you can't change the laws of physics. A complete set of readings along with the knowledge of how to i nterpret them, will leas you to a service call success rate that will astound you.

Let me share a quick story with you.

I, as may of my colleagues, learned this industry from my father. Now that my dad is on the other side of the lawn I can let the cat out of the bag. I refused to do so while he was still alive, mainly because of the fear that the truth would kill him. I no longer live with that fear.

The truth is that my old man should have worked for D'Agostino's, since he was the biggest butcher around. My father coul jerry-rig an AC unit and have it work with moderate success.

Me, being the inquisitive youngster that I was, always inquired about why he was taking particular steps to resolve a problem. The answer I always got was along the lines of, "Shut up and do it, so we can get paid".

It didn;t take me very long to figure out that my old man wasn;t sure why he was doing something, but was doing it because he had reasonable success, or witnessed someone who had reasonable succcess by doing something similar.

My hunger for knowledge grew at an alarming rate and am proud to say that I have learned tons about the industry and enjoy sharing informations with my colleagues and students. I had one teacher colleague a number of years ago who, whaen asked a question by his students would answer with, "well hat do you think?". There is nothing wrong with doing this if, at some point in the future, you actually give the answer. He never did. It became obvious, not only to me but to his students, that he had no clue. As a service to all, he retired from the teaching profession.

In your quest for knowledge, do not be afrais to aske questions. As you can see, there are a lot of people who are eager to share thir knowledge.

Have fun!


  • ed wallace
    ed wallace Member Posts: 1,613
    air flow

    question about cfm have installed a comfortaire a/c its rated at 1,000 cfm it has a 20 x 20 return and a 16 x16 duct after the plenum the blower is prewired on hi speed the unit ran 3 days before it froze up i had installed a pleted filter in the return grill today when i installed a cheap filter the out door temp was 80 degrees the hi side pressure was 245 lbs low pressure was 50 lbs any ideas why it froze up

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  • bill_51
    bill_51 Member Posts: 27

    condenser size?? number of supplies?? not a whole lot of info here but from what little you told it sounds like noncondensibles and a low charge.........whats the delta t for starters??
  • ed wallace
    ed wallace Member Posts: 1,613
    airt flow

    its a 2 1/2 ton system new condenser and air handler both comfortaire units that match at 10 seer there are 3 bed rooms and a bath then at the end of the trunk there is a 12 x 12 grill dumping into living room i had 16 degree delta T across the coil on monday

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  • Empire_2
    Empire_2 Member Posts: 2,343

    If you can, please use a period and an occational comma where you are seperating statements. Seeing that your condenser was precharged and from what you are describing,...3 bed and a 12x12 grill, what are the total # of S/A runs and what do you have for returns? 12x12 grill,...Is that a return or supply. Do you think you have enough S/A rund to take care of the 1000cfm? Was the system evacuated before start up? 80* day @250psig seems hi, espically with a suction of 50 psig...
    The more info the beter so we may answer your question.
    Mike T.
  • ed wallace
    ed wallace Member Posts: 1,613
    air flow

    well excuse me for missing commas and periods. the old system was in use for 15 yrs. cant say what the trunk size is due to the fact its above a finished cieling. but there are 3 bedrooms and a bath all with there own supplys. the the living room has a 12 x12 supply on the end of the ductwork the return is 20x20

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  • Jeff Lawrence_24
    Jeff Lawrence_24 Member Posts: 593
    I've noticed

    That the newer systems are a lot more 'finicky' about air flow than the old systems.

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  • bill_51
    bill_51 Member Posts: 27

    they are more finicky but i'd still go with noncondensables and undercharged..........i'm not sure if comfortaire precharge their condensers for any amout of line set length, how long is the line set?....16 degree sounds a bit low and if airflow was an issue your delta t would show it by being up there(over 20). if the indoor temp and humidity was really high, lets say 85 and about 75% or higher humidity, then i'd expect a delta t of around 16 for the first half hour or so

    i hate these sinus head colds.........keeps me up all hours of the night
  • don_145
    don_145 Member Posts: 1

    guys have to stop bringing up noncondensible everytime you
    see high head.I've never seen noncondensible not effect the low side as well.

    Ed we still love you no matter how you write.

    It sounds like to me a partical restriction and overcharge.
    However that may not be the case if the airflow is not correct.
    Also Ed if I were you I would remove that pleated filter and go back with one of the cheap one.Those filter are very restrictive and if not figure in during design may be hurting you as well.

  • Empire_2
    Empire_2 Member Posts: 2,343

    Sorry bout the comma/period..Iwas just trying to make sense of the Q: It kingda bled together. Anyways, is the evap also new? I was thinking along the lines of restriction. Just out of curriocity, what is the super/heat?
  • don_146
    don_146 Member Posts: 1

    I have one for you..whats your thought on this system?

    Indoor temps 78db and 68 wetbulb.
    Outdoor temps-95F
    Return in 78/ supply out 58 degree
    SST-40 degree.
    SLT-47.8 degree.
    Condenser in 95/condenser discharge temps 108.
    RLA-I forgot to get.


  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Looks Good

    This system looks like a 10 SEER unit that is operating very efficiently.

    YOu have a 20-degree delta-t across the evaporator coil and a 38-degree delta-t of the evaporator coil. Both of these readings are good.

    You are also operating with 7.8 degrees of evaporator superheat and 14.5 degrees of condenser subcoooling.

    THe one thing that seems low is the difference between the condenser saturation temperature and the outside ambient temperature, which is 17 degrees. This is actually indicative of a higher efficiency system, but the 40-degree saturation temperature is an indication of a lower efficeincy model. Higher efficiency systems typically operate with evaporator saturation temperatures in the range of 45 degrees.

    I am confused about the last two pieces of information you provided, ie the "CONDENSER IN 95/CONDENSER DISCHARGE TEMPS 108". Could you please clarify.

    But, from the information you provided, it seems that this system is operating okay. Are you having a problem with it?
  • don_147
    don_147 Member Posts: 1

    Condenser temp split.95 in/108 out=17 degree split.

    No problems, just a pm call.Forgot to mention only 10ft length of line set,8 ft of it inside building envelope as well as the equipment.

    So under these condintion could the 10 seer condenser act more like a higher efficent outdoor unit?

    The condenser split is where I'm confuse,has you said its acting like a higher efficency piece of equipment,however it clearly says 10 seer.

    I would have expect to seem a higher temps split even with
    the lower indoor humidity,even higher as the humidity increase.But I was not going to hang around and wait for that to happen either.LOL.

    Thanks for your time Professor.

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    As I mentioned

    As I mentioned, the system seemed to be a 10 SEER unit, but is definitely operating well. If you can provide me with the temperature at the inlet of the compressor (about 6 inches from the copressor inlet) and the horsepower of the compressor, I can plot out the entire system for you on a pressure enthlpy chart and determine the actual EER at which the sytem was operating when the readings were taken.

    The shorter refrigerant lines definitely add to the higher operating efficiency.
  • don_120
    don_120 Member Posts: 15
    Thanks professor

    I wish I could,however I hope not to return untill next year at this time.

    However, I sure would love to see a example of the math if you would be so kind.
  • Empire_2
    Empire_2 Member Posts: 2,343

    Trying to get used to the New Wall.........
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