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  1. #14
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    Feb 2010
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    How clean is the condenser coil? High head because of a dirty condenser could be throwing off the rest of your numbers.

  2. #15
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    Sep 2007
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    State College, PA
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    I would see if the compressor can pump the system down to at least 0 psi. If not, I would say bad compressor.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  3. #16
    Join Date
    Mar 2008
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    West Monroe, LA
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    I measured the indoor temps at the return grill. I was hoping I was not going to hear weak compressor

  4. #17
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    Jul 2005
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    5,576
    Quote Originally Posted by daviddsims View Post
    I measured the indoor temps at the return grill.
    Unfortunately that doesn't change things.


    I was hoping I was not going to hear weak compressor
    That doesn't either.

  5. #18
    Join Date
    Nov 2010
    Location
    N. Idaho
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    586
    Quote Originally Posted by hvacrmedic View Post
    You have a bad (under-pumping) compressor. High suction, low head, low amp draw, low deltaT. Adding refrigerant will only run the suction pressure up higher, and it's already way to high for those conditions on an old cap tube system.
    So obviously experience comes into play in recognizing the symptoms. As a greenhorn how can I learn to recognize when pressures are too high. Manf. charging charts?

    I would guess a subcool number would be good to know in this situation? Add a bit of refrigerant and watch numbers?
    "I've got my Gas Mechanics license"
    "Yea, well my 16yr old daughter has her drivers license, doesn't mean she knows how to drive"

  6. #19
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    Jul 2005
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    Quote Originally Posted by Damien View Post
    So obviously experience comes into play in recognizing the symptoms. As a greenhorn how can I learn to recognize when pressures are too high. Manf. charging charts?

    I would guess a subcool number would be good to know in this situation? Add a bit of refrigerant and watch numbers?

    Here's the condensed version.

    Part 1, Evaporator TD

    Evaporator temperature (SST) determines the cooling capacity of the system. If you have 400 cfm per ton moving through the coil, and you are trying to get the nominal capacity out of the system, then the coil temperature must be a certain temperature in order to do that. At 80 indoor dry bulb, and 67 indoor wet bulb (rating conditions), the SST is going to have to run at 45 in order to achieve the desired capacity. Let's look at the performance data for a typical evaporator coil.



    As you can see by the highlighted numbers the SST must be running at 45 plus or minus a few tenths in order for the evaporator capacity to match the system's rated capacity. The evaporator capacity changes dramatically when going from 45 to 40 and to 50 SST. At 50 SST the system is running at only 78% or so of nominal capacity. Obviously a 50 SST would correspond to a higher than normal suction pressure on this system. These physical characteristics of the coil define what is called the "Design Evaporator TD". Since physics isn't concerned with brand names, we should expect to get the same results looking at any other evaporator coil's data sheet. It is under these universal test conditions that the coils are sized and get their capacity ratings. IOW, it doesn't matter what brand of unit you're dealing with, if the manufacturer followed protocol, you must have a 45 evaporator at 400 cfm when the return air is at 80 dry bulb/ 67 wet bulb. The "Design TD" is therefore

    80 - 45 = 35

    Part 2, High Efficiency Systems and Over-sized Evaporator Coils

    In this section I'd like to go over the reason that high efficiency systems tend to utilize much larger evaporator coils. The primary reason is that the same capacity can be obtained at a higher SST. Looking back at the performance table in Part 1, you should be able to see that the 3 ton coil produces 2 tons of capacity at a SST of 50 with 300 cfm per ton. So if we stick that 3 ton coil on a 2 ton system, we could achieve 2 tons of capacity at 600 cfm and with a SST of 50. Now if we increase the condenser coil size as well, we can get the head pressure to run lower, and as a result the compression ratio is greatly reduced. Efficiency is greatly improved because we're still generating a full 2 tons of capacity even though the compressor is doing less work. So in a nutshell this is why the coils are over-sized on higher efficiency systems. However, note that the Design Evaporator TD on this new high efficiency system is only 30. Protocol has been broken. Well, so be it, it's impossible to make progress without breaking old rules.

    Part 3, How to use this Information as a Diagnostic Tool.

    The most valuable information in this lesson is this: The Evaporator TD will not change as the indoor temperature changes. If the evaporator entering air temperature drops 5, then the evaporator coil will also drop 5. The rate of sensible heat transfer depends directly upon the TD and is proportional to it. If TD changes, then capacity will also change. So with a constant capacity system, the TD must remain constant. As such, evaporator TD becomes a valuable diagnostic tool. You can use it to determine whether your suction pressure is running too high or too low, you can use it to evaluate system capacity when air flow is known, or you can use it evaluate airflow when the capacity is known, etc. etc.

    Well, that's all good news, but unfortunately it isn't exactly true. In reality system capacity is not a constant, but will vary up and down with both indoor and outdoor temperature changes, and also with indoor RH levels. It does give us a good ballpark figure though, and if the actual TD is different than design TD, well you should be able to determine whether this is due to any of these changes in conditions, or maybe due to something else. For example, if the TD is higher than 35 and ambient is 100 while indoor dry bulb is 70, then there's a problem. The system capacity is lower than the rated capacity under those indoor/outdoor conditions, whereas the higher than normal TD is saying that the capacity is higher than the rated capacity. This presents a contradiction. This should tell you that the indoor airflow is too low, causing a higher than normal TD. This is just one example of how you can use TD as a diagnostic tool in the field. A TEET chart will provide you with the same information in some cases, but it does so from a different angle. You can use both of these tools together to rule out possibilities in some cases, that is, when the data from either is giving you more than one possibility. The possibility that both point toward will likely be the actual issue.

    Part 4, How to determine what the design TD should be for a system when the product data isn't available.

    If the nominal cfm is 325-350 per ton (high SEER), then the design TD should be about 30. If the nominal cfm is 400 per ton, then the design TD should be about 35.

    In practice you'll find that this information isn't 100% accurate all the time, but it has served me well over the past several years. FWIW I use a slightly different version of TD. It goes as follows: Since RH affects the TD, I thought it would be an improvement to use wet bulb temp rather than dry bulb temp to calculate TD. With this change in the formula the temperature difference will remain more constant with changes in latent load. The evap load is after all a direct function of return wb temp, that being a relationship that the SH charging charts all rely upon. That being said, the wet-bulb-SST TD should be 22 for low efficiency (400 cfm per ton nominal) and 17 for high efficiency (325-350 cfm per ton nominal). These numbers were obtained by substituting the design wb temp for the design dry bulb temp in the earlier equations. To wit:

    67(wb) - 45 = 22
    67(wb) - 50 = 17

    Cheers.
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    Last edited by hvacrmedic; 07-29-2012 at 03:47 AM.

  7. #20
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    Jan 2004
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    PA
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    Or he has a piston system with the piston not seating. And thinks the distributor tubes are cap tubes.
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  8. #21
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    Sep 2007
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    State College, PA
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    Quote Originally Posted by beenthere View Post
    Or he has a piston system with the piston not seating. And thinks the distributor tubes are cap tubes.
    That's a good point. I have not seen a cap tube split system in a while.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  9. #22
    Join Date
    Mar 2008
    Location
    West Monroe, LA
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    It could be a piston. I could not get the cover off completely and saw the tubes going into the coils. The unit is a 2004 model so it probably is a piston.

  10. #23
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    Jul 2000
    Location
    NW IL.
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    Quote Originally Posted by daviddsims View Post
    I measured the indoor temps at the return grill. I was hoping I was not going to hear weak compressor
    Measuring the return air at the grille and then again will show if you are sucking air from another source. IE. your return air at the grille is 73 degrees if the temperature at return air of the furnace/handler is 78 degrees that indicates that warmer air is being sucked into the ducting from say the attic through stud wall plates. If the air temperature was 69 degrees it would indicate air being sucked from an unconditioned basement or crawl space.

    You should get into the habit of measuring at both locations to determine if additional loads are being introduced into the system.
    Aircraft Mechanical Accessories Technician. The Air Force changed the job title to Air Craft Environmental Systems Technician. But I've decided I'll always be a Mech Acc.

  11. #24
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    Jul 2000
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    NW IL.
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    Quote Originally Posted by daviddsims View Post
    It could be a piston. I could not get the cover off completely and saw the tubes going into the coils. The unit is a 2004 model so it probably is a piston.
    You'll need to locate the condensing unit information on the required piston size. Have the correct size on hand. Pump the refrigerant back and check to see if the piston is either stuck and not fully seating or is larger than the required before condeming the compressor. Hopefully the piston is either oversized or stuck.
    Aircraft Mechanical Accessories Technician. The Air Force changed the job title to Air Craft Environmental Systems Technician. But I've decided I'll always be a Mech Acc.

  12. #25
    Join Date
    Jul 2005
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    5,576
    Quote Originally Posted by daviddsims View Post
    It could be a piston. I could not get the cover off completely and saw the tubes going into the coils. The unit is a 2004 model so it probably is a piston.
    If you didn't look, then it could also have a TXV. That will definitely change things. If it's a TXV then the system is just undercharged.

    It would be helpful if you would post the model numbers, indoor and out, and even the compressor model number if possible. Your amperage data doesn't appear to be consistent with a 4 ton unit.

  13. #26
    Join Date
    Mar 2008
    Location
    West Monroe, LA
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    69
    Took another look today and sure enough it had a txv on the evaporator. I added a small amount of refrigerant to try and get to 10 degrees subcooling. Getting about a 15 degree split out of the evaporator.

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