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  1. #1
    Join Date
    Aug 2008
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    416

    Tweaking Performance by Indoor Coil Selection

    Thanks, for all the replys about my system replacement. They've helped me decide to stay with the 3 ton design and understand the Manual J calc.
    I wanted to start a thread specifically on the pluses/minuses of tweaking system performance thru coil changes.

    At this point, the contractor is tweaking the performance of the A/C by changing the coil from 3 ton to 4 ton. The Manual J has my house at just over 3 ton and contractor is not pushing upsizing whole system to 4 ton, but would like to "tweak" the performance of the 3 ton system. He is using Trane data (I think its a computer program) to analyze the combinations, so I would think these matches would be Trane approved. What he got:

    3 ton xL16i / 3 ton coil - 33,600 Total and 23,600 Sensible

    3 ton xL16i / 4 ton coil - 34,400 Total and 24,200 Sensible

    Pluses:
    Total up 800 BTUs
    Sensible up 600 BTUs
    SEER went from 15.5 to 16.0
    Same 1200 CFM
    Same physical size of "box" so it won't look funny
    I am paying his cost for larger coil since not in orig. quote

    Questions/Concerns:

    (1) Is Latent simply Total - Sensible??, if so the Latent went from 10,000 to 10,200 which would mean slightly better humidity removal which seems like a plus.

    (2) Did some googling and it seems that upsizing coils is an accepted practice done by other HVAC contractors although +0.5 ton seems more typical. I assume the tuning/matching of coils is mfgr approved as he's using software supplied by Trane and all components are supplied by Trane.

    (3) Suction Pressure would be higher (I think). Would this or anything else affect the life of the compressor, etc.?? I would think that the charging levels for the system would be adjusted and that 10 years from now, a tech who's not aware of the 3ton/4ton match might not charge system proper.

    This contractor is very professional, and is clearly working the numbers and not just using "rule of thumb" and he answers all my questions. I looking for others opinions/experience. Thanks in advance.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,768
    In first stage, the 16i doesn't have good latent capacity.
    I would go with the smaller coil.

    Coil matching should be done by both efficiency and SHR.
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  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    33,392
    Looks like the gain is almost all sensible, great in the desert, not great in a humid area. As been said, the 16i on low is poor on dehumidification already, I've seen as high as 80% sensible (vs 70% for single stage Trane) so you need to do everything you can to get latent up. Also the 16i can't do dehumidify on demand like the rest of the market.

  4. #4
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,506
    Quote Originally Posted by wraujr View Post
    I would think that the charging levels for the system would be adjusted and that 10 years from now, a tech who's not aware of the 3ton/4ton match might not charge system proper.
    If you hire a Pro in 10 years and he uses the proper charging techniques this would not be an issue IMO.
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

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  5. #5
    Join Date
    Aug 2008
    Posts
    416

    Humiditity and xL16i vs. xL15i

    "In first stage, the 16i doesn't have good latent capacity"

    I can certainly beleive this as its not runnning at full capacity. In general, wouldn't this be true of all 2-stage cooling??. When on full his numbers seem to indicate better Latent (+200 BTU) with the 4 ton coil.

    Would it be better to go with xL15i and skip the two stage cooling??

    In my area when temps get in 90's and the humidity is in 90's, I would think that it wouldn't be long before 2nd stage kicks in and I get full Latent performance. This is obviously external humidity from infiltration vs. internal from humans, equipment, etc...

    I know there are other mfgrs, but I'm interested in Trane specifically (mfgr debate seems to always occur in these situations). Just trying to understand the technology. IMHO there is never a "perfect" answer, just shades of grey each with their own pain... Thanks for the replys and keep them coming...

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,768
    The 16i is crippled as far as blower control in first stage. So other units have better latent abilities in first stage.

    The 19i has better latent ability in first stage then the 16i.

    Keep in mind. That the idea of 2 stage is to have longer run times in first stage. So increasing indoor coil size to get more sensible is counter productive in first stage.

    With the smaller coil. When the indoor humidity is lower, you get some of the latent capacity back as sensible. So the smaller coil will do better at lower temps, and still give good performance at higher temps, when its in second stage.

    Remember, lower humidity, can allow you to set your stat higher and save more money then that ½ SEER point will.
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  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by wraujr View Post
    Thanks, for all the replys about my system replacement. They've helped me decide to stay with the 3 ton design and understand the Manual J calc.
    I wanted to start a thread specifically on the pluses/minuses of tweaking system performance thru coil changes.

    At this point, the contractor is tweaking the performance of the A/C by changing the coil from 3 ton to 4 ton. The Manual J has my house at just over 3 ton and contractor is not pushing upsizing whole system to 4 ton, but would like to "tweak" the performance of the 3 ton system. He is using Trane data (I think its a computer program) to analyze the combinations, so I would think these matches would be Trane approved. What he got:

    3 ton xL16i / 3 ton coil - 33,600 Total and 23,600 Sensible

    3 ton xL16i / 4 ton coil - 34,400 Total and 24,200 Sensible

    Pluses:
    Total up 800 BTUs
    Sensible up 600 BTUs
    SEER went from 15.5 to 16.0
    Same 1200 CFM
    Same physical size of "box" so it won't look funny
    I am paying his cost for larger coil since not in orig. quote

    Questions/Concerns:

    (1) Is Latent simply Total - Sensible??, if so the Latent went from 10,000 to 10,200 which would mean slightly better humidity removal which seems like a plus.

    (2) Did some googling and it seems that upsizing coils is an accepted practice done by other HVAC contractors although +0.5 ton seems more typical. I assume the tuning/matching of coils is mfgr approved as he's using software supplied by Trane and all components are supplied by Trane.

    (3) Suction Pressure would be higher (I think). Would this or anything else affect the life of the compressor, etc.?? I would think that the charging levels for the system would be adjusted and that 10 years from now, a tech who's not aware of the 3ton/4ton match might not charge system proper.

    This contractor is very professional, and is clearly working the numbers and not just using "rule of thumb" and he answers all my questions. I looking for others opinions/experience. Thanks in advance.
    I must say my data for latent capability on the 16i (heat pump version) shows a SHR (defined as ratio of sensible capacity to total capacity) much higher than .70 for realistic conditions. In order to get down to .70 from what I read you must get indoor humidity up to 60%RH. Can we explore what are the climate conditions in Illinois where you live? As a homeowner in S.Texas 70-80 miles from the coast I have a lot of exposure to humidity and dwell on it a lot. But that does not mean my obsession should apply to other regions.

    Best of luck -- Pstu

  8. #8
    Join Date
    Dec 2002
    Location
    Houston,Tx.
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    15,506
    Quote Originally Posted by wraujr View Post
    In my area when temps get in 90's and the humidity is in 90's, I would think that it wouldn't be long before 2nd stage kicks in and I get full Latent performance. This is obviously external humidity from infiltration vs. internal from humans, equipment, etc...



    Quote Originally Posted by pstu View Post
    Can we explore what are the climate conditions in Illinois where you live?
    Best of luck -- Pstu
    Is this what your looking for?
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  9. #9
    Join Date
    Aug 2008
    Posts
    416
    "The 16i is crippled as far as blower control in first stage. So other units have better latent abilities in first stage"

    By "crippled", I assume you mean that the blower speed is reduced which I agree would reduce latent but would it not also result in longer run time and remove nearly same humidity. I thought one of the ideas of two-stage was to spend more time in 1st stage where cost of running was cheaper. So reducing blower speed acheives this. Maybe, I'm not sure what you mean by "crippled". Not arguing just trying to understand how all this works.

    I'm in mid-Atlantic (MD) region (not Ill.) and we do have what I consider to be high humidity with some summer days over 90%.

    "Keep in mind. That the idea of 2 stage is to have longer run times in first stage. So increasing indoor coil size to get more sensible is counter productive in first stage."

    Agreed, but designer needs to get Sensible up to match Manual J. So to me it seems as if we have competing requirements that are trying to be balanced.

  10. #10
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,464

    Controlling 75ºF Room Temperatures at 50% RH or less

    The delta-T (Supply Air temperature to Room Temperature) ratio is only part of the humidity control equation.

    A 20 degree drop for 50% RH; In design we use 55ºF Supply Air as a default supply air temperature to maintain a 75 degree space temperature at 50% coincidental RH.

    Air at 55ºF temperature, even if saturated @100% RH has the same specific humidity air, as 75ºF room air at 50% RH.

    However, the quantity of air supplied at whatever sensible temperature has to be sufficient to absorb the heat gains to the space (people, lights, appliances, solar and transmission, etc.)

    If the delta-T is fixed, you will need to adjust the air volume.
    An older EER 7 or less, had a maximum delta-T of 35ºF; 8-EER a max of 32ºF; A 10-EER of 27ºF, the higher the EER normally the lower the evaporator delta-T is for humidity control.

    The reason the old 6-EER units ended up with cold & clammy conditioned spaces, - was because those units were way oversized, with very short runtimes!


    If the air volume is fixed, you need to adjust the (TXV) temperature of that air supplied to the space.

    As for air volume, air only 15 degrees cooler than the room will require 33% more air volume, than air at 20 degrees cooler than the room, to achieve the same cooling load.

    The degrees below the dew point is dependent on the humidity load to the space.

    Given enough airflow quantity, you can control temperature with air cooler than the room itself. However, unless depressed adequately below the served space's dew point, you can forget about humidity control.

    When units are sized right or are two stage; the new digital T-stats with an adjustable differential, of half degree increments from the setpoint, with several degrees range - will have the capability to provide the longer runtimes required for excellent humidity control. - Darrell

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    No, the blower speed of the 16i isn't reduced as much as other 2 stage units.
    Nor can it be controlled by a thermostat like other units can.
    Although it will have a longer run time in first stage, its latent ability is reduced enough, that using a larger coil, will have an adverse effect on moisture removal.

    In first stage, the blower will run at 80% of high speed.
    Most other units can have the blower at 50 to 70% in first stage.
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  12. #12
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    Aug 2008
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    416

    Missing 1st Stage Peformance Data with 4 Ton Coil

    I see what you're saying. Unfortunately, I never asked for the Latent and Sensible performance of the 3 ton condensor with 4 ton coil during the 1st stage cooling. I know that this combo has an ARI # of 1299074 dated 6/18/2008. Is there a way to get these data tables??? and to compare to 3ton/3ton pairing???
    Last edited by wraujr; 08-28-2008 at 05:00 PM. Reason: Additional info

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,768
    Your contractor should be able to print them oout for you.
    Trane has extended data sheets.
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