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  1. #1
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Confused Manual J design conditions, chronically exceeded

    It took me awhile to learn about dewpoint and some basic concepts of psychrometrics. Thanks to the people on this board who steered me in the right direction. The following is something which has been there all the time, but I just figured out what I was seeing.

    My ACCA Manual J book gives design conditions for Houston AP as dry bulb 94F, wet bulb 77F. That equates to 71F dew point or 115 grains/pound absolute humidity. It is very common to have daily high approximately the design temperature, but what bugs me is how very common it is to have dewpoint 75F or higher. That means the difference in indoor vs. outdoor humidity is 20 grains greater than Manual J assumes.

    Is this possibly some kind of a flaw in the theory behind Manual J? Or is it just not important? I understand the concept that the design DB is expected to be exceeded 2.5% of the time. However it seems the DP measure is exceeded rather more than that, while the DB is still up around 94F.

    This is not uncommon in Dallas either. Manual J design conditions are for 65F DP while it sat at 72 DP all through a weekend visit there. Using Weather Underground it appears to be 72 DP as I write this, although the forecast DB high is a few degrees below the Dallas design temp of 100F.

    Is this a thing?

    Thanks -- Pstu

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,316
    As long as my indoor dew point is maintained below 55 degrees, I'm not sweating it. Literally.

    Get some indoor empirical data. When you find outdoor dew points exceeding Manual J expectations, measure your indoor temp and humidity and see if your system is keeping your indoor dew point below 55 at your normal daily dry bulb temperature set points. If it is, you're doing good.

    I understand you had a TXV issue recently, and with it resolved you're reporting much better comfort levels in the house. Almost every time I've encountered "adequate cooling but it feels sticky" scenarios, I've found the refrigerant charge not trimmed properly. Getting the refrigerant charge correct for the real time airflow passing over the evaporator coil is crucial for good dehumidification, particularly with non TXV systems, like so many in Texas are. The difference between an improperly charged a/c that cools but doesn't dehumidify as well as it should, and one with a properly trimmed charge, is night and day in comfort terms.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I've got to say my question is more theoretical than practical. Everything else in the ACCA manuals series seems bound together by a neat set of principles, and this seems an exception. Maybe the latent load from infiltration is all this affects, and since infiltration is almost always a guess, this would lead to something unprovable.

    This season there is no actual complaint in humidity control. I was just musing, wondering what the SHR of my system must be as set up presently, thinking it must be way low. I have not the ability to actually measure performance and SHR but it would be nice.

    It was just a question why ACCA decided on this set of numbers, the other numbers seem quite realistic but not this (I think).

    Best wishes -- Pstu

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,316
    I just remembered something I didn't include in my prior post.

    When you see the design dry bulb for your area, which for Houston is 94 degrees, the wet bulb number is called coincident. This means on a 94 degree day, if the dew point for Houston is around 71 degrees (a 77 degree wet bulb), you are at both expected design dry and design wet bulb conditions for your area. In more familiar terms this translates to 94 degrees dry bulb @ 47% relative humidity.

    I just checked my temperature/humidity readings here at work and we are basically very near your design day conditions right now. It is 94 degrees at 77 degrees wet bulb. Checking conditions in Katy, TX, they aren't much different than here in DFW.

    Interesting thing to watch regarding outdoor dew point. Watch it as the daily temperature goes up. In general you may find higher dew points in the early to mid morning times than during the rest of the day. That is the general summertime pattern in my area, at least. You being much closer to the gulf, it may vary from my pattern. What I'd do is set 94 degrees as your benchmark, and then note any time you see dew points exceeding 71 degrees when it is 94 degrees or hotter outside. It can happen, but I'm thinking it will be few in number.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I was under the impression that in the coastal area at least, DP did not vary much from morning to night. But I did not write readings down and have to rely on memory. Interestingly today's DP has fallen 4-5 degrees now in the hottest part of the day.

    FWIW I like to use airports for data sources, with the rationale that flying is attached to an ethic of being reliable and correct. One is Houston Hull (about 15 miles from Katy):
    http://www.wunderground.com/cgi-bin/...elect=WEATHERl

    and another is North Dallas' Addison Field:
    http://www.wunderground.com/cgi-bin/...Select=WEATHER

    I will have to watch DP conscientiously in the future. Today it went down and that took me by surprise.

    Best wishes -- Pstu

  6. #6
    Join Date
    Jun 2010
    Location
    Houston, TX
    Posts
    178
    Pstu,

    do you think that 85 out. dry, 75 wet and 63% relative humidity with 46 grains difference, is not appropriate parameters for NW Houston?


    Quote Originally Posted by pstu View Post
    It took me awhile to learn about dewpoint and some basic concepts of psychrometrics. Thanks to the people on this board who steered me in the right direction. The following is something which has been there all the time, but I just figured out what I was seeing.

    My ACCA Manual J book gives design conditions for Houston AP as dry bulb 94F, wet bulb 77F. That equates to 71F dew point or 115 grains/pound absolute humidity. It is very common to have daily high approximately the design temperature, but what bugs me is how very common it is to have dewpoint 75F or higher. That means the difference in indoor vs. outdoor humidity is 20 grains greater than Manual J assumes.

    Is this possibly some kind of a flaw in the theory behind Manual J? Or is it just not important? I understand the concept that the design DB is expected to be exceeded 2.5% of the time. However it seems the DP measure is exceeded rather more than that, while the DB is still up around 94F.

    This is not uncommon in Dallas either. Manual J design conditions are for 65F DP while it sat at 72 DP all through a weekend visit there. Using Weather Underground it appears to be 72 DP as I write this, although the forecast DB high is a few degrees below the Dallas design temp of 100F.

    Is this a thing?

    Thanks -- Pstu

  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by crayx4 View Post
    Pstu,

    do you think that 85 out. dry, 75 wet and 63% relative humidity with 46 grains difference, is not appropriate parameters for NW Houston?
    Because of the 85F db, that would be well under the summer design conditions. Do I understand your question? Because I am wondering why you would ask that.

    Best wishes -- Pstu

  8. #8
    Join Date
    Jun 2010
    Location
    Houston, TX
    Posts
    178
    sorry one contractor had made a mistake with the design conditions, before I pointed this out to him, I wanted to make sure..

    Now, other question: what level of a mismatch can I get away with, both on the undersizing and oversizing? 1/2t is still too risky?

    There seem to be a 0.3t lower load between Sensible+Latent and 77% Sensible Capacity...

    Which one do I have to look at?
    If I do decide to go for a 2-stage, and load calculation is X1/2t, does it mean that I have to select the first HIGHER ton available ?

    I am pretty sure the Man. J has been fairly overestimated..



    Quote Originally Posted by pstu View Post
    Because of the 85F db, that would be well under the summer design conditions. Do I understand your question? Because I am wondering why you would ask that.

    Best wishes -- Pstu

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