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  1. #1
    Join Date
    Nov 2008
    Location
    Houston, TX
    Posts
    77

    Summer vs winter A/C delta T

    I live in Houston, TX where we air condition for about 11.5 months a year. During this time of year when the outside conditions are 73°/70%, I can easily get a delta 25°. Occasionally getting as much as delta 27°. Indoor conditions are about 71-72° and 42-45% with the system running. 73-74°/50% with the system off. The air conditioning runs pretty sparsely in February. But it helps to knock the humidity down and drop the temp by a degree or two.


    Now in the summertime, it's a different story. Average outdoor conditions in Houston run about 95°/60%. I get a measly delta 14° during July and August. Inside I'm getting 72°/60%. That's with a supply temp of 58° and 72° at the return. (I can't measure the coil before/after temps). During the mornings, I've tried turning the temp down even farther in an effort to get the humidity down and can only get 68°/57%. The system just can't seem to wring out the humidity. It makes sleeping troubling as I have to turn the temp down to get the humidity in check and then sleep under a somewhat thick blanket. I'd much rather sleep under a thin sheet and have it be 75°/40-45%. (actually I'd like 35%, but that's hard to do)

    I run the fan in the AUTO position always. (Switching it to ON will send the humidity inside up into the 70% range.) Windows and doors are closed. No fresh air intake on the system.

    Now I know dealing with dew points outside in the summer that are in the mid 70's and even touching 80 occasionally, will really give the system a work out. But should deltas really be cut in half during the summer?

    Last summer I was getting $330/month electricity bills (1300 sq ft and 17 cents/kwh prices). If I'm not getting below 60% inside all summer long, something seems not right. I don't make a lot of moisture inside. 5 minute showers, no boiling water on the stove, no fish tank, no drying clothes inside, washers/dryers are located outside by the parking lot.

    Could it be the location of my unit? It's a package unit on our apartment's flat black tar roof. No shade, direct sunlight all day long. It's a Carrier ZP 3.5 ton model (10 SEER). I know 3.5 tons sounds like a lot for 1300 sq ft, but I live in a top floor apartment that's 40 years old with large single pane windows. During the summer, it will run non-stop from 2PM until about 7PM. Cycling on and off (mostly on) throughout the rest of the night and morning. But even running it for 5 hours straight during the afternoon, I'm left with delta 14 (delta 15-16 if I'm lucky) and 57-60% inside. So I don't think it's an oversized/short cycling issue.

    It can't be the condensate drain getting plugged as there is none. The unit simply drips water on to a downward slope that runs to the hole on the side of the unit. I will admit that there isn't a whole lot of water coming from it though.

    I guess I'm basically wondering what people in hot/humid climates are getting for deltas during the worst parts of summer. Inside temp/humidity numbers would be helpful too.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,778
    When was it serviced last.

    May just need a good clean and tune.
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  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    3.5 Tons for a 1300 square foot apartment. Sounds a little large even if it there is little ot no insulation. Maybe there are a lot of leaks so humid outside air is comming in faster than the A/C can remove it.

    Does the unit ever run 100% of the time on 100 degree days? If not, it might be oversized, which makes it hard for the unit to remove humidity. Bigger is not better.

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,302
    An apartment that's forty years old with single pane windows and a flat black tar roof. In Houston, Texas, packed with the double whammy of high solar loading in summer and high outdoor dew points.

    If you don't own this building or unit, there's not a great deal you can do other than press your landlord to hire a competent technician to go over your package unit with a fine toothed comb.

    Your post seems written with a greater level of curiosity toward an a/c unit's performance than is common among apartment dwellers, so I will briefly explain why your delta T varies throughout the year. Delta T through an a/c system is not merely a result of air temperature reduction via refrigeration. It also involves moisture removal, which you seem on the cusp of grasping via your observations posted above.

    Condensing water vapor to liquid water, as you observe via the condensate drain on the roof of the apartment, requires heat removal. Only thing is, it's heat you can't feel...it's known as latent heat. Heat you can feel is called sensible heat. An air conditioning cooling coil of a given capacity has "X" amount of sensible heat removal capacity, and "Y" amount of latent heat removal capacity, under a given set of operating conditions. Let's say your indoor humidity level is in the 30's (don't you wish!) one summer day, but the temperature is 80. You flip on your a/c, let it run awhile to really get going, and then take your delta T. You see it's at or near a 20 degree split. Next, let's boost the indoor humidity level to 60%, but keep the temperature the same, at 80. You remeasure your delta T after some time and notice it's now around 14 to 16 degrees. What? Has the unit laid over and died because we boosted the humidity level? No. It is because more of the cooling coil's cooling refrigerating capacity is being used to convert water vapor to liquid water, with less available to cool the air directly.

    It's possible your apartment's package a/c unit is doing all it can, with its current level of service and maintenance, to keep your apartment habitable. It's further possible to have the thing serviced and not see much improvement in performance (although with how apartment systems are typically maintained, it's far more likely you WILL see some improvement). What the landlord will probably never grasp is how bad of an energy pig his apartment building is, due to its leaky, old construction with questionable thermal boundaries. A building that old does not often receive the upgrades necessary to make it competitive with newer apartment buildings constructed with better insulation, windows, etc.

    Personally, if I were you, and if I could afford to, I'd move out of ANY upstairs apartment in Texas.
    • 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
    Nov 2008
    Location
    Houston, TX
    Posts
    77
    It's a peculiar setup in that there is no accessible attic between my ceiling and the roof and no access from the roof either (aside from where the package unit is and the supply/return ducts underneath). So to check for duct leakage would mean tearing down the drywall from my ceiling.

    As far as service goes, they had a new compressor installed in October 2007. It had started tripping the breaker about 3 times a day. They replaced the breaker, but it kept flipping until the compressor finally gave out. I was glad to get the new compressor as I'd only been getting delta 12° during the summer of 2007.

    A service check here consists of "the low/high pressures look good." No superheat or subcooling check. The coils get cleaned with a green spray bottle of coil cleaner about once a year. There are no garden hose hook ups outside our complex though. So you can't rinse them off. I'm not sure how well waterless coil cleaners work. I suppose you could take a pot of water up there and toss it on it......

    I replace the air filter monthly. No problems there.

    I can't really comment on if it runs continous or not at 100°. We only had two 100° days last year. But it will be 95° for weeks on end.

    The electricity is included in the rent (thank goodness). But the manager here says we use too much electricity. It's the classic catch 22. They don't want to spend any money for an A/C tech to come out here and service/replace parts because the apartment maintnance guys say it works fine. So it must be my fault for using too much electricity. The maint. guys that work here say you don't want it too dry because then you suffer from dry skin. I used to live in Vancouver where the summer conditions outside were 80°/30% Nobody had dry skin. It felt wonderful. Hardly anybody had air conditioning. Didn't need it. The maint. guys would be comfortable at 78°/55%. I asked them to turn the fan speed down a notch awhile back. It's a single speed fan with 3 speed choices. (350/400/450cfm/t) They couldn't find the fan speed wire even though I gave them a printed out manual and schematic for the unit. I'm guessing it's probably on the default 400 speed. But that's 1400 cfm for 1300 sq ft.

    I think I probably will just pay for a tech to come out here this year. Assuming they'll let me. Granted it will be for comfort only as the electrical bill isn't mine. But the summers here are just too long to be uncomfortable.

    Shophound, you pretty much nailed it.

  6. #6
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,159
    Something as simple as a dirty condenser can cause high humidity. All pressures go up and the evaporator coil stays warmer. A good thorough cleaning of the condenser coil is where I'd start. This requires plenty of water to wash out the condenser, you may have to extend the garden hose to reach.

  7. #7
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,485
    It may be pulling 120f air off the roof into return or above ceiling. 40 years ago they didn't have mastic duct sealant. I'm sure the duct is leaking to some degree probably substantially. Also your home could be in a negative pressure due to excessive supply air leaks thus intensifying the thermal envelope leaks that a 40 yr old construction would have and pulling in 80% humid hot air into the home. It's probably a combo of all that and a filthy condenser coil.

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,302
    Wow, a four year old thread finally gets more replies!

    I've corresponded with Gord since this thread drifted off into slumberland, and I think he's made some progress with the system.
    • 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.

  9. #9
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,485
    I didn't even realize it was that old. He's probably moved out by now. Lol

  10. #10
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,302
    Quote Originally Posted by jtrammel View Post
    I didn't even realize it was that old. He's probably moved out by now. Lol
    He's still there, far as I know. And he still tracks the split across the unit pretty closely. He's probably got more data along that aspect than I do for my own unit at home. But as I said before I think some work has been done to the unit since the OP and it is working better, now.
    • 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.

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