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  1. #14
    Yeah, I'll give the humidistat a try. I do have a concern about how effective it'll be with whatever is going on with my system, though. My understanding is that opening the humidistat jumper on the VS AH drops the CFM to 80% of what it normally is, until the humidistat closes the jumper again. Last summer, while I was tinkering trying to figure out what was going on, I changed the CFM jumper setting to various levels. I even set it down as low as it would be if I used the humidistat, and I saw absolutely no change in the condensate flow. That is what really freaked me out. Lower air flow should lead to a cooler coil, which I had thought would lead to greater condensation.

    I'm not completely educated on how the TXV behaves in these various situations, and I was just wondering if perhaps the issue is, as beenthere suggested, a problem with the TXV itself.

    I think my first attempt will be to pull the humidistat jumper and just see if that makes any difference, before actually going to the expense of getting a humidistat installed.

    Thanks!

  2. #15
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,535
    With a well insulated home and an high SEER a/c, maintaining <50%RH is difficult. Over-cooling the home or operating the fan at speeds that do not circulate the air properly are not a comfortable solution. As the air flow is decreased, the SEER decreases and the duct temperatures decrease. Colder ducts/grills cause condensate with potiential mold growth. Even a low SEER a/c is unable to provide dehumidification without a high cooling load. Using a whole house dehumidifier(WHD) provides humidity control without any a/c load. Are you providing any fresh ventilation? Fresh air is important in a tight home. The WHD provides fresh air as an option.

    The ULtra-Aire 135H WHD handles 4,000 sqft. plus basement space. Humidity control and fresh air ventilation are as important for indoor air quality, for purging indoor polutants and controlling dust mites and mold growth.

  3. #16
    Thanks, Teddy Bear. Sounds like I'm damned if I do, damned if I don't. Looks like maybe I shot myself in the comfort department by putting so much effort into insulation and incidental sealing.

    With 6 people (2 adults and 4 children) in the house pretty much round the clock (we home school, and I work from home) there is plenty of coming and going, so I'm not too concerned about the fresh air supply.

    But yeah, if I want to get below the 55% humidity we're running at right now, without freezing our tails off, it may just be that I need to go the in-line dehumidifier route. That's been in the back of my mind, but I was hoping to avoid the expense, but that may not be possible.

    Just to be clear, though. If I do have a 12F subcooling, with a VS AH with a TXV, is is normal for the condensate to be only at a slow drip when the system is running, or should I continue to push on this issue?

  4. #17
    Join Date
    Feb 2004
    Location
    Wadsworth, OH
    Posts
    316
    I would expect to see a steady flow of condensate after a few minutes of operation. Normally enough to fill a cup in about 3-5 min.
    What is the difference in temperature from the return to the supply? With a high latent (humidity) load you should be around 18 to 22 degrees.

  5. #18
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,320
    Originally posted by jramunni
    With a high latent (humidity) load you should be around 18 to 22 degrees.

    The higher the latent load, the lower the temp delta.
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  6. #19
    I remeasured just now, first measurement after the service call.

    Outside temp: 83F, 41% RH
    Temp before the coil: 76.5F
    Temp after the coil: 54.1F
    deltaT: 22.4F
    RH: 55%
    CFM: 1250
    Condensate rate: 3 drips/sec (100 drips in 31 sec)

    Maybe it truly is that I've insulated so well that the AC just doesn't run long enough to do its job...

  7. #20
    Originally posted by teddy bear
    Humidity control and fresh air ventilation are as important for indoor air quality, for purging indoor polutants and controlling dust mites and mold growth.
    Yes, that is precisely my concern, even beyond personal comfort. I have a couple of children who have dust mite and mold allergies...

  8. #21
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,320
    Get the humidistat, set the blower to 400 cfm per ton.
    When the humidity is high the blower should slow to 80%, which would be 1120 cfm.

    Your seeing the effects of a well insulated house with a oversized a/c.

    Are you sure your hydrotherm is acurate?
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  9. #22
    Originally posted by beenthere
    Get the humidistat, set the blower to 400 cfm per ton.
    When the humidity is high the blower should slow to 80%, which would be 1120 cfm.

    Your seeing the effects of a well insulated house with a oversized a/c.

    Are you sure your hydrotherm is acurate?
    I'm not positive of its absolute accuracy, but I am of its relative accuracy. It shows 42%RH at 78F in my father-in-law's house across the street, and 55% at 77F in my house.

    The 78F and 77F are in agreement with the digital stats in both houses. I've not done an ice-water bath to verify the 32F point for the thermometer, but I'm presuming it's relatively linear over such a small range.

    Thanks for the advice on setting up the speed and humidistat. I think your assessment of the oversized issue is completely accurate. It's pretty amusing to think that a 3.5T unit would be oversized for a 4000sqft house, but that's certainly what could be interpreted from the HVAC-Calc calculations, especially given the fact it almost never hits the design limit of 100F here. HVAC-calc actually recommended a lower limit, but none of the HVAC professionals here would accept anything other than a 0-100F limit.

  10. #23
    Join Date
    Feb 2004
    Location
    Wadsworth, OH
    Posts
    316
    Originally posted by beenthere
    Originally posted by jramunni
    With a high latent (humidity) load you should be around 18 to 22 degrees.

    The higher the latent load, the lower the temp delta.
    beenthere,
    I was looking at it from a "Desired" delta T perspective to remove more moisture, being his issue is humidity. Does that make sense?

    gstovall,
    How long does the A/C run durring a typical cycle and how long is it off.

    Something isn't making sense, high RH with a 22 degree delta-T, my 17 yrs in the field tells me that you should be seeing much more condenstate than 3 drips/sec. But then again I have been behind a desk for the last 11yrs, maybe things have changed.

  11. #24
    Originally posted by jramunni
    Something isn't making sense, high RH with a 22 degree delta-T, my 17 yrs in the field tells me that you should be seeing much more condenstate than 3 drips/sec. But then again I have been behind a desk for the last 11yrs, maybe things have changed.
    That's exactly what I keep thinking! All the data I have collected directly contradicts what I have learned from reading this board over the last few years, and contradicts what I've read in my Dad's HVAC textbooks. To my understanding, the water should just be pouring out under these conditions...

    The system typically doesn't run all that long when any zone calls, maybe 10 minutes tops usually, and it's usually off for quite a long time. About the only time it runs for any major length of time is in the afternoon on a hot day; the family room zone takes quite a thermal load then. However, even when I took two zones yesterday and dropped them both by 4 degrees to make the system run (and it ran for about 45 minutes), the ending output air temperature of the coil was 53F, while the input was 75F at the end.

    Hmmm...55% RH in at 75F. What is the dewpoint for air that has this moisture content? About 60F? Should be enough of a drop to get SOME KIND of moisture flow out of the air...

    [Edited by gstovall on 06-03-2005 at 10:14 AM]

  12. #25
    Join Date
    Feb 2005
    Posts
    344

    Wet bulb

    Would it be possible to get a wet bulb for enetering and leaving air?

    If you do not have a psychrometer you might try wraping the temp probe with a COTTON pipe cleaner or slipping a short section of COTTON shoe lace over the probe. Of course you will want to wet the cotton.

    Also read the dry bulb for each sample.

    This may give us more insight on what the process line looks like.

    I wonder if this application/problem is a perfect reason to consider sizing equipment for the largest zone instead of the whole house?

    Thanks...

  13. #26
    Join Date
    Jul 2004
    Posts
    253
    Originally posted by gstovall
    Originally posted by jramunni
    Something isn't making sense, high RH with a 22 degree delta-T, my 17 yrs in the field tells me that you should be seeing much more condenstate than 3 drips/sec. But then again I have been behind a desk for the last 11yrs, maybe things have changed.
    That's exactly what I keep thinking! All the data I have collected directly contradicts what I have learned from reading this board over the last few years, and contradicts what I've read in my Dad's HVAC textbooks. To my understanding, the water should just be pouring out under these conditions...

    The system typically doesn't run all that long when any zone calls, maybe 10 minutes tops usually, and it's usually off for quite a long time. About the only time it runs for any major length of time is in the afternoon on a hot day; the family room zone takes quite a thermal load then. However, even when I took two zones yesterday and dropped them both by 4 degrees to make the system run (and it ran for about 45 minutes), the ending output air temperature of the coil was 53F, while the input was 75F at the end.

    Hmmm...55% RH in at 75F. What is the dewpoint for air that has this moisture content? About 60F? Should be enough of a drop to get SOME KIND of moisture flow out of the air...

    [Edited by gstovall on 06-03-2005 at 10:14 AM]
    You can measure the latent heat BTUs and compare it to the specs for your system for the wetbulb temperature and outside temperature that the condensor sees. Measure the condensate for 10 minutes ( probably want the system to be running for 20 minutes or so before you start the measurement) and multiple the number of ounces of water that you collect in 10 minutes by 690. This will give you the BTUH of latent capacity. Compare with your equipment specs.

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