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  1. #1
    Join Date
    Dec 2004
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    509

    Question for the residental pros around here

    ok guys dumb reefer guy here. I typically work on supermarket racks so dehumidifing is not my strong suit.

    I read something in another thread that intrigued me a bit. This was a residental system with high humidity and the recomendation was to slow thier fan down. This got me to thinking. I get slower airflow will increase moisture removal but I would think that would create cooling problems.
    1. Im guessing if I slow my fan I would need to close my txv down some to keep my SH or is that wrong because of the latent heat factor for condensing water from the air?

    2. What would that do to air distribution throughout the house? would thier still be enough to cool the house effectivly? Would the unit just run a lil longer to acomplish the desired effect?

    3. Does the evaporator need to be resized for this to be done right?

    4. I read in a post you should not leave your fan in the on postion if you have a humidity problem which I find intriuging as well. Why not? Would that not tend to create positive air pressure in the house if everything is sized right and layed out propperly? Also wouldnt it keep a more even temp and humidity level in your house because of it being mixed.

    5. My stores use my racks to dehumidify through heat reclaim. Do they sell residental sys equiped with reclaim features? Im in florida so there is a heavy demand on a/c systems here and alot of power coservation incentives here as well. Another big product down here are water cooled systems to heat swimming pools. It would seem to me a system that could offer to cool your home while heating your pool, or providing you with hot water, or dehumidifcation would be marketable.

    6. Sorry if Im asking dumb questions just curious where your guys of the trade are at in technology and practices.

    7. If anyone has any links reguarding any of these topics please share them Im always intrested in new technolgies

    Thanks in advance for all your incitefull inputs

  2. #2
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by gas_n_go View Post
    ok guys dumb reefer guy here. I typically work on supermarket racks so dehumidifing is not my strong suit.

    I read something in another thread that intrigued me a bit. This was a residental system with high humidity and the recomendation was to slow thier fan down. This got me to thinking. I get slower airflow will increase moisture removal but I would think that would create cooling problems.
    1. Im guessing if I slow my fan I would need to close my txv down some to keep my SH or is that wrong because of the latent heat factor for condensing water from the air?

    2. What would that do to air distribution throughout the house? would thier still be enough to cool the house effectivly? Would the unit just run a lil longer to acomplish the desired effect?

    3. Does the evaporator need to be resized for this to be done right?

    4. I read in a post you should not leave your fan in the on postion if you have a humidity problem which I find intriuging as well. Why not? Would that not tend to create positive air pressure in the house if everything is sized right and layed out propperly? Also wouldnt it keep a more even temp and humidity level in your house because of it being mixed.

    5. My stores use my racks to dehumidify through heat reclaim. Do they sell residental sys equiped with reclaim features? Im in florida so there is a heavy demand on a/c systems here and alot of power coservation incentives here as well. Another big product down here are water cooled systems to heat swimming pools. It would seem to me a system that could offer to cool your home while heating your pool, or providing you with hot water, or dehumidifcation would be marketable.

    6. Sorry if Im asking dumb questions just curious where your guys of the trade are at in technology and practices.

    7. If anyone has any links reguarding any of these topics please share them Im always intrested in new technolgies

    Thanks in advance for all your incitefull inputs
    1) I am not being a smart ass on purpose but the TX valve should take care of itself.

    2) The unit will run longer to respond to temperature with a lower airflow. Slowing the fan increases latent capacity but drops total capacity at the expense of sensible coling. My gut feeling is with a single zone system if it worked fine at 400 CFM per ton it will work fine at 300 CFM per ton. Maybe have a hard time getting air to go where you want with the multiple zone residential systems.

    3) People 'resize' the evaporator from the get go in many places, but typically telling the HO to slow the fan and run it in Auto Mode is an attempt to fix something that did not work in the first place.

    4) A 4 ton coil sized for a face velocity of 500 FPM could have half a pound of moisture on it that will re-evaporate in maybe 10 minutes. Horizontal air handlers have a little standing water in the drain pan that will also re-evaporate. Supply duct that leak can cause problems when they are in the vented attic or vented crawlspace, they can cause infiltration and boost humidity, so running the fan will exaggerate that effect also. Dumbest one, a fresh air intake to the return air without a motorized damper cycling with the compressor will pump in the humidity also.

    5) Lennox has a system out now, not very familiar with it but seems to subcool liquid line for reheat. Heat Pipe Technology has a minor pre-cool reheat system. York sells packaged units that use hot gas reheat. In my opinion something with partial hot gas reheat would be great. Some thing that could blow out dry air cooler than 80F and warmer than 70F. Something blowing out air at the same temperature it entered at would be perfect when there was a call to dehumidify.

    6) There are only dumb answers
    Last edited by Carnak; 07-03-2007 at 10:04 AM.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  3. #3
    Join Date
    Apr 2002
    Posts
    11,808
    The fan on vs auto should be easy enough for you to test in your home
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  4. #4
    Join Date
    Dec 2004
    Posts
    509
    Ty you for your answer

    Makes total sense the way you have explained it to me. I was only speaking theoreticaly not facing this issue. I like to understand things so I asked. I am very surprised noone is marketing reheat in southern climates where a/c is being used in high demands.


    To me its silly not to be using a preheat tank for you hotwater. Why not move those btus into your hot water system and let that run less as well maybe getting better performance from your a/c when your discharge gas is being water cooled before its air cooled. The only thing you might need to watch is that your head pressure dosnt crash on you but I highly doubt that would occur and a simple control can be put in place to ensure that would not occur.

  5. #5
    Join Date
    Apr 2002
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    11,808
    Would have to have a preheated tank for sure, use up the Btus pretty quick, what you could desuperheat while the system was running would not keep up.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  6. #6
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    heat pipe technology will be catching on soon for dehumidification

    its been a proven win win situation for big buildings in miami fl.

    i used to install "heat pipes" about 5 years ago when the service company i worked for got slow



    .

  7. #7
    Join Date
    Apr 2002
    Posts
    11,808
    I just came across a link on the Auto Fan vs Constant fan, is a paper published by ASHRAE that I believe can be downloaded for a few dollars.

    its called The impact of part load air conditioner operation on dehumidification performance

    The info on the paper that they give you when you are deciding on whether or not you want to buy it is as follows

    The Impact of Part-Load Air-Conditioner Operation on Dehumidification Performance: Validating a Latent Capacity Degradation Model
    Constant-volume air-conditioning systems provide space conditioning in many residential and commercial buildings. In these applications, the compressor typically cycles on and off to meet the cooling load, while the supply fan operates constantly to ensure the zone is well mixed. Under this scenario, it has been demonstrated that the dehumidification capacity of an air-conditioning system degrades at part-load conditions because moisture on the cooling coil surface evaporates back into the airstream when cooling is deactivated. Henderson and Rengarajan (1996) developed an engineering model of this phenomenon suitable for use in hourly building simulations as well as other types of analysis. The model considers conditions entering the coil, thermostat cycling rates, air-conditioner transient performance, and the moisture- retaining characteristics of the cooling coil.

    Previous laboratory measurements have shown that during the OFF cycle, the cooling coil essentially acts as an evaporative cooler, adiabatically providing both sensible cooling and moisture addition. This paper uses that knowledge to determine the moisture evaporation characteristics of a cooling coil installed in a residence with relatively simple temperature measurements. Field-monitored data for a residential water-to-air heat pump are used to determine required parameters for the latent degradation model. The model was able to properly predict the measured latent degradation effects measured over the cooling season. The paper also discusses the other types of experimental data that would be required to further validate the model for various classes of cooling coils and operating scenarios.
    Conclusion, constant fan running when the compressor cycles off functions like an evaporative cooler. A compressor running for 20 minutes straight and then off for 10 minutes is two steps forward and one step back. Compressor running less than 20 minutes, barley removes more water than what gets re-evaporated. If the AC is oversized enough, it will remove next to nothing when operated in conjunction with constant fan.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  8. #8
    Join Date
    Apr 2002
    Posts
    11,808
    My automatic first two steps to treat a complaint of high humidity is to set the fan to Auto, and to slow the blower down.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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