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  1. #14
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    Jun 2004
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    Kathleen GA
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    236
    http://optimalbuilding.com/files/ele...n_the_room.pdf

    Interesting read. However, a real world test (as opposed to theories and thoughts and hearsay) by the Florida Solar Energy Center found that oversized HVAC systems provided better humidity control than right sized HVAC units - at least in Florida.

  2. #15
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    4,311
    I think what is happening with the oversized systems is airflow is well below 400cfm/ton due to poor ductwork, therefore better dehumidification.

  3. #16
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
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    1,781
    Quote Originally Posted by alex_in_fl View Post
    http://optimalbuilding.com/files/ele...n_the_room.pdf

    Interesting read. However, a real world test (as opposed to theories and thoughts and hearsay) by the Florida Solar Energy Center found that oversized HVAC systems provided better humidity control than right sized HVAC units - at least in Florida.
    Is there ever really a time when the latent load exceeds sensible load in traditional florida construction? I think that is the key problem here. So much work has been done (especially in these foamed houses) to limit/nearly eliminate sensible heat gain, that the latent load exceeds sensible load most the year. The only way to effectively remove latent load (humidity) without also removing too much sensible load is by cooling and reheating (which is essentially what whole home dehumidifiers do). In commercial systems, they instead would use a chilled water coil for humidity control followed by a hot water coil for temperature control. This is not something that is impossible to do on a conventional residential AC system also - but I'm not sure of an off-the-shelf item that would tackle it. In theory, if you used even domestic hot water as the heat source, you could add a reheat to a traditional ac - and if that domestic hot water was heated by, say, a desuperheater on the condenser, it would essentially add a whole home dehumidification capability to the conventional residential split system.

    The heat pump water heater doesn't help in this situation, as it is essentially just another evaporator pulling out heat (sensible AND latent). In fact, I'm guessing since it is used for heating water to 120-140F, they prefer to keep the evap at a higher temp for efficiency, so they likely don't remove very much latent heat anyways, potentially making the situation worse.

  4. #17
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,470
    Quote Originally Posted by jtrammel View Post
    I agree that fresh air is needed in any home and controlled ventilation is a must in a tight home. I also agree that dehus are great. Although when it is cooler out outside rh is lower and internal loads alone in milder weather will produce enough gain in a typical house for a true variable capacity hvac system such as the iq drive or mini split to run on low almost constantly thus removing enough humidity to keep comfortable at a reasonable temp setting. These higher end systems also have dehumidify functions that will run the unit long enough to maintain rh.
    That is the discusion. The outside dew point is +65^F and it is raining for a week, the fresh air contains the +65^F dew point. The occupants are adding a 2 lbs. of moisture per hour. There is no sensible cooling load. If you want a <55^F indoor dew point, 75^F,50%RH, you need 4.000-5,000 btus of latent heat per hour. How will any a/c without reheat or a dehumidifier maintain the desired <50%RH without severe overcooling?
    Also consider that a/c removes 1-2 lbs. of moisture per KW while the high efficiency dehus remove as high as 7 lbs.per kw. The ideal strategy is setup the a/c to maintain <50%RH during significant cooling loads. During short cooling cycles, overnight, and cool wet weather, the indoor %RH rises above 50%RH, the dehumidifier will cycle. This applies to any green grass climate with adequate fresh air ventilating and typical varible weather. During high cooling loads, dehumidification is needed. Also when the home is unoccupied for longer periods, the a/c can be setup and the dehum will maintain <50%RH at a fraction of electricity as an a/c.
    In my Cape Coral home, the a/c is off during the summer and the dehu maitnains <50%RH for less than $20 per month.
    When the home is occupied, the dehu is also able to provide fresh filtered air, blend with the house air, and circulate throughout. When the a/c is not operating, the dehu dries the air to maintain <50%RH.
    The data graph show the inverter a/c amps verses the temp/%RH in the home.
    If you got an a/c that will do better, I would volunteer to monitor it.
    Regards TB

    VS AC RH Control .pdf
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #18
    Join Date
    Nov 2004
    Location
    SW FL
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    6,296
    Quote Originally Posted by teddy bear View Post
    That is the discusion. The outside dew point is +65^F and it is raining for a week, the fresh air contains the +65^F dew point. The occupants are adding a 2 lbs. of moisture per hour. There is no sensible cooling load. If you want a <55^F indoor dew point, 75^F,50%RH, you need 4.000-5,000 btus of latent heat per hour. How will any a/c without reheat or a dehumidifier maintain the desired <50%RH without severe overcooling?

    Also consider that a/c removes 1-2 lbs. of moisture per KW while the high efficiency dehus remove as high as 7 lbs.per kw. The ideal strategy is setup the a/c to maintain <50%RH during significant cooling loads. During short cooling cycles, overnight, and cool wet weather, the indoor %RH rises above 50%RH, the dehumidifier will cycle. This applies to any green grass climate with adequate fresh air ventilating and typical varible weather. During high cooling loads, dehumidification is needed. Also when the home is unoccupied for longer periods, the a/c can be setup and the dehum will maintain <50%RH at a fraction of electricity as an a/c.

    When the a/c is not operating, the dehu dries the air to maintain <50%RH.

    The data graph show the inverter a/c amps verses the temp/%RH in the home.

    VS AC RH Control .pdf
    The attachment shows average R.H. at 57%.
    Which area is this _ Lower level in WI?
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #19
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    Sep 2008
    Location
    Western PA
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    25,718

  7. #20
    Join Date
    Jul 2008
    Location
    Rochester NY
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    Quote Originally Posted by teddy bear View Post
    a/c removes 1-2 lbs. of moisture per KW while the high efficiency dehus remove as high as 7 lbs.per kw.
    A gallon vs a pint? That gets MY attention.

    TB, could you please tell me more about this?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  8. #21
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    Quote Originally Posted by tedkidd View Post
    A gallon vs a pint? That gets MY attention.

    TB, could you please tell me more about this?
    Yes I want to know more also. Does the A/C calculation include the sensible heat removed as waste or useful work?

  9. #22
    Join Date
    Oct 2004
    Location
    The Zone
    Posts
    392
    Trane makes a 2.5 Ton two stage XL20i
    It's Hammer Time!

  10. #23
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,736
    Quote Originally Posted by mrfixinthangz View Post
    Trane makes a 2.5 Ton two stage XL20i
    good to know!
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  11. #24
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,781
    Quote Originally Posted by tedkidd View Post
    A gallon vs a pint? That gets MY attention.

    TB, could you please tell me more about this?
    I'm pretty sure the reason the dehumidifiers can remove so much more moisture per kW is that the condenser is so much colder. The condenser in the units is cooled by the cold air from the evaporator. The VERY low delta-t is what allows them to get such a low kW/ton. They are, if I remember correctly (they are not at all prevalent where I live), simply a small refrigeration system, with a blower, evap, and cond coil. The air passes through the evap, cools down below dewpoint, removing sensible AND latent heat, then passes through the condenser, which adds sensible heat back in. The result is warm, dry air leaving the unit. They do add energy to the space, but by removing the moisture, improve comfort. (If humidity without high temp is a concern - which, like I said, is not a problem here in so cal, where the highest humidity season is late summer, where there is always plenty of sensible heat to be removed).

  12. #25
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,296
    Quote Originally Posted by CraziFuzzy View Post
    I'm pretty sure the reason the dehumidifiers can remove so much more moisture per kW is that the condenser is so much colder.

    The VERY low delta-t is what allows them to get such a low kW/ton. They are, if I remember correctly (they are not at all prevalent where I live), simply a small refrigeration system,.
    VeryHIGH delta T
    a.k.a. , Very LOW coil temperature
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  13. #26
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
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    Quote Originally Posted by dan sw fl View Post
    VeryHIGH delta T
    a.k.a. , Very LOW coil temperature
    I was referring to the delta between evap and condenser - which is the key factor in the efficiency of any heat pump.

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