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  1. #14
    Join Date
    Feb 2006
    Posts
    468

    you had to ask...

    I only dimly recall the report on oversized dehus - it may possibly have been a footnote or sidebar in the 15-20 house dehu optimization study done in Houston and sporadically posted here. I just remember convincing language to the effect that dehu capacity has lately grown such that it way exceeds demand in a typical homeowner app, and that this serves no one other than bigger-is-better marketing twits.

    I agree that the oversizing downside is not nearly as problematic as with AC, but it does mean overspending on, and putting up with a larger and heavier unit than needed. I also strongly suspect that the liters/ kWh efficiency numbers fall way off in intermittent conditions owing to startup and shutdown inefficiencies including but not limited to refrigerant pressure reequalization and condensate reevaporation from the evap coil. The Energystar savings is printed on the box but not really actually installed in the home.

    Now that you mention it, I'm not sure I agree that dehu capacity can't reasonably be modeled via something like Manual J? Just off the top of my head why couldn't one run a Man J calc using reasonable numbers for infiltration and just look at the latent portion of the result?

    OTOH, a portable dehu only costs $150-$250, so chuck one into the crawl space or basement, maybe along with a hygrometer and a Kill-a-Watt if you have above-average curiousity, monitor results and go from there - it's not like it's a multi-kilobuck ducted HVAC install...one can probably sell a fairly new one for half the purchase price so the risk is pretty low.

    I'm cool with opposing posts, especially yours. I've mentioned here before that I'm an engineer who has wandered out of field - some of my posts here are trial balloons designed to draw comment. I've been whacked a couple times but learned something each time.

  2. #15
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Eureka, perhaps

    >>Now that you mention it, I'm not sure I agree that dehu capacity can't reasonably be modeled via something like Manual J? Just off the top of my head why couldn't one run a Man J calc using reasonable numbers for infiltration and just look at the latent portion of the result?

    I think you got something here. This is one of those simple ideas that sounds obvious when you say it. Except that I never thought to say it before.

    I submit that the efficiency factor is reduced because dehu controls always appear to operate in wide swings. Perhaps this is designed into them to minimize the efficiency issues. While a heat/cool system will spring into action with one degree temperature difference from setpoint, a dehu seems to operate as if it accepts a swing of 5 points RH. This is merely my estimate from observing a hygrometer in a room where one works.

    Another challenge using Manual J is... it is designed to size AC/heat systems according to peak. They do not address partial loads quantitatively, while a dehu is all about periods which are neither hot nor cool.

    Best wishes -- Pstu

  3. #16
    Join Date
    May 2006
    Location
    Warren, MI
    Posts
    912

    Re: Eureka, perhaps

    Originally posted by pstu
    >>Now that you mention it, I'm not sure I agree that dehu capacity can't reasonably be modeled via something like Manual J? Just off the top of my head why couldn't one run a Man J calc using reasonable numbers for infiltration and just look at the latent portion of the result?

    I think you got something here. This is one of those simple ideas that sounds obvious when you say it. Except that I never thought to say it before.

    I submit that the efficiency factor is reduced because dehu controls always appear to operate in wide swings. Perhaps this is designed into them to minimize the efficiency issues. While a heat/cool system will spring into action with one degree temperature difference from setpoint, a dehu seems to operate as if it accepts a swing of 5 points RH. This is merely my estimate from observing a hygrometer in a room where one works.

    Another challenge using Manual J is... it is designed to size AC/heat systems according to peak. They do not address partial loads quantitatively, while a dehu is all about periods which are neither hot nor cool.

    Best wishes -- Pstu
    I asked Thermastor about my Santa Fe Advance and they responded that +/-5%RH was normal for the built in sensor.

    EDIT: How accurate is your hygrometer too? Another point is that a dehumidifier operating raises room temperature also and that in itself lowers RH a bit.

    [Edited by wptski on 08-07-2006 at 11:05 AM]
    Bill

  4. #17
    Join Date
    Feb 2006
    Posts
    468
    I was thinking of applying only the latent load calculated by Man J to the dehu sizing - that should dispense with the peak OD temp issue.

    I concur that dehus run +- 5% - my Kenmore /LG digital setpoint and measurements step in 5% increments.

    Regarding Hygro accuracy, I cam across a nifty hygro accuracy checking technique that is concurred on websites geared toward cigars (humidors) and string instruments (carrying cases) Seems both camps have a need to maintain some humidity for prized stogies and violins.

    What you do is put a couple teaspoons of salt in a small open container (I used a 1 oz cup used to dose kids with cold meds) and just enough water to dampen (not submerge) the salt. Put that and the hygro into a sealed bag (A gallon ziploc freezer bag, in my case) such that air in the bag freely circulates around the salt paste cup opening. Seal it and leave it overnight.

    The hygro should read 75 (actually 75.5%) owing to the hygroscopic properties of the salt paste.

    I have a single data point - a Radio Shack digital hygro I suspect reads high went into the bag a couple days ago and read 85% the next morning, and still read 85% while still in the bag a day later. I'm looking forward to testing other hygros.


  5. #18
    Join Date
    May 2006
    Location
    Warren, MI
    Posts
    912
    Originally posted by watercop
    I was thinking of applying only the latent load calculated by Man J to the dehu sizing - that should dispense with the peak OD temp issue.

    I concur that dehus run +- 5% - my Kenmore /LG digital setpoint and measurements step in 5% increments.

    Regarding Hygro accuracy, I cam across a nifty hygro accuracy checking technique that is concurred on websites geared toward cigars (humidors) and string instruments (carrying cases) Seems both camps have a need to maintain some humidity for prized stogies and violins.

    What you do is put a couple teaspoons of salt in a small open container (I used a 1 oz cup used to dose kids with cold meds) and just enough water to dampen (not submerge) the salt. Put that and the hygro into a sealed bag (A gallon ziploc freezer bag, in my case) such that air in the bag freely circulates around the salt paste cup opening. Seal it and leave it overnight.

    The hygro should read 75 (actually 75.5%) owing to the hygroscopic properties of the salt paste.

    I have a single data point - a Radio Shack digital hygro I suspect reads high went into the bag a couple days ago and read 85% the next morning, and still read 85% while still in the bag a day later. I'm looking forward to testing other hygros.

    Here's a hygrometer thread that received little attention: http://hvac-talk.com/vbb/showthread....readid=110066. The calibrated unit was just returned and works no differently than the original. The situation difies a digital hygrometer.
    Bill

  6. #19
    Join Date
    Jul 2004
    Posts
    322
    I have a Sears and it works fine except for the humidistat. If humidity falls below the set point, it cycles every few minutes instead of just staying off. This strikes me as a very inefficient way to remove humidity, the same problem with a short-cycling AC. Maybe I got a dud though.

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