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  1. #1
    Join Date
    Feb 2006
    Posts
    168
    I have gained much knowledge in these threads by learning about the need to deal with the house envelope and the value of a heat pump for my area, West Texas. I would like to pose another question. I have read about the value of variable speed air handlers in dealing with humidity concerns, especially on the higher SEER equipment. But, are variable speed air handlers needed in climates which are dryer?

    Thanks,
    jdb

  2. #2
    Join Date
    Dec 2005
    Posts
    4,043
    It may not be needed for humidity concerns. But it will be more energy efficient and quieter.

  3. #3
    Join Date
    Feb 2006
    Posts
    168
    Thank you Newoldtech,
    On energy savings, would you know how to quantify the savings? What would be the draw on an old style single speed motor vs the newer variable speed?

    Thank you,
    jb

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

    Maybe single stage as good

    I am a homeowner in the hot-humid part of Texas. For me, I would tend to want 350 cfm/ton in a single speed blower, that gives more latent capacity and less sensible. You would benefit from the other extreme: consider just using a single speed blower with air speed 450 cfm/ton. According to what I have read, you would get about 12% greater sensible capacity, and I am guessing you won't need the latent capacity.

    The above would be true whether your air handler is single speed or VS. The VS supports pushing a given CFM through your duct system "no matter what", but if you have good ducts it shouldn't matter much and if you have bad ducts then VS is not a proper fix. My VS works nice and gently with two stage heating, probably would also with two-stage cooling. Other than that, single speed *is* cheaper. Most Texans have single stage heating and cooling, will you?

    It crossed my mind I might be speaking jargon to you, it would have been jargon to me a couple years ago. The "sensible" capacity reflects the ability to move your thermostat. The "latent" reflects moisture removal. We near Houston worry about latent all the time, must be nice to be able just to ignore it with impunity.

    Best of luck -- Pstu

  5. #5
    Join Date
    Feb 2006
    Posts
    168
    Thanks PSTU,
    I used to live in north Houston, so I remember the humidity. It does feel different out here. I have heard so much about variable speed. It's interesting to see a recommendation for single speed. I have no doubts about variable speed in higher humidity environments.
    I read that variable speed technology is more costly in terms of upfront and replacement costs, but if it's more cost efficient to run year after year and you stay in one place, maybe, it breaks even in the long run. That's why I posted the question about operating expenses.
    Thanks for your comments regarding sensible/latent issues. In the cooling season we stress our systems day after day at the extremes of design conditions.

    Thanks again,
    jb

  6. #6
    Join Date
    Feb 2006
    Posts
    168
    PSTU,
    You mentioned the issue of sensible vs latent heat in a dryer climate, and also mentioned a way to configure the system to enhance sensible capacity. Do you know of equipment brands that do a better job of sensible heat removal as a percentage of their total capacity. Particularly, in the range of a cooling load that required a 4 ton system but was largely a sensible need. I hesitate to bring up the issue of brands, but I think I remember comments on the threads which mention that some brands do a better job of with sensible capacity in hotter, more demanding climates.

    Thanks,
    jb

  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    JB, I have never heard of one brand doing a better job than another with respect to humidity control. The pros concensus on this board is that design of the system, selection of the right equipment, and craftsmanship of the installation are the big issues. Nothing I have ever heard from a good credible source, makes me disagree. I *have* heard of one brand having thinner metal in a cabinet, and another model having a part which is more labor intensive to remove/replace, but you are not too very interested in that, are you?

    There is a human desire to boil the issue down to one or two brands which do everything right (or are to be avoided). If I could identify the HVAC tech who is ace in his field, I would go with whatever brand he chooses, and sleep at night.

    It's hard enough for a homeowner to judge the tech's abilities, though. You might look for brand-bashing and probably avoid those techs (that's kinda opposite your thinking with this very question). You might ask if he will measure ESP with his installation and make it a good number -- I find a lot of techs don't want to measure it, and I try not to use them. This is an attempt to see if he thinks like a well trained tech would. You might ask about the issue of duct leakage, and see what he intends to do about it.

    Still, techs will throw you a curveball in my experience. My best current one, installed a VS furnace on iffy ductwork, using the old adage that it would "do better" with the ductwork problem, if not fix it. Where he did me wrong, is failing to consider what happens when you push 1110 CFM thru before, and 1400 CFM after. A homeowner cannot be expected to see all these things in advance, but you would hope a tech would. Despite this he is still the guy I work with.

    Myself I would look for his vocabulary to include Manual J, Manual D, and Manual S. If he bad-mouths the ACCA methods, take that into account. There might be things in the contract which you can value, e.g. one guy offered to guarantee 55% relative humidity with his system. I myself would ask the tech to prepare the workpapers for Manuals J,D, and S and hang onto them, even thought I am inexpert at reading them.

    Best of luck -- Pstu

  8. #8
    Join Date
    Feb 2006
    Posts
    168
    Thanks PSTU,

    That's good advice. My general strategy is to ask enough questions to get enough answers so that I can ask the right questions of my contractor/installer when the time comes. I am working on the aspect of our needs in West Texas which stem from our long, hot, dryer summers, and how that relates to sensible capacity as a function of total capacity of a system. Of course, our local HVAC pros have been dealing with that for decades.

    jb

  9. #9
    Join Date
    Nov 2005
    Location
    Kansas
    Posts
    4,434
    In the long run a VS is cheaper. The blower moter uses only 1/5 of electricity as a standard motor.

  10. #10
    Join Date
    Nov 2005
    Location
    Kansas
    Posts
    4,434
    VS has a higher tolerance to inadequate ductwork than standard blower. CFM and static pressure has to be measured and not only estimated after installation and when needed ductwork needs to be adjusted to run system in spec.

  11. #11
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    Originally posted by tostaos
    In the long run a VS is cheaper. The blower moter uses only 1/5 of electricity as a standard motor.
    A VS uses less electricity only when running at less than full power. At full power (maximum CFM) a VS may use more.

  12. #12
    Join Date
    Nov 2005
    Location
    Kansas
    Posts
    4,434
    …and a residential system does not run all the time on full load. The statement above, even it is the truth, is irrelevant.

  13. #13
    Join Date
    Aug 2002
    Posts
    1,651
    Tost,

    I do agree with Panama to a point..

    A VS is cheaper to run than a PSC motor at design TESP..

    However, as static pressure rises a VS will raise the rpms to try to move the proper cfm and amp draw rises,, whereas when TESP rises on a PSC motor the rpm slows and amps drop and cfm drops..

    The point being is that a VS on a real high Static(TESP) duct system will cost more to operate but will try move the proper cfm but may be noisy.. A PSC motor may cost less but will not move the proper cfm.. Neither are correctly installed and may have blower problems.. The duct system should be reviewed and upgraded for either system to properly operate within design specs..

    VS is not a cure-all for a poor duct system.. A VS does have more capabilities to provide the proper cfm over a wider TESP but does have serious down falls outside those parameters.. Tests and evaluation should be done to any duct system to make sure it will move the required cfm within the TESP range of the blower whether a PSC or VS system will be installed..

    Just my opinion,
    J

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