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  1. #1
    anybody think that present or future variable speed technology may make the whole house dehumidifier a thing of the past? we have reheat options available to address no sensible load / high latent load... an all in one system?

  2. #2
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    Certainly, a/c with full reheat eleminates the need for a dehumidifier. Several commercial expensive units are available. Lennox is introducing a partial reheat residential unit at a premium price. According to their presentation, a partial cooling load is required to avoid over cooling. It's a matter of investment, operating cost, fexibility, and meeting specific needs. The installed cost of the more complex reheat a/c is more than a high SEER a/c + ventilating dehu. High efficiency dehus remove 5 pints per kwh verses a/c with reheat 2 pints per kwh. The ventilating dehu is more refined for accurate amounts of fresh, filtered air ventilation. Dehumidify the fresh air before adding it to the home is more efficient. The controls on the a/c with reheat are complex requiring top notch techs. We here the problems of set up and diagnosing problems on the current premium a/c systems.
    Adding a dehu to a problem home is much more effective than replacing an entire a/c system that provides marginal results.
    The industry does not want to talk about fresh air ventilation, because of the high latent loads. The refinement of controlling fresh air to make minimal fresh air ventilation work in a home is a science equal to humidity control. Currently, the best a/c system will not provide the same benefits. Someday the best a/c will do everything we do. Hopefully, we will be able to improve function and decrease the investment to deserve a share of the future bussiness. Why is there resistance to being part of an improved way providing ideal comfort and IAQ? TB

  3. #3
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    With all proper respect, I think this is a losing argument. This product's capital cost is less than the additional cost of a two-stage AC, is it not? In my experience its ongoing cost to operate ranges between 50 and 200 KWH per month. What results is a more comfortable house, where the thermostat setpoint can be a couple degrees higher.

    This is a technology that can compete with AC for humidity control all right. It took awhile to sink in to my mind, but by (partially) separating the cooling and humidity control functions, one can do each better and possibly save money too. This is echoed by publications for commercial AC, I believe the principle is equally true for residential.

    I have read that with a tight house in a hot and humid climate, you are looking for a SHR (sensible heat ratio) perhaps as low as 0.5. Maybe other pros would disagree and use a different number, I would like to hear from them. There simply are not AC systems you can buy which accomplish this low a SHR. You might find some interesting technical reading from the website of Central City Air in Houston, the (deceased) owner David Debien had some aggressive ideas on how to get super humidity removal without a dehumidifier. He did not advocate a two stage AC but rather a two stage coil of his own design. He posted as "Airman1" if you are searching the archives on this board. Central City Air is still in operation.

    To the best of my knowledge, his innovations and inventions did not invalidate the pints/kwh cost numbers that Teddy Bear quoted. But just to do the job at all, is an accomplishment.

    Teddy Bear, how hard would it be to express the pints/day work done by a dehumidifier, into BTUH for comparison with Manual J and S requirements? I am expecting a sizable BTUH off the latent needs, and a lesser BTUH might be added to the sensible needs. I imagine this is trivially easy for someone who knows this stuff, but I could use a little help.

    Best wishes -- Pstu

  4. #4
    (brand name withheld) 3 ton two stage S/C 18seer split system VS AH 410A......# in btus....

    stage / sensible / latent
    1 20100 6300
    2 27700 9900
    1 dehum. 19950 6550
    2 dehum. 26050 10550 = .4049 ratio
    total cap. 37600

  5. #5
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    SHR=0.5 would require air much more humid than you would want in your home, would be about a 50/50 mix of return and outside air here
    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
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    Wish I had never mentioned that 0.5 SHR, honest I read it somewhere but don't have the background to discuss it respectably. Looked at my own Manual J and certainly that does not support 0.5, more like 0.87 which surprises me on the high side (assumes design conditions are 99 degrees and 114 grains humidity outside, 75 degrees and 50% RH inside). So I am confused but you already knew that.

    A definition of SHR is in this article "IN AIR CONDITIONING, IF BIGGER IS NOT BETTER, WHAT IS?":
    http://www.proctoreng.com/articles/better.html

    The article repeats the familiar principles which many of us have heard repeatedly. It discusses what makes an AC good at humidity removal, although it does not discuss two stage designs.


    Still think Kmills would get more satisfying response with a similar thread on the Residential AC board. Double posting threads is supposed to be frowned upon, but this time it seems like a good idea to me. Lots of good techs who might comment.

    Best wishes -- Pstu

    [Edited by pstu on 03-16-2006 at 10:24 PM]

  7. #7
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    SHR is the ratio of sensible heat to the total heat that a coil removes.

    You can also think of it as the SHR that is needed, how the perfect air conditioner could divide its cooling power between controlling temperature and removing moisture.

    The problem is that when the required SHR is low, is that it can create an "IMPOSSIBLE PROCESS" this means that a coiling coil by itself cannot take one set of entering air conditions and turn it into one specific set of leaving air conditions.

    To get rid of the moisture then, a system has to over cool the air. But if this air was delivered to the space at the low temperature, it would either overcool the space or it would satisfy the thermostat and shut off the cooling before enough moisture is removed.

    So the air gets reheated to make up for the over cooling. They can use electric heaters, gas heat, hot water heat or do like a dehumidifier does and re-heat the air with the hot gas off of the compressor.

    The dehu is just an air conditioner with hot gas reheat. All the sensible and latent heat removed from the air, plus the energy to run the compressor ends up reheating the dehumidified air.

    I think teddy sells one where you can dump the heat to the outside of the home using ductwork, his competition sells one where you run refrigerant lines outside to a remote condenser.

    The competition's product will work like a dehumidifier blowing hot dry air into the house until a thermostat calls for cooling, at which point it dumps the heat outside to the remote condenser and just blows cool air in the house.

    I think teddy has a system where the heat can be dumped via duct work, which he would be more than happy to explain.

    York has some packaged units that will function as a dehumidifier as well, the DR series.


    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
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    SHR of 0.87 sounds suspsiciously high for your area p-student
    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/

  9. #9
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    Apr 2002
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    That sounds more like Dallas than Houston.

    It seems like manual j data, lists summer grains as the amount of grains above what you are trying to maintain indoors, rather than the actual grains in the outside air.

    Looking at it from the standpoint of the total amount of moisture held in outside air, as a worst case 0.4% scenario

    Dallas/FTW airport is published as 132 grains and 82F, Houston Hobby 147 grains and 84F.

    Just goes to show the most humid time often does not coincide with the hottest time of year.



    [Edited by Carnak on 03-17-2006 at 08:06 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/

  10. #10
    Join Date
    Apr 2002
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    11,808
    Originally posted by teddy bear
    Certainly, a/c with full reheat eleminates the need for a dehumidifier. Several commercial expensive units are available. Lennox is introducing a partial reheat residential unit at a premium price. According to their presentation, a partial cooling load is required to avoid over cooling. It's a matter of investment, operating cost, fexibility, and meeting specific needs. The installed cost of the more complex reheat a/c is more than a high SEER a/c + ventilating dehu. High efficiency dehus remove 5 pints per kwh verses a/c with reheat 2 pints per kwh. The ventilating dehu is more refined for accurate amounts of fresh, filtered air ventilation. Dehumidify the fresh air before adding it to the home is more efficient. The controls on the a/c with reheat are complex requiring top notch techs. We here the problems of set up and diagnosing problems on the current premium a/c systems.
    Adding a dehu to a problem home is much more effective than replacing an entire a/c system that provides marginal results.
    The industry does not want to talk about fresh air ventilation, because of the high latent loads. The refinement of controlling fresh air to make minimal fresh air ventilation work in a home is a science equal to humidity control. Currently, the best a/c system will not provide the same benefits. Someday the best a/c will do everything we do. Hopefully, we will be able to improve function and decrease the investment to deserve a share of the future bussiness. Why is there resistance to being part of an improved way providing ideal comfort and IAQ? TB
    The cadallac system controls humidity and temperature separately, not ever job needs a cadallac. But to be fair, just because a person only needs a k-car does not mean he can't have a cadallac if he wants one.

    Teddy mentioned commerical units, and York has had a DR series packages out for a while.

    http://www.yorkupg.com/NewInfo.asp?id=455&t=c&PID=7

    The larger ones are dual circuit, meaning they can run both compressors on a call for dehumidification, and use the heat of rejection of one compressor as hot gas reheat.

    End up with dry air in the 70s being supplied.

    They make smaller packages but I THINK they are only single circuit, one compressor. So when they go into reheat mode the air coming out is quite hot.

    http://www.yorkupg.com/NewInfo.asp?id=942&t=c&PID=7

    Lennox and Carrier have their packages with dehumidification options, getting reheat by liquid line subcooling or hot gas reheat as well.

    Addison has been building units for a while, designed to even go 100% outside air, they are expensive.

    I have had success with custom built stuff by Engineered Air down here, not cheap by any means either.


    The small copeland compressor in the photo is for supplemental dehumidification, and its condensing coil is in the air stream as well for reheat, will supply 63F air with a 50 dewpoint. If the space temp rises, it will shut off the small copeland and pull temperature down in about 2 minutes time

    They are building small residential sized 100% outside air units in Florida right now, the manufacturer's name escapes me at the moment but I bet it starts at 6 or 7 grand to be able to handle a couple hundred CFM.

    Dectron offers 100% outside air equipment as well and so does heat pipe technology, but these are also made for the commercial market and expensive.

    Maybe one day teddy will be selling the total cooling package, I see a niche market for it.
    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/

  11. #11
    Join Date
    Apr 2002
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    11,808
    Heat Pipe Technology has a Z-Coil, a cooling coil with wrap around heat pipes, that is matched up with a York standard and VS blower modules.

    This is enhanced dehumidification but does not cycle on just to control humidity

    Z-coil info

    http://www.heatpipe.com/mktg_materia.../dinhzcoil.pdf

    A look at the coil match with a York Blower

    http://www.heatpipe.com/ProductsServ...AhrSeriers.htm


    These guys like to push the dust mite angle too

    When people ask me to spec out the system I would put in my own home I go with the heat pipe air handler. Air handler costs as much if not more than the condensing unit.

    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/

  12. #12
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    Apr 2002
    Posts
    11,808
    Originally posted by pstu

    A definition of SHR is in this article "IN AIR CONDITIONING, IF BIGGER IS NOT BETTER, WHAT IS?":
    http://www.proctoreng.com/articles/better.html

    The article repeats the familiar principles which many of us have heard repeatedly. It discusses what makes an AC good at humidity removal, although it does not discuss two stage designs.

    The point that article makes on proper sizing is evident in the FSEC study you posted.
    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/

  13. #13
    the idea came from a supermarket setup i did with lots of display cases and refrigeration... it had the same problem -no AC load high humidity-it used hot gas from the refrig. equipment to reheat air so the cooling load was handled primarily by the display cases and the AC primarily removed moisture... in a res set-up we could run a hotgas reheat the same way with two coils..

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