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  1. #1
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    Jun 2004
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    There is a HVAC tech who seems otherwise very good, tells me that a fresh air inlet duct would allow a lot of humidity into the house. I live in S.Texas and the climate is indeed hot and humid. The FA duct I imagine, would be probably 5-inch flex from the AC return plenum, to a pickup point under the eaves on the north side of the house. There would be a filter, a damper and I would expect airflow to be under 100 CFM when the AC is running full speed (possibly 2-stage system).

    The counter argument that I have heard is: the extra humidity load will be less than you think, in most houses. That there *already* is a sizable infiltration and that is worse because of being unfiltered, untreated air. The argument says that having a FA inlet increases the pressure inside the house (or at least makes it suck less) and there will be a reduction in infiltration.

    Certainly the FA inlet provides at least one pass thru the AC system to dehumidify. And it will be filtered whereas the infiltration air won't. Is there a common sense way to measure what's going on? Possibly to estimate the additional humidity load on the AC system?

    And last, is there any literature which attempts to give answers? Something from Lstiburek would be ideal, or Florida's FSEC maybe.

    Thanks very much -- Pstu

  2. #2
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    24/7 or long run times I'd be concerned.Why not use an ERV??

  3. #3
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    Why

    It is my understanding that the central air handler is a source of de-pressurization and I cannot change its nature. The ductwork is in a vented attic. If the ductwork leaks only 5% the EPA will pat me on the back with Energy Star rating, but still with a 1400 cfm air handler I will be leaking 70 cfm every minute it is on (assuming non-VS for this logic). Having a passive FA inlet is the simplest way I can think of, that will have the inlet air supplied when needed and only then. Unless I get kinda Rube Goldberg, it seems the only way.

    If you are concerned, then I am concerned. Just trying to figure out what is knowable here.

    I get the impression that one can only expect a modest amount of humidity transfer from an ERV. This is from reading a few specs, am I in error here?

    Best wishes -- Pstu

  4. #4
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    Jun 2003
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    Madison, WI/Cape Coral, FL
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    6,592

    Re: Why

    Originally posted by pstu
    It is my understanding that the central air handler is a source of de-pressurization and I cannot change its nature. The ductwork is in a vented attic. If the ductwork leaks only 5% the EPA will pat me on the back with Energy Star rating, but still with a 1400 cfm air handler I will be leaking 70 cfm every minute it is on (assuming non-VS for this logic). Having a passive FA inlet is the simplest way I can think of, that will have the inlet air supplied when needed and only then. Unless I get kinda Rube Goldberg, it seems the only way.
    I get the impression that one can only expect a modest amount of humidity transfer from an ERV. This is from reading a few specs, am I in error here?
    Pstu,I agree with your logic and this is a move in the right direction. The main problem with negative pressure is outside high dew point air sucked into the wall/insutlation. High dew point air condenses on the cool drywall/insulation, growing mold and peeling paint. ERV is a balanced air flow device. An ERV takes out 70 cfm, adds 70 cfm and transfers half of moisture from the wet air to the dry air. If you add the ERV ventilation to the home, the airhandler leakage/infiltration negative pressure problem continues. The home is over ventilated by the combination with additional humidity load. This not a solution to the negative pressure problem. The make-up air suggestion solves the negative air pressure problem. To get fresh air without cooling/heating load, you could operate the airhandler when the home is occupied on a timer. This would also help the bath exhaust and clothes drier operation. Maintaining <50%RH is last important point avoid growing mold/dust mites and to providing indoor air quality. Supplemental dehumidification is a must in any greed grass climate for high humidity during low/no cooling load.
    There are ventilating dehumidifiers that bring make-up air, blend the make-up with house air, filter both, and dehumidify when needed. Enough! TB

  5. #5
    Join Date
    Nov 2004
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    2,927
    have you tried get the honeywell "gray manual"
    it was available at their site to download. its a big 300 or so pages of this type stuff ,and the formula you need might be in there

  6. #6
    Join Date
    Nov 2004
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    Originally posted by pstu
    and I would expect airflow to be under 100 CFM when the AC is running full speed (possibly 2-stage system).


    Is there a common sense way to measure what's going on? Possibly to estimate the additional humidity load on the AC system?

    And last, is there any literature which attempts to give answers? Something from Lstiburek would be ideal, or Florida's FSEC maybe.

    I looked around for the ready made formula but didnt find it.
    Dont know how to accurately predict what will happen.

    it shouldnt be too hard measure after the fact.
    use your psychrometric chart,and compare what your system does with and without the 100 cfm of unconditioned air. the results will vary with outdoor conditions

    as for predicting ,I havent figured that out yet,but I am close to a breakthrough.Math isnt kind to me.

  7. #7
    Join Date
    Jun 2001
    Location
    Louisville, KY
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    12,189
    From a mathematical perspective, the additional latent load can be calculated.

    H(L) = CFM * 0.68 * (W2-W1)

    where

    H(L) = Latent Heat Btuh
    CFM = air flow
    W2 - W1 = change in Grains of water per lb. of dry air


    TEDDY: Why couldn't you unbalance the ERV? There are balancing dampers in the connecting ducts. You deal with these auxillary devices regularly. So, I'd like your opinion.

  8. #8
    Join Date
    Apr 2002
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    11,808
    In a commercial application and an economy of scale, you can run the ERVs unblanced and maintain a slight positive pressure in a humid climate.
    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
    Join Date
    Apr 2002
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    11,808
    Originally posted by pstu
    There is a HVAC tech who seems otherwise very good, tells me that a fresh air inlet duct would allow a lot of humidity into the house. I live in S.Texas and the climate is indeed hot and humid. The FA duct I imagine, would be probably 5-inch flex from the AC return plenum, to a pickup point under the eaves on the north side of the house. There would be a filter, a damper and I would expect airflow to be under 100 CFM when the AC is running full speed (possibly 2-stage system).

    The counter argument that I have heard is: the extra humidity load will be less than you think, in most houses. That there *already* is a sizable infiltration and that is worse because of being unfiltered, untreated air. The argument says that having a FA inlet increases the pressure inside the house (or at least makes it suck less) and there will be a reduction in infiltration.

    Certainly the FA inlet provides at least one pass thru the AC system to dehumidify. And it will be filtered whereas the infiltration air won't. Is there a common sense way to measure what's going on? Possibly to estimate the additional humidity load on the AC system?

    And last, is there any literature which attempts to give answers? Something from Lstiburek would be ideal, or Florida's FSEC maybe.

    Thanks very much -- Pstu
    First off pstudent, decent indoor air quality can be maintained with intermittent ventilation. Others argue for a constant lower level of fresh air.

    Keeping it intermittent and coinciding with the compressor cycling on and off then allows a system to draw in, cool and dehumidify this outdoor air and in the process pressurizes the home with dry air.

    As you will see on the Florida Solar Site, ASHRAE's humidity design and control manual and other published reports, 2 pascals of depressurization or being depressurized continuosly by 0.008" WC with respect to the outdoors has caused millions of dollars of moisture damage in the hotel/motel industry alone.

    Florida Solar documents how being pressurized by a mere 2 Pascal tends to reverse the process so that dry air tends to leak out.

    If your ducts were not leaking in the attic, a slight pressurization could tend to stop the humid air from infiltrating in during the cooling season when the system was running.

    So when the system cycles off you are prone to natural wind driven infiltration but it is not a constant problem. Running continuous exhaust fans(even attic power venters) or leaking supply ducts in the attic is a problem. The net difference between supply duct leaks in the attic and a fresh air intake could be too small as well. The leaking ducts are the problem.

    I do not like a constant lower level of fresh air in a humid climate as it pumps in the humidity when the system is off, drives up humidity, then you are getting forced to run a dehumidifier to control it else deal with higher than required indoor RH. So promoting this approach sells more dehumidifiers.

    If you are under 60% RH you are doing fine. Saying that you must always be under 50% is a good pitch to mandate selling a dehumidifier.

    The only time I see it recommended to keep it under 50% at all times is for archives and libraries, paper loves to absorb mositure and hates to let go of it. Paper is predigested mold food as well.

    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
    Dec 2000
    Location
    Michigan
    Posts
    724

    Hmm Stack Affect?

    "there *already* is a sizable infiltration"


    pstu, can you please explain "stack affect" in a home when the inside temperature is lower then the outside temperature?

    I am not aware of a home with AC operating going negative, unless there are exhaust fans running or leaking duct work in a non-conditioned space. Therefore, why do you think you require additional outside air to be brought into the home?

    Finally, if a home has positive pressure, summer or winter, it will increase the possibility for mold in the exterior cavities!

    Fix the problem, do not treat the symptoms.
    The quality of my performance, sometimes depends on the quality of my audience.
    Imitation (Plagiarism) is the best compliment one can get -- "Open A Window"

    To improve Indoor Air Quality: Control Indoor Air QUANTITY = "I.A.Q.Q."

  11. #11
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    Estimating "natural" infiltration

    I appreciate every one of the replies. Overall it seems there exists no plain and simple answer to my queries. It might help that a blower door test reported 69 CFM ventilation would be provided by building leakage -- in the context that 64 cfm more would be required to meet ASHRAE 62P ventilation level.

    The same report said air changes at 50 Pa = 5.3, and said Manual J air change rate is 0.24/hr or 149 CFM summer, 0.40/hr or 249 CFM winter (same section said C=261 and N=0.650, the meaning of those last two are completely unknown to me). One thing of note: the summer conditions are visited upon us day after day, reliably between June and September. We can count on that. The winter conditions will only exist very sporadically -- typical weather in winter months bears no resemblance to design temperatures. This is very unlike colder climates, but in this hot-humid climate I would mostly neglect the winter problem.

    I suppose that model contains a professional opinion that this house has approx. 70 CFM infiltration average? Under those assumptions I would guess that up to 35 CFM via fresh air inlet, would essentially displace existing infiltration and add very little humidity to the house. And a trivial amount of positive pressure. If fresh air were upped to 70 CFM the existing infiltration would be reduced but not eliminated, and I would guess the total humidity load would be increased slightly. But as 70 CFM of outside air (114 grains humidity at design conditions) would go directly to the AC, it would be substantially conditioned by the time it got to any room. I would expect the AC to do slightly more work, but the IAQ to be improved also.

    There remains the problem of air handler runtime being very high in summer, rather low in spring, fall and winter. I would think the correct design would size the duct small enough to avoid over-ventilation in the peak summer. That would leave the rest of the year well short of ASRAE standards, however those milder months are exactly the sort where opening a window makes sense. On average anyway.

    Tentatively, I think a fresh-air inlet would be endorsed by some FSEC papers, and if kept on the skinny side would not hurt the humidity problem. Are there any golden nuggets of info, perhaps from the blower door report, that I have overlooked?

    Thanks as always! -- Pstu

    [Edited by pstu on 02-12-2006 at 02:35 PM]

  12. #12
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    A few scattered thoughts

    From that blower door test, the report tells me I need 409 CFM to obtain 2.0 Pa pressure difference. With the two air handlers of 2600 CFM combined capacity, it would take over 15% leakage and simultaneous running to produce that. The CFMs for any reasonable FA inlet, will have less than 1.0 Pa consequence, perhaps far less. Although I will seal every leak I can find, I doubt this house will get to the level to ever really worry about mold or other problems in the structure, traceable to infiltration. In my opinion the problem is limited to IAQ.

    Thinking this through, makes me see the extra benefits of a ventilating whole-house dehumidifier such as Teddy Bear promotes. A more direct solution to the humidity issue, if not the pressure issue.

    Xavier: Excellent and interesting question, but I cannot explain "stack effect" in summer. Rather think it is academic. Take note there is far less difference between indoors and outdoors in summer, than there is in cold weather -- I submit it is a small phenomenon as a result. As stated earlier, I most certainly do have ducts and air handler in non-conditioned space, they can never be sealed 100% or even 97% actually, that would be the main source of pressure issues. And I believe your statement about positive pressurization is correct in cold climates where exfiltrated air meets cold objects below its dew point. In hot-humid climates that cannot happen with exfiltration, it can happen with infiltration.

    Researched who is near that can do Aeroseal. Remarkably, there is a franchise 65 miles away in "Aggie-Land" (College Station TX), but none whatsoever in the Houston metro region. Care to travel a bit, Dash? <g>

    Best wishes -- Pstu

    [Edited by pstu on 02-12-2006 at 03:06 PM]

  13. #13
    Join Date
    Apr 2002
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    11,808

    Re: Stack Affect?

    Originally posted by Xavier
    "there *already* is a sizable infiltration"


    pstu, can you please explain "stack affect" in a home when the inside temperature is lower then the outside temperature?

    I am not aware of a home with AC operating going negative, unless there are exhaust fans running or leaking duct work in a non-conditioned space. Therefore, why do you think you require additional outside air to be brought into the home?

    Finally, if a home has positive pressure, summer or winter, it will increase the possibility for mold in the exterior cavities!

    Fix the problem, do not treat the symptoms.
    Not likely in a house but possible in a taller building, kind of an iverted stack. Cold air falls to lower levels, pressurizes low level, depressurizes upper level.
    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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