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  1. #1
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    This is probably an old question, but it is important to me:

    Is it unwise to have a fresh air inlet on the AC return plenum, because of being in a hot-humid climate?

    Would that really increase the humidity in the house, or would the fresh-air intake tend to displace infiltration elsewhere in the home? Is there any good info on this?

    Thanks in advance -- P. Student

  2. #2
    Join Date
    Aug 2003
    Posts
    626
    It all depends on how tight the home is. Fresh air intakes to the return work well, in very tight homes (less than .30 NACH), with an electronic damper control unit (ex. Aircycler) that lets one control the amount of air coming into the home.

    [Edited by uktra on 10-15-2004 at 10:19 AM]

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,336
    Originally posted by perpetual_student
    This is probably an old question, but it is important to me:

    Is it unwise to have a fresh air inlet on the AC return plenum, because of being in a hot-humid climate?

    Would that really increase the humidity in the house, or would the fresh-air intake tend to displace infiltration elsewhere in the home? Is there any good info on this?

    Thanks in advance -- P. Student
    A frseh air inlet on the return is better than no fresh air. Operating air handlers create possitve/negative pressures throughout the ducts and rooms. The pressure difference cause fresh air ventilation. Adding a fresh air return decreases the negative pressure on the home. This stops humid air from being sucked in to wall cavities etc. That's good. But it is not an effective ventilation device because it only works when the air handler operates on high speed. We need fresh air whenever the home is occupied, regard of heating/cooling needs. Opeating an air handler during the occupied hours is expensive compared to a small dedicated fan. A small dedicated fan could be setup to blend, filter, and circulate the fresh air throughout the home. Fresh air is critical to health regardless of humidity, so humidity must be controlled(<50%RH). Humid climates should have reheat or dehumidification to avoid mold potiential and have fresh air ventilation.

  4. #4
    Join Date
    Apr 2002
    Posts
    11,808
    Use the fresh air intake with fan on Auto
    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/

  5. #5
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Air infiltration measurements

    The house I am particularly wondering about, is 3430 sqft, in Houston area. A blower door test was done and it tells me "Estimated Manual J Air Change Rate" was

    Winter = 0.40 per hour or 249 CFM
    Summer = 0.24 per hour or 149 CFM

    This would be in the "medium" category for tightness, would it not? The summer figure would seem to meet the criteria Uktra mentioned, for being tight enough (in summer at least) that a fresh air intake would perform as recent literature says.

    I suppose there would indeed be some extra latent load, or is that not necessarily so? I am wondering if the natural infiltration would tend to be via the dozen old fashioned (i.e. leaky by design) ceiling canister lamps. My hypothesis is air from that source may carry even more humidity from the attic air, and a fresh air intake may conceivably not add latent load.

    Does this make sense to you? Does this modify any of your advice? I really appreciate everyone who has responded.

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    With natural infiltration the humidity from the outside air has to be removed from the room air by the air conditioner. You try to remove extra humidity out of air that is already dry.

    With a fresh air intake and the air handler fan in auto mode, the house gets pressurized intermittently meaning dry air is trying to escape the house therefore humidity is not entering the house. At the same time the evap coil is facing warmer and more humid air. There is an additional load on the coil but it is easier for the coil to remove sensible and latent heat from the miced air, than it is for it to remove extra humidity from the dry air.

    Using a fresh air intake is like wearing a condom. Not using the intake is like getting an abortion. Keep the humidity out of the house in the first place.
    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/

  7. #7
    Join Date
    Jun 2000
    Location
    NC
    Posts
    108
    I have done some work for a mech. engineer that did a blower test on my duct work and a blower door test on the whole house at start up and we set a balancing damper on the 6" run that was about 15' from the blower itself. I cant remember what the setting was but when I questioned him he said that he included it in his heat load calc.

  8. #8
    Join Date
    Feb 2004
    Location
    NC
    Posts
    143
    P student
    Remember law 1. One cfm in equals 1 cfm out. When ventilation air eclipses ach natural, the ventilation rate dominates the moisture load. They are not cumlative.

  9. #9
    hey carnac What the hell are you smoking out there?

  10. #10
    Join Date
    Apr 2002
    Posts
    11,808
    I smoke whatever I can get my hands on supplies little scarce here these days. The fresh air intake is the best info provided on your site. You looking to rumble?

    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
    May 2004
    Location
    south louisiana
    Posts
    3,161
    .35 ACH per hour is the standard in Louisiana,
    Florida standards are .25 ACH
    Even with the Fla standard your house is not 'too tight'.
    Was a recommendation to add fresh air suggested by the
    person who tested you home?
    Air from your recessed lights should be stopped.
    Airtight trim kits do the job well. Hot humid attic
    air enters through these areas and reduces comfort
    and IAQ.
    Once these cans are sealed, adding a fresh air may
    become necessary.
    A cost effective way to do this is to add a fresh
    air intake, with a filter back grill, to filter air
    before bringing it inside. Here hvac companies are
    using a 6" duct with a manual damper ducted to the
    return. Air passes though the filter usually located
    under a soffit, to the return where it is dehumidified
    before entering the conditioned areas.
    Makeup air is 10 cfm per person, and 15 cfm per gas
    appliance in conditioned areas.
    I have seen barometric dampers also. They seem to work
    well, but there is some discussion as to the long term
    solution.

    The cure of the part should not be attempted without the cure of the whole. ~Plato

  12. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,336
    Originally posted by energy_rater_La
    .35 ACH per hour is the standard in Louisiana,
    Florida standards are .25 ACH
    Even with the Fla standard your house is not 'too tight'.
    Was a recommendation to add fresh air suggested by the
    person who tested you home?
    Air from your recessed lights should be stopped.
    Airtight trim kits do the job well. Hot humid attic
    air enters through these areas and reduces comfort
    and IAQ.
    Once these cans are sealed, adding a fresh air may
    become necessary.
    A cost effective way to do this is to add a fresh
    air intake, with a filter back grill, to filter air
    before bringing it inside. Here hvac companies are
    using a 6" duct with a manual damper ducted to the
    return. Air passes though the filter usually located
    under a soffit, to the return where it is dehumidified
    before entering the conditioned areas.
    Makeup air is 10 cfm per person, and 15 cfm per gas
    appliance in conditioned areas.
    I have seen barometric dampers also. They seem to work
    well, but there is some discussion as to the long term
    solution.

    How does this strategy get fresh air into the home when routinely occupied? This type of ventilation only works when heating/cooling at peak load which is a small percent of the time that you need ventilation. In addition, you are over-ventilating when unoccupied during peak load heating/cooling. In a modern house, I suggest a small 100 watt fan to blend a specific amount of fresh air with house air, filter the blend, and circulate the blend throughout the house using the heating/a/c ducts. The fan may be operated on a routine time schedule to match at at home schedules or constant operation. Natural infiltration or controlled ventilation in green grass climates requires supplemental dehumidification to avoid high indoor humidity during cool damp weather. If your climate only has occasional cool wet weather, a simple residential dehumidifier may do the job. In humid climates, it is much more efficient to use a dedicated high efficiency ventilating dehumidifier to optimize indoor air quality.

  13. #13
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Couple of replies

    ENERGY_RATER_LA wrote:

    >>Was a recommendation to add fresh air suggested by the person who tested you home?

    No, in fact nobody I have talked to locally has been very encouraging. Mostly I perceive they are unfamiliar with the concept, so I don't consider them expert opinion compared to a number of experienced people from this forum.

    >>Air from your recessed lights should be stopped.

    I totally agree, just have not done it yet. It seems wrong to me these lights are even offered for sale, but we seem to live in an unenlightened world.



    TEDDY_BEAR wrote:

    >>How does this strategy get fresh air into the home when routinely occupied?

    One solution would be a timer device which makes the air handler run at a minimum duty cycle. I believe both Aprilaire and Lipidex (Aircycler) make such things.
    http://www.aircycler.com/
    http://www.aprilaire.com/pdfs/10005994.pdf

    As I understand it, both will refrain from adding any runtime if the airhandler is already doing enough, and some models can operate a motorized damper to put a maximum on air intake in any given hour.

    Your preferred method of a dedicated intake fan sounds good also. My main observation is -- the passive intake to the air handler return, will tend to give positive house pressurization at those precise times the air handler might tend to produce negative pressurization (e.g. via supply leaks). That sounds like a benefit to me.

    Another observation about the 1st method is -- will a variable speed air handler make it exceedingly difficult to understand the correct size for a passive intake?

    Thanks -- P.Student

    [Edited by perpetual_student on 10-25-2004 at 11:53 AM]

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