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  1. #1
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    Two Questions About HVAC Operation

    1. Assuming one had their home and AC Unit in order, proper unit specs for the home, good insulation, house sealed properly, etc, if the outside conditions became hot enough with humidity, could a unit struggle to keep the house at a certain temperature during the day?

    2. Is there anything that could be added to a system to add air freshner to it? Something to fill the house when the air handler is on?

    Thanks in advance,
    Lee

  2. #2
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    When the outdoor ambient temperature rises above the design conditions, the unit will run constantly and very well could loose ground to the current set point. Even though nothing is wrong.
    I think the big box stores sell some air freshener thing that fits on the air filter. Never tried one. Many systems are undersized with duct/filter area anyway so adding something that blocks more area hasn't ever really interested me.
    I havent failed. Ive just found 10,000 ways that wont work. - Thomas Edison

    Its not whether you get knocked down, its whether you get up. - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  3. #3
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    Quote Originally Posted by rider77 View Post
    When the outdoor ambient temperature rises above the design conditions, the unit will run constantly and very well could loose ground to the current set point. Even though nothing is wrong.
    I think the big box stores sell some air freshener thing that fits on the air filter. Never tried one. Many systems are undersized with duct/filter area anyway so adding something that blocks more area hasn't ever really interested me.
    Thanks rider.

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  5. #4
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    Quote Originally Posted by leehop71 View Post
    1. Assuming one had their home and AC Unit in order, proper unit specs for the home, good insulation, house sealed properly, etc, if the outside conditions became hot enough with humidity, could a unit struggle to keep the house at a certain temperature during the day?

    Lee
    Yes, during the peak of the day when (if) heat gain exceededs the a/c's capacity, expect the temp/%RH to rise. There is a undersizing craze in the industry because a/cs are unable to maintain low %RH during times of low/no cooling loads and high outdoor dew points.
    Better to have an a/c slightly bigger than peak sensible/latent cooling loads including real fresh air ventilation.


    Quote Originally Posted by leehop71 View Post
    1
    2. Is there anything that could be added to a system to add air freshner to it? Something to fill the house when the air handler is on?

    Thanks in advance,
    Lee
    Did you mean adding fresh air or air freshner? Tell us about this.
    Most experts tell us we need a fresh air change every 4-5 hours to purge indoor pollutants and renew oxygen when occupied.
    Nothing to do with air freshners?
    Starting from scratch, we need a big enough a/c to handle this load at peak load. The a/c should be setup with a cold enough cooling coil to remove latent load (moisture) at this condition. Peak typically is 75% sensible and 25% latent. This about 3 lbs. of dehumidification per ton of cooling capacity. Fresh air and occupants moisture in green grass climates is about 10-15 lbs. per hour even during the evening hours of the day with low/no sensible cooling load.

    Thus you need reheat or supplemental dehumidification to control these variable cooling loads.
    Homes have become sealed containers that require mechanical fresh filtered air devices.
    Enter the small whole house ventilating dehumidifier. Using this unit, set the temperature you want and the %RH you want and be comfortable while controlling mold and dust mites.
    There are months when no sensible cooling is needed but you have 1-4 lb. per hour dehumidification load.
    Straighten out this "air freshner" thing, so I understand.
    Keep us posted, its a good discussion.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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  7. #5
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    I was referring to air freshener.

  8. #6
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    Teddy Bear has a good point about regular fresh air purging...
    Houses can be 'too tight'... and get 'stale' inside.
    And the addition of chemical 'perfume' is not a healthy thing to do.

    Design temperature (design conditions), noted above in post #2 above... is a standard average temp for the summer. We know temps get warmer (hotter) than averages... The reason for average design temperature is so the system will not be SOOO oversized the house is damp from humidity all the time... as systems need adequate run time to dry out the air.

    There are multiple ways to accomplish both low %RH AND adequate cooling on the HOT days...
    Installing a whole house dehum unit is one of them.
    Another is supplemental AC capacity (multi-stage).
    Another would be a high tech inverter drive AC system that adjusts to the load.

    Considerations and PRO/CON's for each solution should be thought out before choosing one.
    GA-HVAC-Tech

    Quality work at a fair price with excellent customer service!

    Romans Ch's 5-6-7-8

    2 Chronicles 7:14

  9. #7
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    Looks like since all 3 of us have allergies, dehumidifier would be the way to go. Research says I would need a 70 pint machine to cover my 1500 sq ft.

    What is the benefit of one with a built in pump?

    Assuming it would be best to locate centrally in the house as much as possible?

  10. #8
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    As rider77 said, "Even though nothing is wrong". Here on LI,NY the older design temp was based on a 20*TD between indoor/outdoor temp( 95* out,75*in or 100* out/80* in). Then the newer design TD is based on a 15*F TD or 95*F outdoor/80* indoor.

  11. #9
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    The preferred method of dehumidification would be to locate the dehumidifier next to the a/c and route the dry air from the dehumidifier to the ducts down stream of the a/c cooling coil. This gets you distribution of dry air throughout the home without operating the a/c blower. The ducted dehumidifiers are more durable and efficient than the typical residential types. Also many have the fresh air ventilation option. Air freshners are not a solution to indoor air quality problems.
    I am not aware of any method of keeping a home <50%RH without supplemental dehumidification.
    Check out the Ultra-Aire 70H, a sponsor of this site for a durable high efficiency whole house system. Hopefully you can plumb the condensate drain into the a/c drain. If you have a basement/crawlspace, make sure to incorporate the humidity of these spaces as part of humidity control to provide indoor air quality.
    A free standing dehumidifier is a last resort and better than high humidity in the home. You must have auto draining to make dehumidification possible.
    Keeping the home <50%RH controls dust mites and mold.
    Keep us posted.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  12. #10
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    Hey Teddy Bear,

    Can you post a chart of what 'issues' happen at each higher humidity level...
    As:
    At 55%, these issues become an issue...
    At 60%, these issues become an issue...
    At 65%, these issues become an issue...
    Etc...

    Would be interesting...

    THX

    GA
    GA-HVAC-Tech

    Quality work at a fair price with excellent customer service!

    Romans Ch's 5-6-7-8

    2 Chronicles 7:14

  13. #11
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    Quote Originally Posted by ga-hvac-tech View Post
    Hey Teddy Bear,

    Can you post a chart of what 'issues' happen at each higher humidity level...
    As:
    At 55%, these issues become an issue...
    At 60%, these issues become an issue...
    At 65%, these issues become an issue...
    Etc...

    Would be interesting...

    THX

    GA
    These are great questions. Lets see if I can give answer that makes sense. This will be a challenge for both of us. Some did this for many years ago.
    My statement is that the a/c is unable to maintain 50%RH with low/no sensible cooling loads and high outdoor dew points. Supplemental dehumidification is required.

    Assuming 75^F, 50%RH, a 55^F dew point inside condition is desired. Also assuming a fresh air change in 4-5 hours which minimum, occurs.
    In most homes this about 100 cfm of fresh air. Also assume 4 occupants which has a latent cooling load of 1,000-2,000 buts. Lets say 1.5 lbs. of dehumidification per hour.
    The biggest factor is the moisture in the outdoor air. For simplicity lets use ^F dew point to describe it.
    The neutral point would be a outdoor dew point of 55^F without occupants. There is not moisture load from outdoor air or occupants.
    The wind blows and/or the clothes drier operates, make no difference to indoor moisture content. When the home is 75^F, the indoor %RH will be 50% regardless how hard the wind blows or what operates. Add occupants with a 2 lbs. per hour moisture load, up goes the indoor %RH and dew point. If the wind is calm and outdoor temps are neutral, no cooling load what will the indoor %RH and dew point be after 12 hours? The occupants add 1.5 lbs. per hour without a/c cooling or air passing through the home.
    You get the point, this is going to be a damp home. 18 lbs of moisture into a 20,000 ft^3 air space raises the indoor %RH and dew point to near saturation extremes. The building materials will absorb moisture short term and help. This is not good. Adding a 1.5 lbs. per hour dehumidification will neutralize the problem.
    Adding a 100 cfm of fresh outdoor air with a 43^F dew point will also maintain a the desire 50%RH or 55^F dew point by purging the moisture from the occupants.
    Lets kick it up a notch.
    Its 70^F, 90%, 67^F dew point outside and with have the same indoor occupants moisture load with a 100 cfm of infiltration/ventilation and we want 75^F, 50% RH, a 55^F dew point inside.
    Passing the outside air through the home purges indoor pollutants, renew oxygen and the occupants add 1.5 lbs. of moisture per hour. The interior and exfiltrating air from the home the interior materials absorb moisture from the increased %RH will be 75^F, 80%RH, a 68^F dew point. The a/c will not run enough to remove any moisture. A 2 speed or VS a/c will not remove any significant moisture with substantial reheat.
    We need 2.6 lbs./hour of moisture removal to get the 100 cfm outdoor dew point air down to 55^F dew point plus 1.5 lbs/hour for the occupants moisture removal. Total 3.1 lbs. per hour to maintain 50%RH.
    Come back for claification on any points.

    A Chart for 100 cfm of air infiltration/ventilation
    Outdoor dew points------ Indoor %RH @ 75^F____ +- Lbs./hr of moisture 50%RH

    45^F _______________ 34%RH _________________1.4
    50^F_________________ 41%RH ________________1
    55^F _________________50%RH ________________ 0
    60^F__________________59%RH________________ -1
    65^F__________________71%RH_______________ -1.8
    70^F__________________ 84%RH________________-2.9

    Hope this does for you what it did for me.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  14. #12
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    Quote Originally Posted by leehop71 View Post
    1. Assuming one had their home and AC Unit in order, proper unit specs for the home, good insulation, house sealed properly, etc, if the outside conditions became hot enough with humidity, could a unit struggle to keep the house at a certain temperature during the day?

    2. Is there anything that could be added to a system to add air freshner to it? Something to fill the house when the air handler is on?

    Thanks in advance,
    Lee
    Something seemed off about your posts and I feel more confident about my hunch.

    You can get a heat exchanging air exchanger which lets you bring in outside air and cool it/dehumidify it using the indoor air on it way out the vent. If pollens or other particulate matters affect you, you could use the same grade filter you would use in the air handler to avoid them from coming in, or do you have some issues regarding business process effluence that may cause community concerns or violate emission standards and attract attention of authorities?

  15. #13
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    Sounds like you ASSumed wrong. A properly sized unit will dehumidify first and then reach set point. Water also known as R-718 will be collected on the evaporator and actually act as an insulator. Once the insulator is removed the the evaporator can then collect more heat.

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